Hawaii Energy and Environmental Technologies (HEET) Initiative

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1 Hawaii Energy and Environmental Technologies (HEET) Initiative Office of Naval Research Grant Award Number N Assessment of Desiccant Dehumidification: Literature and Technology Review Prepared by: Prepared for: University of Hawaii at Manoa, July 2016

2 ASSESSMENT OF DESICCANT DEHUMIDIFICATION Project Deliverable 2: Literature and Technology Review July 9, 2016 Final Prepared by: Manfred J. Zapka, PhD, PE James Maskrey, MEP, MBA Sustainable Design & Consulting LLC Prepared for: Hawaii Natural Energy institute

3 ASSESSMENT OF DESICCANT DEHUMIDIFICATION Project Deliverable No. 2: Literature and Technology Review FINAL Prepared for July 9, 2016 Prepared by: Manfred J. Zapka, PhD, PE (1) James Maskrey, MEP, MBA, Project Manager (2) (1), Honolulu, Hawaii (2) Honolulu, Hawaii

4 ACKNOWLEDGEMENTS This research was supported and funded by the Office of Naval Research grant award N in collaboration with the at the University of Hawaii at Manoa. The authors believe that desiccant cooling applications can play a significant part making building conditioning more energy efficient and foster the implementation of more environmentally friendly ways to provide occupant comfort in buildings.

5 ABBREVIATIONS AC Air conditioning ACHR Air Conditioning, Heating and Refrigeration NEWS AHU Air handling Unit ASHRAE American Society of Heating, Refrigerating, and Air Conditioning Engineers BET Brunauer Emmett Teller CHP Combined Heat and Power CA conditioned air (also CA after treatment) COP Coefficient of performance DP Dew point DX Direct Expansion Air Conditioning System DEAC Direct Evaporative Air Coolers DEC Direct evaporative cooler EER Energy Efficiency Ratio Engelard Titanium Silicate (ETS) EAC Evaporative Air Cooling external source of energy (Q) GTI Gas Technology Institute GCI Green Cooling Initiative HNEI HR Humidity Ratio HVAC Heating Ventilation Air Conditioning IC Internal Combustion IEAC Indirect Evaporative Air Coolers LDDX Liquid Desiccant DX System IDECOAS Indirect and Direct Evaporative Cooling Assisted Outdoor Air System IRR Internal Rate of Return LD Liquid Desiccant LDAC Liquid Desiccant Air Conditioner LD IDECOAS Liquid Desiccant Indirect and Direct Evaporative Cooling Assisted Outdoor Air System IEC Indirect Evaporative Cooler LiCl Lithium Chloride LCST Lower Critical Solution Temperature M cycle Maisotsenko (M) Cycle MIT Massachusetts Institute of Technology

6 ABBREVIATIONS (CONT.) O&M OA ph. PV RH SHR R SAC SDC SI TAT TSA VAC WBT VAV DEVAP Operations & Maintenance Outdoor (or outside) Air (also OA after treatment) Numeric Scale Used to Specify the Acidity or Basicity (alkalinity) of an Aqueous Solution Photovoltaic System, also solar PV Power System Relative Humidity Sensible Heat Ratio Separation Factor Solar air conditioning International System of Units Thermally Activated Technologies Temperature Swing Adsorption Vapor Compression Air Conditioning Wet bulb temperature Variable Air Volume Desiccant Enhanced Evaporative Air Conditioning UNITS atm Atmosphere pressure BTU British Thermal Unit BTUH/sqft BTU per square feet C Degree Celsius CFM Cubic feet per minute F Fahrenheit K Kelvin kw kilo Watt kj kilo Joule MPa Mega Pascale square feet Square Feet

7 TABLE OF CONTENTS TABLE OF CONTENTS SECTION 1 EXECUTIVE SUMMARY AND OVERALL CONCLUSIONS... 1 SECTION 2 INTRODUCTION OF DESICCANTS IN REFRIGERATION AND COOLING... 3 SECTION 3 BASICS OF THE DEHUMIDIFICATION PROCESS Methods of Dehumidification Review of Psychrometrics Principles Application of Psychrometric Chart Introduction to Desiccants Desiccant Cycle Desiccant Material SECTION 4 METHODS OF DEHUMIDIFICATION Cooling based Dehumidification Desiccant Dehumidification Comparing Desiccant to Cooling based Humidification SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Basic Considerations of Physical Properties in Desiccant Dehumidification Isotherms Types of Isotherm Models Adsorption Energy Chemical and Physical Stability Adsorption Rate and Performance Basic Heat and Mass Transfer Modelling by Example of Desiccant Wheel Heat and Mass Transfer Characteristics of Desiccant Polymers SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES Desiccant Regeneration with Heat from Natural Gas or other Fossil Fuels Desiccant Regeneration with Waste Heat Desiccant Regeneration with Solar Heat and other Renewable Heat Sources July 9, 2016 Project Deliverable No. 2.: Literature and Technology Review Page i

8 TABLE OF CONTENTS TABLE OF CONTENTS (CONT.) SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Thermal Cooling Technologies Evaporative Cooling General Principle Direct Evaporative Cooler Working Process Indirect Evaporative Cooler Working Process Maisotsenko (M) Cycle Enhanced Indirect Evaporative Cooling Absorption Chillers Adsorption Chillers Comparison between Absorption and Adsorption Cooling Technology Magnetic Refrigeration as an Alternative to Vapor Compression Refrigeration SECTION 8 SOLID DESICCANT COOLING SYSTEMS Solid Desiccant Cooling Systems using Different Cooling Devices Solid Desiccant Cooling with Evaporative Cooling Solid Desiccant Cooling to Increase Conventional AC System Solid Desiccant Colling with Solar Heat SECTION 9 LIQUID DESICCANT COOLING SYSTEMS General System Configuration of Liquid Desiccant Cooling Systems Liquid Desiccant Systems to Enhance Conventional AC System Liquid Desiccant Cooling with Evaporative Cooling and Solar Energy Desiccant Enhanced Evaporative Air Conditioning SECTION 10 GENERAL FINDINGS OF THE LITERATURE REVIEW REVIEWED LITERATURE July 9, 2016 Project Deliverable No. 2.: Literature and Technology Review Page ii

9 SECTION 1 EXECUTIVE SUMMARY AND OVERALL CONCLUSIONS SECTION 1 EXECUTIVE SUMMARY AND OVERALL CONCLUSIONS The present literature and technology review is the deliverable of Part 1 of the project Assessment of Desiccant Dehumidification which Sustainable Design & Consulting (SDC), LLC is performing for Hawaii Natural Energy Institute (HNEI). Part two of the project follows the literature review and will assess the feasibility of using candidate desiccant dehumidification systems in conjunction with building conditioning in the hot and humid sub tropical climate of Hawaii Under Part 1 of the project, an extensive literature review has been performed on the subject of desiccant dehumidification with an emphasis on cooling and dehumidification applications for buildings. A further emphasis was placed on building and space conditioning in humid climates, such as is found throughout the Hawaiian islands. Since the literature review addresses many technical and applied scientific papers and publications discussing intricate thermodynamic concepts, the literature review also presents some important thermodynamic concepts to facilitate understanding by the non technical or the non HVAC expert reader. This literature and technology review presents a succession of dehumidification technology developments over the past 20 to 30 years. Desiccant dehumidification is being extensively used in specific industrial and commercial applications where dry air is required below the dew point that could be achieved with cooling coil based dehumidification. This literature and technology review focuses on the use of desiccant dehumidification technology in conjunction with building conditioning. To date these desiccant applications have mostly used solid desiccant material installed in vessel swing operation or rotary wheel structures. The use of desiccants in conventional AC has been mostly concerned with energy savings by recovering sensible and latent loads in central AC systems. Such desiccant applications have become standard applications for conventional AC systems. Liquid desiccant dehumidification in conjunction with conventional has only received limited attention in the literature. The use of desiccants in conjunction with evaporative cooling, often referred to in the literature as desiccant cooling, was introduced about two decades ago and has not seen a wide spread use in building designs and operations, although such systems can generate significant energy savings. Both solid and liquid desiccant systems are suitable for integration with evaporative cooling applications. While solid desiccant dehumidification integrations have the advantage of longer track record in actual building installations, it is liquid desiccants which have attracted much attention in recent developments of desiccant cooling operations. In fact, since 2010 several groundbreaking liquid desiccant cooling technologies have been proposed. These recent liquid desiccant technologies developments suggest significant application potential for Hawaii, because of their favorable process performance even in July 9, 2016 Page 1 of 171

10 SECTION 1 EXECUTIVE SUMMARY AND OVERALL CONCLUSIONS Hawaii s humid climate and their ability to work with low grade, unconventional heat sources, including solar heat and waste heat from combined heat and power applications. July 9, 2016 Page 2 of 171

11 SECTION 2 INTRODUCTION OF DESICCANTS IN REFRIGERATION AND COOLING SECTION 2 INTRODUCTION OF DESICCANTS IN REFRIGERATION AND COOLING Over the past decade, the worldwide demand for residential and commercial cooling has been growing at a strong pace. Most of the near term and long term growth will come from developing countries and countries in transition. Industry related assessments for the short term (five year) development of the world wide refrigeration and air conditioning market (ACHR, 2016) indicate an overall annual growth of 6%. Under this projection, the biggest growth is projected with eight percent in both the Americas and Asia and two percent in Europe and the Middle East, India & Africa. Asia Pacific is reported as the largest market in absolute sales, with China and Japan representing 82 percent of that market by value. A long term prediction of the global market by the Green Cooling Initiative (GCI) predicts that between 2010 and 2030 the market volume of the refrigeration and air conditioning sectors for these countries is projected to increase by factor of nearly four (GCI, 2016). While the demand for cooling is increasing, conventional cooling technology causes significant environmental impact mainly due to harmful refrigerants and high energy consumption. This fact coupled with the market trend suggests that impacts from cooling applications will likely increase in the years to come. The logical method for impact mitigation would be to reduce the two main causes by significant reduction in energy demand and release of harmful process agents, meaning harmful refrigerants. Another possible mitigation measure is to develop and implement environmentally friendly sustainable cooling technologies as a way to mitigate negative impacts. GCI promotes the use of Green cooling, an overall strategy that leads to impact mitigation for the cooling sectors (GCI, 2016). Green cooling uses a combination of three approaches, which include maximizing energy efficiency, encouraging the use of natural refrigerants, and fostering a sustainable approach to private and commercial energy consumption. Green cooling helps to protect the environment, resources and the climate and supports the use of renewable technologies within cooling. It thereby contributes towards a sustainable reduction of fossil fuel consumption. The Heating Ventilation and Air Conditioning (HVAC) industry has acted upon the need for impact mitigation with the introduction of various technology innovations. Innovations include, but are not limited to, variable capacity compressors, system improvements in humidity control, high efficiency fans and building construction improvements (Kammers,2010). Among these innovations, improved humidity control provides significant benefits, in particular for the residential and commercial building sector. Innovative humidity control extends beyond the conventional humidity management technology of providing cold surfaces for below dew point separation of humidity in conditioned spaces. This innovative humidity control separates the processes of heat rejection and humidity control in space conditioning applications, which means the removal of sensible and latent loads, respectively. In this July 9, 2016 Page 3 of 171

12 SECTION 2 INTRODUCTION OF DESICCANTS IN REFRIGERATION AND COOLING application, desiccant humidification replaces in of cold coil dehumidification. Using conventional HVAC technology, three space condition functions of ventilation, sensible heat and latent heat removal are performed concurrently, by means of air flow forced over external cooled coils. This conventional process invariably causes energy losses and frequent over cooling of spaces, as excess cooling must be provided to remove humidity, especially in hot and humid climates. Desiccant dehumidification avoids some of the drawbacks by providing a means to dehumidify spaces without the need of cold coils for condensation. As fundamental shift in space condition, desiccant dehumidification allows for a separation of sensible and latent heat removal in space conditioning, as desiccants remove humidity, and therefore the latent load, and a separate cooling technology removes the sensible load. Desiccant dehumidification is not a new technology used in the refrigeration and cooling industry. In fact, numerous industrial and commercial processes rely on low humidity process conditions, which can be achieved more economically with desiccants rather than with cooling based dehumidification. In the building industry, desiccant dehumidification has seen a limited range of applications. The reason for the limited acceptance of desiccant dehumidification can be attributed to initial installation costs, the fact that operational benefits are not fully understood, lack of technology awareness, and company priorities, which are not focused on benefits of new technology (ECW, 2000). The main benefits reported by the desiccant industry are the ability for precise humidity control and for the utilization of low grade heat sources, such as solar and waste heat. One of the other main reasons why desiccant systems have not been used to a great extent in space conditioning is the fact that in high dew point applications, vapor compression systems are usually more energy efficient than desiccant systems. The typical dew point in space conditioning, e.g. airconditioning, is about 50 F, for a standard conditioned space temperature and relative humidity of 72 F and 50%, respectively. Therefore, dew point reduction applications in buildings favor cooling based dehumidification, based on cost advantages and technology familiarity of conventional vapor compression cooling technology by building management. When efficiency gains through building system integration and the building industry movement towards green building technologies become more important considerations, benefits for the use of desiccant systems can outweigh reasons to stick with the conventional HVAC. Apart from the key advantage of independent sensible and latent heat removal, the use of waste or solar heat for desiccant regeneration can lead to very favorable operational conditions. In fact, desiccant systems can compete if large amounts of outside air must be processed, if electric demand charges are high, or if large amounts of low costs solar and waste heat are available. The use of low cost solar energy and waste heat from industrial processes for regeneration of desiccant material is making the dehumidification system more cost effective. The initial cost of solar energy can be significant, but life cycle costs typically contribute to a favorable present value analysis. Solar July 9, 2016 Page 4 of 171

13 SECTION 2 INTRODUCTION OF DESICCANTS IN REFRIGERATION AND COOLING radiation is intermittent and weather dependent. Therefore, back up heat sources or heat storage is required to continue the drying process when solar energy is not available. Besides desiccant dehumidification as a means to augment conventional cooling technology, another technology approach is the integration of desiccant dehumidification with evaporative cooling. This combination offers significant energy saving and environmental impact mitigating potential. Evaporative cooling is the adiabatic increase in absolute humidity with a simultaneous reduction in dry bulb temperature. Here, air is subjected to free water surfaces which enhances evaporation while keeping the enthalpy of the air constant. Typically, evaporative cooling is only effective at high temperatures and low humidity levels, since the driving forces for evaporation are reduced in humid climates. Since the Hawaii climate is the representative environmental condition for this literature review, stand alone evaporative cooling would not be considered as applicable. Desiccant dehumidification can, however, favorably enhance conditions for evaporative cooling by reducing the dew point of the external air upstream of the evaporative cooler. Apart from the purely technical considerations, the success of solar/desiccant cooling technologies will depend on the encouragement and promotional schemes offered by the policymakers, and the efforts undertaken by the manufacturers to improve the cost efficiency as well in developing better technologies. July 9, 2016 Page 5 of 171

14 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS SECTION 3 BASICS OF THE DEHUMIDIFICATION PROCESS This section introduces basic processes of dehumidification. Dehumidification is an essential process with a wide range of industrial and commercial applications, most of them with specific dehumidification technologies. The focus of this literature and technology review is, however, on dehumidification in conjunction with building conditioning. Therefore, only processes and technologies which are pertinent for space conditioning are discussed in more detail. 3.1 Methods of Dehumidification There are two basic methods of dehumidification, cooling based and desiccant dehumidification. Apart from these there are more specialized niche methods, which use pressure and electric field properties. These dehumidification technologies have, however, only very limited application potential in building conditioning and are not further discussed in this review Cooling based Dehumidification The method of dehumidification in conventional air conditioning systems uses cooling coils which are maintained at a temperature below the desired dew point of the supply air. As air flows past the cooling coils the air temperatures drop and water vapor condenses on the coils. Lowering air temperature decreases the air s ability to hold moisture. Thus, the air can be made drier by cooling it. The water condensate drains into a pan by means of gravity and is thereby removed from the system. The amount of moisture removed from the air is a function of the temperature of the coil. The coils are cooled by chilled water, refrigerants, glycol solutions, or engineered fluids circulating through the coil. Since this method reduces the temperature of the HVAC system supply air, spaces could be subcooled if the rooms do not have sufficient internal heat gain to offset the low temperature air. Typically, the supply air is therefore reheated downstream of the cooling coils to provide the desired indoor air supply temperature. Reheating the supply air requires a significant amount of energy, depending on the target room temperature. In applications that require low air temperature for dehumidification purposes, while the requirement for sensible heat removal would not require such low air supply temperatures overcooling can be a problem. In these applications a heat energy recovery strategy could be effectively used to lower the energy input for reheat the air downstream and for pre cooling the air up stream of the cooling point. A suitable strategy would be a heat pipe run around system, where coils on either side of the cooling coil and use a refrigerant to economically pre cool and reheat the air stream. Another limitation to this technique is the freezing point of water. When air is dried via refrigeration, the cooling surfaces of the coils may attain sub freezing temperatures. Under these conditions ice can form on the coils, which reduces the efficiency of the cooling system. July 9, 2016 Page 6 of 171

15 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Desiccant Dehumidification Desiccant dehumidification provides an effective method for dehumidification through the use of sorption of water vapor to and into a material that has a natural affinity to water. Desiccant dehumidification uses adsorption and absorption, which use physical processes to take up the additional moisture given up by the air without changing the size or shape of desiccants. Absorbents take up humidity by allowing water vapor to enter into the desiccant material. Adsorbent materials, on the other hand, attract and hold water molecules in pores at their surface. Absorbents generally can attract and hold greater quantities of water per pound of desiccant material. The desiccant based dehumidification does not require elaborate cooling equipment as used in the cooling based dehumidification process. In typical technical dehumidification application, the desiccant agent takes up moisture until an equilibrium state is reached between the desiccant material and the air. At this point the desiccant is regenerated, typically through thermal processes, after which the desiccant is again available for the dehumidification process. There are two technical processes under which desiccant dehumidification is carried out, liquid and solid desiccant systems. Liquid desiccant systems: In liquid desiccant dehumidification, a liquid desiccant solution is brought into contact with the air stream and water vapor is entering the desiccant solution through a difference in vapor pressure. The method of providing free surfaces of liquid desiccant is dependent on the design of the process apparatus, which can be based on spray, packed column or bubble bed. When an unsaturated desiccant solution is exposed to air, the desiccant absorbs moisture from the gaseous phase. The process conditions can be varied by changing the concentration of the liquid desiccant, pressure and temperature. Upon saturation, which means a process threshold in the diminishing ability to take up more moisture, the liquid desiccant is regenerated by heating it and subsequent desorption of captured moisture to a waste air stream. An additional benefit of using selected liquid desiccant systems is that the desiccant solutions can act as a biocide for the conditioned air, which is beneficial in applications for which bacteria or viruses are the least desirable. Solid desiccant systems: In solid desiccant dehumidification, a solid desiccant material is brought into contact with the air stream and water vapor attaches to the surface of the desiccant material. Moist air is drawn through the spaces around desiccant material, which absorbs the moisture. As the desiccant reaches its capacity, the material is moved into a warmer reactivation air stream. Moisture is rejected from the desiccant surface through a difference in vapor pressure and is expelled by a waste air stream. The regeneration of the desiccant material requires a heated or dry air stream. There are different configurations of solid desiccants devices, such as the often used desiccant wheel. July 9, 2016 Page 7 of 171

16 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Other dehumidification methods There are other dehumidification methods for specialized industrial and commercial applications in use. One of these methods is dehumidification of air by means of compression. As the pressure in air is increased, the dew point or temperature at which water will condense is raised. In order to get dry air within the vessel, a heat sink is needed to cool the compressed air. Typically, costs can be prohibitive when used outside the specialized process, because of the equipment, space, and auxiliary equipment required for the process. If compressed air is already available and only very small amounts of dry air are needed for humidity control, compression could be feasible route to dry air. Dehumidification by compression of air is, however, not feasible for application of space conditioning. One new humidification method is electrostatic dehumidification technology. Rather than removing humidity from the air by means of cooling based phase change or physical sorption processes, moisture is removed through electrostatic fields which separate moisture vapor from air streams. The reported benefit is a significant reduction in energy used for dehumidification, by eliminating energy needed for a phase change or the regeneration of desiccants. This literature review has not identified advanced prototypes or even commercially available of electrostatic dehumidifiers at this time. 3.2 Review of Psychrometrics Principles Psychrometrics describes the physical and thermodynamic properties of gas vapor mixtures, whereby the properties of moist air, e.g. a mixture of water vapor and air, is of significance to human comfort and conditions in the built environment. In the process of humidification and dehumidification properties of the moist air are changed, sometimes with or without transfer of heat or matter between the air and its surroundings, which is referred to as isothermal or adiabatic processes. The review of dehumidification methods necessitates a basic understanding of psychrometric principles. The following provides an overview of some of the most important topics and metrics of moist air processes related to dehumidification. A more comprehensive introduction to moist air properties and processes used in the HVAC is given by ASHARE (2009). Psychrometric Chart The analysis of moist air properties in HVAC processes is facilitated by the psychrometric chart and related property tables. There are multiple psychrometric charts available, each providing graphical interpretations for constant pressures. Figure shows the ASHRAE psychrometric chart for sea level (ASGRAE, 2009). The chart is in SI units. If any two properties of the moist air are known, the chart allows for a quick assessment of all other properties of moist air. July 9, 2016 Page 8 of 171

17 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Dry Bulb Temperature The dry bulb temperature is also referred to as the sensible temperature of the air the heat which can be sensed by a dry thermometer. In the chart below, the dry bulb temperature of the air is displayed at the bottom, increasing from left to right. Constant dry bulb temperatures are depicted as vertical lines. (refer to Figure 3.1.2). Figure 3.1.1: Psychrometric Chart for pressure at sea level (ASHRAE, 2009) Presented in SI (metric units) Figure 3.1.2: Dry bulb temperature displayed in the psychrometric chart, (ASHRAE, 2009) In the depicted psychrometric chart temperature are given in degrees Celsius ( C). Base psychrometric Chart by ASHRAE; modified for illustration example July 9, 2016 Page 9 of 171

18 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS The dry bulb temperature is usually referred to as air temperature. The dry bulb temperature is measured with a thermometer that is exposed to the air but shielded from radiation, and refers basically to the ambient air temperature. It is called "Dry Bulb" because the air temperature is indicated by a thermometer not affected by the moisture of the air. Wet Bulb Temperature The Wet Bulb temperature is the adiabatic saturation temperature. This means the temperature at which water, by evaporating into moist air at a certain dry bulb temperature and absolute humidity ratio, can bring air to saturation adiabatically, at a constant pressure. The adiabatic evaporation of water and resulting the cooling effect causes the wet bulb temperature to be lower than the dry bulb temperature. The wet bulb temperature can be measured by a psychrometer, which consists of two thermometers, where one thermometer s sensor is covered by a wetted wick. Placing the instrument in an airstream promotes evaporation and cooling, with the equilibrium temperature being the wet bulb temperature. While this procedure is not strictly adiabatic saturation, the resulting error compared to the exact thermodynamic wet bulb temperature is sufficiently small enough to allow use of the experimental procedure for technical applications. Figure illustrates wet bulb temperatures as lines in the psychrometric graph. Figure 3.1.3: Wet bulb temperature displayed in the psychrometric chart Base psychrometric Chart by ASHRAE; modified for illustration example July 9, 2016 Page 10 of 171

19 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Relative Humidity Percent of Saturation In the technical literature relative humidity is often referred to as the moisture content of air, expressed as a percent value, which air "can hold". This definition is not exact since it does not express a temperature dependency, and sometimes causes confusion with the absolute humidity. As can be seen in Figure On the psychrometric chart, relative humidity is displayed as a series of curves, increasing from the bottom of the chart. The saturation curve is the left boundary and represents 100% relative humidity. (see Figure 3.1.4) Figure 3.1.4: Relative humidity of airwater vapor mixtures Base psychrometric Chart by ASHRAE; modified for illustration example The more exact definition of relative humidity in air can be expressed by vapor and air partial pressure, density of the vapor and air, or by the actual mass of the vapor and air. Since partial vapor pressure is an important property for all dehumidification processes, we define relative humidity as the ratio of vapor partial pressure in the air to the saturation vapor partial pressure of the air at a certain dry bulb temperature. The following equation calculates the relative humidity (The EngineeringToolBox). ϕ = p W / p WS * 100% (1) where: ϕ = relative humidity (%) p W = vapor partial pressure (mbar) p WS = saturation vapor partial pressure at the actual dry bulb temperature (mbar) July 9, 2016 Page 11 of 171

20 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Dew Point Temperature: With decreasing temperatures moist air can hold less moisture than at higher temperatures. The dew point refers to the temperature at which water vapor starts to condense out of the air. The dew point depends on the absolute amount of water vapor in the air. The higher the amount of water vapor in the air the higher is the dew point. Figure shows an example of a dew point for a sample of moist air, whose properties are defined by dry bulb and wet bulb temperature of 20 C and 16 C, respectively. Consequently, dew point temperature t dp is the temperature of moist air saturated at pressure p, with the same absolute humidity as that of the given sample of moist air. Cooling based dehumidification systems remove moisture from air by cooling it below its dew point temperature, at which water vapor condenses and is separated from the air as liquid phase draining from the cold coils. Figure 3.1.5: Dew point temperature The example shows a sample of moist air with dry bulb and wet bulb temperatures at 20 C and 16 C, respectively (A). The value of absolute moisture of the sample is determined by the horizontal line (A) to (C). As the sample is cooled (sensitive cooling) the state point of the sample moves along the horizontal line towards the saturation line (B). The dry bulb temperature of the intersection of the horizontal line of constant absolute humidity and the saturation curse is the dew point for the sample. Base psychrometric Chart by ASHRAE; modified for illustration example Humidity Ratio The humidity ratio can be defined as the ratio between mass of water vapor present in moist air to the mass of dry air. x = m w / m a where x = humidity ratio (kg/kg or g/kg) m w = mass of water vapor (kg or g) m a = mass of dry air (kg) July 9, 2016 Page 12 of 171

21 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Figure illustrates the use of humidity ratio for a sample of moist air, whose properties are defined by dry bulb and wet bulb temperature of 20 C and 16 C, respectively. Figure 3.1.6: Humidity Ratio The example shows a sample of moist air with dry bulb and wet bulb temperatures at 20 C and 16 C, respectively (A). The resulting humidity ratio can be read from the graph as approximately 9.7 g/kg, which indicates that are 9.7 g of water vapor per kg of dry air. The humidity ratio can also be expressed as kg/kg. In our case 9.7 g/kg converts to kg/kg. Base psychrometric Chart by ASHRAE; modified for illustration example Enthalpy Enthalpy is the measure of the total energy in air. The enthalpy of moist air includes sensible and latent heat, which is the enthalpy of the dry air and the enthalpy of the evaporated water in the air, respectively. The total enthalpy, e.g. the sum of sensible and latent, is used when calculating cooling and heating processes. The specific enthalpy of moist air is defined as the total enthalpy of the dry air and the water vapor mixture per unit mass (of dry air. Therefore, the specific enthalpy has the units of kj/kg da. Specific enthalpy of moist air can be expressed as (ASHRAE, 2009): h = ha + x hw where h = specific enthalpy of moist air (kj/kg, Btu/lb) ha = specific enthalpy of dry air (kj/kg, Btu/lb) x = humidity ratio (kg/kg, lb/lb) hw = specific enthalpy of water vapor (kj/kg, Btu/lb) The determination of difference in enthalpy when going between state points in the psychrometric chart is of important in determining energy requirements in heating and cooling processes. Figure illustrates the use of enthalpy in moist air applications for a sample of moist air, whose properties are defined by dry bulb and wet bulb temperature of 20 C and 16 C, respectively. July 9, 2016 Page 13 of 171

22 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Figure 3.1.7: Enthalpy The example shows a sample of moist air with dry bulb and wet bulb temperatures at 20 C and 16 C, respectively (A). The resulting specific enthalpy is shown as 44.5 kj/kg da Base psychrometric Chart by ASHRAE; modified for illustration example Specific Volume Often it is required to convert between mass and volumetric flow rates or to calculate energies required to move between state points. Therefore, the specific volume v of a moist air mixture is expressed in terms of a unit mass of dry air: v = V/M da [m 3 /kg da ] where V is the total volume of the mixture and M da is the total mass of dry air Figure illustrates the determination of specific volume for a sample of moist air, whose properties are defined by dry bulb and wet bulb temperature of 20 C and 16 C, respectively. The use of psychrometric chart in solving mass and heat transfer processes in cooling is best carried out with illustrative examples. Section 3.2 presents three representative cases, which helps in the understanding of cooling and dehumidification processes presented in this literature review. For a more analytical explanation of the examples refer to ASHRAE (2009). July 9, 2016 Page 14 of 171

23 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Figure 3.1.8: Humidity Ratio The example shows a sample of moist air with dry bulb and wet bulb temperatures at 20 C and 16 C, respectively (A). The resulting specific volume is shown as m 3 /kg da Base psychrometric Chart by ASHRAE; modified for illustration example 3.3 Application of Psychrometric Chart This section presents three case applications of the psychrometric chart, which helps the reader to understand cooling and dehumidification processes presented in this literature and technology review. Application of psychrometric chart Illustrative Case A; Sensitive Cooling Statement: Moist air at dry bulb and wet bulb temperature of 30 C and 18 C, respectively, enters a cooling coil assembly at a rate of 5 m3/s. The chilled air leaves the coil at a dry bulb temperature of 20C. Determine the rate of heat that has to extracted from the air flow. Solution: Figure shows the heat and mass flow diagram of the example. Figure shows the psychrometric properties of the moist air at state points (1) and (2). July 9, 2016 Page 15 of 171

24 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Figure 3.2.1: Heat and mass flow diagram Moist air with properties at (1) and at 5 m 3 /s flow rate enters the system interface cooling coils assembly (1) and is cooled by an external heat sink. The rejected heat through the cooling coils results in the air properties at system interface (2). m da is the mass flow in m 3 /s h is the specific enthalpy in kj/kg da W is the humidity ratio in g/kg da 1q 2 is the heat rate in kw that is extracted from the air flow between interfaces (1) and (2) Figure 3.2.2: Psychrometric property for illustrative case A The following properties are read from the chart: h 1 = 50.5 kj/kg da h 2 = 40.0 kj/kg da v 1 = m 3 /kg da W 1 = W 2 = 8 g/kg da Base psychrometric Chart by ASHRAE; modified for illustration example Since there is no phase change the moisture ratio of conditions (1) and (2) remain constant at 8 g/kg da The air mass flow rate is calculated with m da = 5.0 m 3 /s / m 3 /kg da = 5.75 kg da /s. The rate of heat that is extracted from the air is calculated as follows: 1q 2 = m da *( h 1 h 2 ) = 5.75 [kg da /s] * ( ) [kj/kg da ] = 60.4 [kj/s] = 60.4 kw July 9, 2016 Page 16 of 171

25 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Application of psychrometric chart Illustrative Case B; Moist air cooling and dehumidification: Statement: Moist air at 32 C dry bulb temperature and 60% RH enters a cooling coil at 10 m 3 /s and is processed to a final saturation condition at 10 C. Find the required rate of heat extraction (refrigeration). Solution: Figure shows the heat and mass flow diagram of the example. Figure shows the psychrometric properties of the moist air at system interfaces (1) and (2). Figure Heat and mass flow diagram for illustrative case C (ASHRAE, 2009) Moist air with properties at (1) and at 10 m 3 /s flow rate enters the system interface (1) and is cooled by an external heat sink. The rejected heat through the cooling coils results in the air properties at system interface (2) and a mass of water (condensate) m da is the mass flow in m 3 /s h is the specific enthalpy in kj/kg da W is the humidity ratio in g/kg da h w is the specific enthalpy of water at saturation at final saturation temperature m w is the mass flow rate of condensed water 1q 2 is the heat rate in kw that is extracted from the air flow between interfaces (1) and (2) Figure indicates the energy equation as follows: Heat flux incoming Heat flux outgoing m da * h 1 m da * h 2 + m w * h w + 1 q 2 July 9, 2016 Page 17 of 171

26 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Figure 3.2.4: Psychrometric property for illustrative case B The following properties are read from the chart: h 1 = 70.5 kj/kg da h 2 = 29.5 kj/kg da v 1 = m 3 /kg da W 1 = 15.0 g/kg da W 2 = 7.6 g/kg da Base psychrometric Chart by ASHRAE; modified for illustration example The air mass flow rate is calculated with m da = 10.0 m 3 /s / m 3 /kg da = 11.3 kg da /s. The tabulated value of specific Enthalpy, of Water at Saturation at (2) is kj/kg w The rate of heat that is extracted from the system is calculated as follows: m da * h 1 = m da * h 2 + m w * h w + 1 q 2 1q 2 = m da (h 1 h 2 ) m w * h h w With: m w = m da * (W 1 W 2 ) 1q 2 = m da (h 1 h 2 ) m da * (W 1 W 2 ) * h W 1q 2 = m da ((h 1 h 2 ) (W 1 W 2 ) * h W ) 1q 2 = 11.3 [kg da /s] * (( ) [kj/kg da ] (( )/1000) [kg w /kg da ] * [kj/kg w ]) 1q 2 = kj/s = kw July 9, 2016 Page 18 of 171

27 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Application of psychrometric chart Illustrative Case C; Adiabatic mixing of two moist air streams Statement: Two streams of moist air are adiabatically mixed. Stream (1) has 4 m 3 /s with 6 C and 4 C dry bulb and wet bulb temperatures, respectively and Stream (2) has 10 m 3 /s with 25 C dry bulb temperature with 60% RH. Find the dry bulb temperature and RH of the resulting moist air mixture. Solution: Figure shows the heat and mass flow diagram of the example. Figure shows the psychrometric properties of the moist air at system interfaces (1) and (2). Figure 3.2.5: mass and energy flow diagram for illustrative case C Two streams (1) and (2) of moist air are adiabatically mixed. The resulting mixed stream (3) is a combination of the two psychrometric conditions. m da is the mass flow in m 3 /s h is the specific enthalpy in kj/kg da W is the humidity ratio in g/kg da Under the adiabatic conditions conservation of energy and mass results in the following three relationships: Heat flux incoming Heat flux outgoing m da1 * h 1 + m da2 * h 2 m da3 * h 3 Dry air mass incoming Dry air mass outgoing m da1 + m da2 Water vapor mass incoming m da3 Water vapor mass outgoing m da1 * W 1 + m da2 * W 2 m da3 * W 3 July 9, 2016 Page 19 of 171

28 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Figure : Psychrometric properties for illustrative case C The following properties are read from the chart: h 1 = 17.2 kj/kg da h 2 = 55.7 kj/kg da v 1 v 2 = m 3 /kg da = m 3 /kg da W 1 = 4.3 g/kg da W 2 = 12.0 g/kg da Base psychrometric Chart by ASHRAE; modified for illustration example The three relationships can be rearranged: h 2 h 3 W 2 W 3 m da1 h 3 h 1 = W 3 W 1 = m da2 This results in a weighted distribution of properties, where the resulting properties of condition (3) as follows: m da1 = 4.0 [m 3 /s] / [m 3 /kg da ] = 5.2 [kg da /s] m da2 = 10.0 [m 3 /s] / [m 3 /kg da ] = 11.6 [kg da /s] m da1 / m da2 = 5.2 / 11.6 = [ ] h 3 = (h *h 1 ) / = 43.9 [kj/kg da ] W 3 = (W * W 1 / = 9.6 [g/kg da ] Figure shows the results depicted in the psychrometric chart July 9, 2016 Page 20 of 171

29 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Figure 3.2.7: Resulting psychrometric properties for illustrative case C h 3 = 43.6 kj/kg da W 3 = 10.0 g/kg da T db3 = 18.5 C RH 3 = 74% The property of (3) is on line between (1) and (2); the distance (2) to (3) is 44.4% and (3) to (1) is 55.6% of the distance (1) to (2) Base psychrometric Chart by ASHRAE; modified for illustration example 3.4 Introduction to Desiccants Desiccants are types of sorbents, where sorbents are materials characterized by their ability to attract and hold gases or liquids. Desiccants have a particular affinity to water. Virtually any material acts as desiccants and attracts water. Building materials can attract sizable amounts of water, such as woolen carpets, which can hold up to 23% of their dry mass in water vapor. Commercial desiccants must have much higher ratios of water attraction, as high as 1000% their dry mass in water vapor, depending on the type and on the moisture available in the environment. In addition, desiccants continue to attract moisture even when the surrounding air is quite dry and the driving forces for sorption are small, a characteristic that other materials do not share. The process of water sorption is basically the same in all types of commercial desiccants. The conditioned desiccant material is subjected to a moist air stream and attracts moisture until saturated, reaching equilibrium with the surrounding air. The desiccant is then regenerated by applying heat to drive out moisture. Since the process of sorption generates heat due to latent heat of water vapor and, to a smaller amount, due to additional heat of sorption, the desiccant material has to be cooled before the next cycle of sorption can commence. There are two basic processes of sorption, referred to as adsorption or absorption, depending on whether the desiccant undergoes a chemical change as it takes on moisture. Adsorption does not change the desiccant, except by adding water vapor mass. Absorption changes the desiccant as water enters into and changes properties of the material. July 9, 2016 Page 21 of 171

30 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Typical Desiccant Applications Desiccants have been used in the HVAC industry to a certain extent, to dry moist ambient air. Their market penetration is, however, well behind cooling based dehumidification, due to cold coil dehumidification being a relatively simple and low cost alternative. While lacking in the number of installed units compared to conventional HVAC applications, desiccant dehumidification can offer tangible benefits under the following operating conditions: The latent load in the moist air is large in comparison to the sensible load; using only coolingbased dehumidification could overcool the building. Energy cost to regenerate the desiccant is low compared to the energy costs of refrigeration dehumidification, including the energy for reheating of low dew point air supply. If too low dew point air is required, refrigeration dehumidification coils would need to be operated at subfreezing dew points resulting in icing problems. 3.5 Desiccant Cycle The driving force of desiccant dehumidification is the difference between water vapor pressures at the desiccant s surface and of the surrounding air. Sorption of moisture occurs with a positive vapor pressure gradient at the desiccant surface, which means a higher vapor pressure in the surrounding air than at the exposed desiccant surface. Desorption occurs with an opposite pressure gradient, when the desiccant surface vapor pressure is higher than that of the surrounding air. The nonlinear relationship between desiccant moisture content and vapor pressure at the desiccant surface is depicted in Figure As the moisture content of the desiccant rises, so does the water vapor pressure at its surface. At some point, an equilibrium condition occurs, when the vapor pressure at the desiccant surface is the same as that of the surrounding air. At this point neither sorption nor desorption occurs, since the pressure difference has reached a minimum threshold which is an insufficient driving force. At this point the desiccant has to be regenerated to restore the desiccant s ability for sorption of water vapor. Figure illustrates the effect of temperature on vapor pressure at the desiccant surface. Both higher temperature and increased moisture content increase surface vapor pressure. This indicates that desiccants can attract moisture better at lower temperatures, since the pressure gradient between the surrounding air and the desiccant surface is larger. At higher temperatures the vapor pressure at the desiccant surface is higher, which results in less sorption or, in case of desiccant regeneration, in better desorption when moisture is driven from the desiccant to the surrounding air. Figure illustrates the sorption and desorption cycle used in desiccant dehumidification. The cycle starts at (1) with a cold and dry desiccant coming into contact with a moist air stream. During process July 9, 2016 Page 22 of 171

31 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS step (1) to (2) moisture is attracted and the moisture content of the desiccant increases. The desiccant surface temperature increases due to latent heat and heat of sorption. As the desiccant enters equilibrium conditions at (2) sorption stops. During process steps (2) to (3) external heat is added and the vapor pressure at the desiccant surface increases, creating a pressure gradient to drive moisture out of the desiccant. At (3) the moisture content of the has fallen to a desired process condition. During process step (3) to (1) the desiccant is cooled to attain the lower vapor pressure of the initial state at (1), where a new cycle starts. The effectiveness of desiccant cycle depends on the energy required to move through this cycle. The process step (1) to (2), when dehumidification occurs, requires only relatively small fan or pump energy to bring the desiccant and the air stream into physical contact. The process steps (2) to (3) and (3) to (1) requires significant heating and cooling energy, respectively. Figure 3.4.1: Desiccant Water Vapor Pressure as Function of Moisture Content and Temperature (ASHRAE, 2009) Figure 3.4.2: Desiccant Cycle (ASHRAE, 2009) During process step (2) to (3) heating requires the sum of the following heat input: Sensible heat input to raise the desiccant to a temperature high enough to make its surface vapor pressure higher than that of the surrounding air, the higher the Latent heat input to evaporate moisture July 9, 2016 Page 23 of 171

32 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Latent heat for desorption of water from the desiccant (a small amount) During process step (3) to (1) cooling requires sensible heat rejection to the surrounding, where the required cooling capacity is proportional to the desiccant mass properties and difference between its temperature at (3) and (1). Regeneration in the desiccant cycle also works with pressure differences. This is used in certain industrial and commercial applications but not in HVAC applications. Summarizing, the greater the difference between the air and desiccant surface vapor pressures, the greater is the ability of the desiccant material to attract moisture from the air. The selection of the most effective desiccant for a particular application depends on the range of water vapor pressures in the air, the temperature of the regeneration process, and moisture sorption and desorption characteristics of the desiccant. 3.5 Desiccant Material There are two forms of desiccant materials, solid and liquid desiccants. Liquid Desiccants act as absorbents to attract and hold moisture. In a liquid absorption dehumidifier, air is brought into contact with liquid desiccant solution. If the water vapor pressure of the liquid desiccant surface is lower than that of air, moisture is absorbed at the surface film surface and therefore moisture migrates from the air to the desiccant; this process is called dehumidification. Conversely, if water vapor pressure at the desiccant surface is higher than that in the air moisture migrates to the air; this process is called desorption. The vapor pressure of the liquid desiccant is directly proportional to its temperature and inversely proportional to its concentration. Figure illustrates the effect of temperature and concentration on the vapor pressure of various solutions of water and lithium chloride (LiCl), a common liquid desiccant. In the sample condition shown in Figure the solution that is 25% lithium chloride has a vapor pressure of 1.25 kpa and 3.3 kpa at temperatures of 21 C and 37.8 C, respectively. This indicates that the warmer the liquid desiccant is, the less moisture it can absorb from the air. In standard practice, performance of liquid desiccants is controlled by adjusting process temperature, concentration, and pressure. The desiccant process temperature is controlled by simple heaters and coolers to adjust the temperature of the desiccant. Liquid desiccant concentration is controlled by heating the desiccant to drive moisture out into a waste airstream or directly to the ambient. Commercially available liquid desiccants have an especially high water holding capacity. Selected products have water absorption capacity of more than 1,000 % on a dry mass basis. In order to achieve effective sorption and desorption processes, the two phase flow through the process reactor has to provide sufficient free surfaces of desiccant and a good flow distribution. High surface renewal rates and July 9, 2016 Page 24 of 171

33 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS maximum contact time between the gaseous and liquid phase allow the desiccant agent to approach its theoretical sorption and desorption capacity. Solid desiccants attract moisture as a surface active sorption process. The moisture does not change the nature of the solid adsorbent. The condensed adsorbed moisture is confined into capillaries, and the individual adsorbents attract moisture through electrical field charges at the desiccant surface. The electric field is not uniform in magnitude and polarity, therefore specific sites on the desiccant surface attract water molecules that have a net opposite charge. When the complete surface is covered with moisture, the adsorbent can hold still more moisture because vapor condenses into the first water layer and fills the capillaries throughout the material. The ability of solid adsorbents to attract moisture is a function of vapor pressure difference between the adsorbent surface and the surrounding air. Generally, solid adsorbents have a smaller capacity to attract water per dry weight than liquid absorbents. ASHRAE (2009) suggests that, a typical molecular sieve adsorbent can hold 17% of its dry mass in water when the air is at 21 C and 20% RH, whereas a LiClsolution can hold 130% of its mass at the same process conditions. Solid adsorbents have an advantage with respect to temperature dependency. Molecular sieves, for example, can adsorb moisture in warm airstreams, making dehumidification possible at elevated outdoor temperatures. Solid adsorbent can be manufactured with specific pore diameters to increase adsorption capacity for selected ingredient in the air stream. For example, water has an effective molecular diameter of 0.32 nm. A molecular sieve adsorbent with an average pore diameter of 0.40 nm adsorbs water but has almost no capacity for larger molecules, such as organic solvents. This selective adsorption characteristic is useful in many applications. For example, several desiccants with different pore sizes can be combined in series to remove first water and then other specific contaminants from an airstream. Intrinsic parameters that affect adsorption performance of different desiccant material include total surface area, total volume of capillaries, and range of capillary diameters. A large surface area gives the adsorbent a larger capacity at low relative humidity. Large capillaries provide a high capacity for condensed water, which gives the adsorbent a higher capacity at high relative humidity. A narrow range of capillary diameters makes an adsorbent more selective in holding vapor molecules. Figure illustrates the effects of the three parameters shown by three noncommercial silica gel adsorbents. Each of the adsorbents has a different internal structure. Gel 1, shown in Figure has large capillaries, making its total volume large but its total surface area small. Gel shows large and small adsorption rates at high and low relative humidity, respectively. For example, materials with large capillaries necessarily have a smaller surface area per unit of volume than those with smaller capillaries. As a result, adsorbents are sometimes combined to provide a high adsorption capacity across a wide range of operating conditions. July 9, 2016 Page 25 of 171

34 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Figure 3.5.1: Adsorption and Structural Characteristics of Some Experimental Silica Gels (ASHRAE, 2009) The figure illustrates the effect of capillary diameters on the capacity of adsorbents to attract and hold moisture. The adsorbent Gel 1 has a high capacity to attract condensation, which means at high relative humidity. Types of solid desiccants: Sorbent Systems (2016) presents five types of solid desiccants which are often used dehumidification applications: Montmorillonite Clay ( Clay ) is a naturally occurring porous adsorbent with good regeneration characteristics at low temperatures without substantial deterioration or swelling. Clay is inexpensive and effective within normal temperature and relative humidity ranges. Variations in adsorption performance are due to differences in obtaining this naturally occurring material. Silica gel has an amorphous micro porous structure with a distribution of pore opening sizes of roughly 3 60 angstroms, which results in a very large surface area that will attract and hold water by adsorption and capillary condensation. Silica gel is efficient at temperatures below 77 F, but loses July 9, 2016 Page 26 of 171

35 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS adsorbing capacity at higher temperatures, similar to Clay. Silica gel is non corrosive and nontoxic and has received approval for use in food and drug packaging. Molecular sieves (also Zeolite) has a larger adsorption capacity than either silica gel or clay. A molecular sieve is synthetic rather than naturally occurring, and therefore higher in cost per unit, but due to its extremely large range of adsorptive capabilities, it might often be the best value. Calcium oxide (CaO) can adsorb much greater amounts of water at low relative humidity than other materials. It is also effective in retaining moisture at high temperatures. This desiccant is relatively inexpensive as compared to many other desiccants. Calcium sulfate (CaSO 4 ), also known commercially as Drierite, is an inexpensive manufactured desiccant alternative general purpose desiccant geared mainly toward laboratory use. It is chemically stable, non disintegrating, nontoxic, non corrosive, and does not release its adsorbed water when exposed to higher ambient temperatures. CaSO 4 is a low cost desiccant but has a comparably smaller small adsorbent rate compared to dry weight. CaSO 4 has regeneration characteristics that tend to limit its useful life. Table 3.5.1: shows important properties of the five solid desiccants. Table 3.5.1: Properties of the five desiccants. (Sorbent Systems, 2016) Property Adsorptive Capacity at low H20 Concentrations Molecular Sieve Silica Gel Clay CaO CaSO4 Excellent Poor Fair Excellent Good Rate of Adsorption Excellent Good Good Poor Good Capacity for F, 40% RH Separation by Molecular Sizes Adsorptive Capacity at Elevated Temperatures High High Medium High Low Yes No No No No Excellent Poor Poor Good Good Figure shows the moisture adsorption capacity of the five desiccants as a function of relative humidity. Figure illustrates activation conditions of several desiccants. Activation conditions are of importance for regeneration. Figure illustrates the adsorption rates as a function of time. July 9, 2016 Page 27 of 171

36 SECTION 3 BASICS OF DEHUMIDIFICATION PROCESS Figure 3.5.2: Adsorption capacity for moisture as a function of relative humidity (Sorbent Systems, 2016) Figure 3.5.3: Temperature dependence of activation conditions for regeneration (Sorbent Systems, 2016) Figure 3.5.4: Equilibrium Capacity (H 2 O) of solid desiccants (Sorbent Systems, 2016) July 9, 2016 Page 28 of 171

37 SECTION 4 METHODS OF DEHUMIDIFICATION SECTION 4 METHODS OF DEHUMIDIFICATION This section presents main methods of dehumidification. While the topic of this literature report is desiccant dehumidification and related technology applications, a brief summary of cooling based dehumidification is provided. There are potential applications where desiccant and cooling based dehumidification can be combined to achieve effective cooling and space conditioning. 4.1 Cooling based Dehumidification The basis for cooling based dehumidification is the compression refrigeration cycle. Figure shows a simple process diagram of the cooling based dehumidification, where humid air passes over cooling coils, moisture condenses and is removed by gravity separation. The low temperature in the cooling coils is maintained by a refrigeration system. Typically, cooling based dehumidification occurs in conjunction with an air conditioning systems. Figure illustrates the psychrometric process of cooling the air below the dew point. The figure indicates that there are two thermal processes, sensible and latent cooling, where latent cooling is the process of dehumidification. Figure 4.1.1: Basic process cycle for cooling based dehumidification. (Harriman, 2002) Humid air passes over cooling coils which are at a temperature below the dew point of air. The humidity condenses on the coils Figure 4.1.2: Psychrometric air path (Harriman, 2002) Cooling systems first chill the air to its dew point (100% relative humidity). After that point, further chilling removes moisture. The more the air is cooled, the more will it be dried. Figure shows the type of refrigeration cycle which is used in most commercial and residential air conditioning systems. The system basically conveys and rejects heat from the conditioned space to the outdoor environment, using the refrigerant to transport the heat energy. July 9, 2016 Page 29 of 171

38 SECTION 4 METHODS OF DEHUMIDIFICATION Heat is removed from the conditioned space by first transferring its heat energy to the expanding and vaporizing refrigerant in the evaporator. The latent heat of the refrigerant evaporation in the evaporator creates a significant heat sink. From the evaporator, the refrigerant vapor flows to the compressor, where pressure in the refrigerant is increased. Further downstream the high pressure refrigerant vapor enters the condenser where condensation occurs creating high temperatures resulting in heat rejection to the external environment or another external heat sink. The now liquid refrigerant flows to an expansion valve which creates a pressure differential across the valve as the in the refrigerant travels downstream to the low pressure evaporator. As the refrigerant enters the evaporator, the refrigeration cycle is completed. Figure 4.1.3: Basic refrigeration cycle used in The type of refrigeration cycle shown is used in most commercial and residential air conditioning systems. Cooling based dehumidification systems can have different component configuration. There are three basic working principles to provide the cooling capacity and heat sink for water vapor condensation: Direct expansion cooling system Chilled liquid cooling system Dehumidification with reheat system For the direct expansion cooling and chilled liquid cooling systems dehumidification occurs in conjunction with air conditioning systems where cooling and dehumidification occurs. The dehumidification with reheat system does not contribute to space cooling, but only contributes to dehumidification. Direct expansion cooling: This type of cooling system is often referred to as DX or direct expansion system. This system is mostly used in residential or roof top cooling system installations. Figure illustrates the DX system diagram. Moist supply air flows over cooling coils, which serve as the evaporator. Chilled liquid cooling: Chilled liquid systems use a secondary cooling loop to cool and dehumidify air. In the secondary cooling loop chilled water, or other secondary cooling fluid, is used to transport heat from the cooling coils to the evaporator. The secondary cooling loop adds complexity to the system July 9, 2016 Page 30 of 171

39 SECTION 4 METHODS OF DEHUMIDIFICATION and therefore it is typically used in larger system installations, where they can gain advantages of installed cost and operating efficiency over DX systems. Figure illustrates the chilled liquid cooling system diagram. Dehumidification with reheat: It is common practice that air conditioning systems use reheat to increase the temperature of the supply air downstream of the cooling coils. This is done to assure occupant comfort, since cold air can lead to draft and overcooling of conditioned spaces. This section presents a dehumidification configuration that most residential, but some commercial, dehumidifiers use. This system is depicted in Figure 4.1.6, and only controls humidity level in spaces humidity without cooling. The dehumidification reheat systems uses combined cooling and to achieve great efficiency. As illustrated in Figure 4.1.6, the dehumidification reheat system has a condenser coil for reheat immediately downstream of the evaporator, which is the cooling coil where moist air sheds humidity. This configuration results in an efficient dehumidification process performance, since the low air temperature after the cooling coil makes the refrigerant condenser very efficient. The reheat energy from the condenser is essentially free, since it is heat rejected by the cooling process. While for most dehumidification applications cooling based process provides good system performance, there are problems with frozen condensate that can occur with very low dew point conditions. In these cases, frozen condensate reduces heat transfer effectiveness and can clog coils, thus reducing the airflow. For systems that cool air below 32 F dew point defrost coil systems and suitable controls can provide some mitigation. Figure 4.1.4: Direct expansion cooling process diagram (Harriman, 2002) The DX cooling cycle is mostly used in commercial and residential cooling applications. Here dehumidification occurs in conjunction with space cooling. In the DX system the cooling coils over which the moist air flows function as the evaporator. July 9, 2016 Page 31 of 171

40 SECTION 4 METHODS OF DEHUMIDIFICATION Figure 4.1.5: Chilled liquid cooling process diagram (Harriman, 2002) The chilled liquid cooling cycle uses a secondary cooling loop to transport heat between the cooling coils and the evaporator. The secondary cooling loop complexity and but offers benefits. The chilled liquid cooling loop is typically used in larger installations. Figure 4.1.6: Dehumidification reheat system (Harriman, 2002) This dehumidifier configuration uses a cooling reheat scheme to remove moisture from air. The arrangement is energy efficient because it uses condenser reheat downstream of the evaporator. The reheat energy is essentially free, and the condenser is most efficient in the low temperature air that comes from the evaporator coil. The source image (Harriman, 2002) was edited by adding comments 4.2 Desiccant Dehumidification This section introduces several process characteristics of desiccant dehumidifying. Section 5 will present more detail about the physical properties of desiccants and the desiccant dehumidification process. The process of desiccant dehumidification can be separated in three main process steps. These three main process steps are described in Figure July 9, 2016 Page 32 of 171

41 SECTION 4 METHODS OF DEHUMIDIFICATION Desiccant Dehumidification process step A: Sorption This process steps describes the process from point (1) to point (2). The surface vapor pressure of the desiccant material is low because it is dry and cool and has a low moisture content (conditions at point (1)). As the desiccant draws moisture from the surrounding air, the desiccant surface becomes saturated and hot, because of the heat of condensation and sorption (conditions at point (2)). At point (2) vapor pressure is equal to that of the surrounding air and the desiccant cannot collect more moisture because there is no pressure difference between the surface and the vapor in the air. Desiccant Dehumidification process step B: Desorption This process steps describes the process from point (2) to point (3). The desiccant to conditions of state point (2) requires energy (e.g. heat) input to go to point (3). For this the desiccant is taken out of the moist air, heated, and placed into a different airstream. Due to an increase of temperature the vapor partial pressure at the desiccant surface increases to a point where the vapor pressure is higher than in the surrounding air. Therefore, moisture moves off the desiccant surface to the air to equalize the pressure differential. At the conditions at point (3) the desiccant is dry and hot. Therefore, although the desiccant is dry, its vapor pressure is still too high to collect moisture from the air, due to the desiccants high temperature. July 9, 2016 Page 33 of 171

42 SECTION 4 METHODS OF DEHUMIDIFICATION Desiccant Dehumidification process step C: Cooling This process steps describes the process from point (3) to point (1). This completes the desiccant basic process cycle. The desiccant is cooled to restore a low vapor pressure, returning it to conditions at point (1). Figure 4.2.1: Three main process steps A through C of the basic desiccant dehumidification cycle, (Harriman, 2002) These three basic process steps are used in basically all desiccant equipment configurations. Harriman (2002) presents five typical process equipment configurations for desiccant dehumidifiers: 1. Liquid desiccant packed column 2. Solid desiccant packed tower 3. Solid desiccant rotating horizontal bed 4. Solid desiccant multiple vertical bed 5. Solid desiccant rotating Honeycomb Liquid desiccant packed columns: In the packed column dehumidifiers liquid desiccant solution is brought into contact with moist air. Figure shows a typical configuration with two process vessels. The larger of the two vessels is used for the dehumidification (sorption) process step and the smaller vessel for the regeneration (desorption) of the desiccant solution. In both vessels, liquid desiccant solution is either sprayed or distributed on the packing inside the vessel (e.g. packing of rings and other geometries). The liquid desiccant solution runs down on the packing material and shear forces stimulate surface renewal. Moist air is flowing upwards and is brought into contact with the liquid desiccant solution. The desiccant solution drips down into a pan and from where a portion is recirculated and the remaining portion is pumped to the smaller regeneration. Control indicates how much desiccant has to be recirculated. In the smaller vessel liquid desiccant is regenerated by adding heat, letting it run down over packing like in the larger vessel and blowing a separate air stream, the reactivation air, through the smaller vessel. Moisture leaves the desiccant and moves to the reactivation air. An important process parameter is the creation of active surfaces, also referred to as surface renewal. In this process surfaces of droplets renew frequently as droplets break up, form new drops and are mixed into the mass of the liquid desiccant. Some inert packing or baffles inside the sorption process vessel increases the July 9, 2016 Page 34 of 171

43 SECTION 4 METHODS OF DEHUMIDIFICATION magnitude of surface renewal. The numbers on the diagram indicates how the process equipment perfumes in regard to the basic desiccant dehumidification process. Figure 4.2.2: Liquid desiccant packed column process vessel These units are like air washers, except they spray liquid desiccant into the process air instead of simply water. The heat and moisture from the dehumidification process is transferred to the desiccant. Heat is rejected through an external cooling system and moisture is rejected in the desiccant regenerator, which re concentrates the diluted desiccant solution. (Harriman, 2002) As the desiccant returns from the regenerator to the sump on the dehumidification vessel, it is dry but still hot, which results in a high vapor pressure. The desiccant is circulated through a cooler, which is connected to a chilling system or cooling tower. Downstream of the cooler, the vapor pressure of the desiccant is lowered since it is both dry and cool and is ready to absorb more moisture. Among the advantages of the packed column is good control of heating and cooling. As an added benefit, independent of the thermal treatment, the liquid desiccant can eliminate particles from the supply air flow. Disadvantages of the system include process inertia, which makes adaptation of rapidly changing humidity conditions difficult. Larger quantities of desiccant solution may be distributed throughout a large piping system with large reserve sump. This can result in significant time to respond to fast changing internal moisture loads or different necessary outlet conditions. The slow response on the July 9, 2016 Page 35 of 171

44 SECTION 4 METHODS OF DEHUMIDIFICATION outlet conditions, however, can also be a benefit since to creates process inertia which means slow responses to inlet changes. A large mass of recirculating desiccant is used to protect the internal process from rapid changes in weather moisture. Typically, liquid spray dehumidifiers are employed in large, central systems rather than small, freestanding units. The main reason is the somewhat higher complex of the liquid spray dehumidifier in comparison to solid desiccant units. Some large systems connect several dehumidification process vessels to a single regenerator reactor. This can have advantages of first cost, at the expense of complexity of controls. Maintenance varies between the type of liquid desiccants used, since some desiccants solutions are more corrosive than others. Solid desiccant packed tower: The solid desiccant packed tower dehumidifier system consists of pairs of process vessels which are filled with solid granular desiccant material, such as silica gel or molecular sieve. The pairs of vessels operate in a batch wise, or process swing mode. Figure illustrates the working principles. One process vessel operates in dehumidification mode while the other regenerates the desiccant material after it becomes saturated with moisture. In Figure 4.2.3, the two vessels are identified as process vessels A and B. In dehumidification mode, moist process air starts to pass through process vessel A. The desiccant material in vessel A is cool and is effective to adsorb humidity. The moisture content and temperature of the desiccant increases as moist process air flows through vessel A. At the same time when moist process air flows through vessel A, vessel B is in desorption mode. Reactivation air, heated by the so called desiccant heater, is passed through vessel B driving out the moisture from the desiccant and transferring the moisture to the reactivation air. After the desiccant in vessel A becomes saturated and/or the desiccant in vessel B is fully regenerated, the 4 way valves switch the air flow and the functions of vessels A and B are reversed. Since drying and reactivation take place in separate, sealed compartments, the packed tower dehumidifier is frequently used to dry pressurized process gases. The process can achieve very low dew points, as low as below 40 F. Desiccant dehumidifiers for compressed air are frequently the packedtower type. Since the system operates batch wise and not continuous, conditions of the process air change during one cycle of the swing operation. At the start of dehumidification, dry and cool desiccant is exposed to the process airstream. Since the vapor pressure is at a maximum, the process air can be deeply dried. As its moisture capacity fills up, the air is not dried as efficiently. In order to limit the changes in the desiccant, controls are provided to ensure the process vessels are changed before the process air condition become too wet. The air flow velocities inside the process vessels are kept low in order to avoid uneven air flow distribution and the disturbance of the granulated desiccants. As a result, the solid packed tower units can be quite large. July 9, 2016 Page 36 of 171

45 SECTION 4 METHODS OF DEHUMIDIFICATION Figure 4.2.3: Solid packed tower dehumidifier (Harriman, 2002) with annotations added Moist air is dried in pairs of process vessel, which operate batch wise. The process vessels are filled with granulated solid desiccant. The two process vessel alternate as between dehumidification and desiccant regeneration mode. The desiccant is regenerated by hot airstream. The system is used frequently for compressed air and pressurized process gasses. It is less common in ambient pressure applications. Rotating horizontal bed The rotating horizontal bed dehumidifier consists of granular desiccant that are held in a series of shallow, perforated trays which rotate continuously between sections the process and reactivation airstreams. In dehumidification mode, trays rotate through a section where moist process air passes vertically through the trays. The desiccant adsorbs moisture. The trays then rotate into a position where reactivation airstream heats the desiccant, raising its vapor pressure and releasing the moisture into the reactivation air. The hot desiccant material is cooled by cold air flowing through the trays before the tray reaches the section where the desiccant is again exposed to the moist process air. Figure illustrates a typical system configuration. July 9, 2016 Page 37 of 171

46 SECTION 4 METHODS OF DEHUMIDIFICATION The system configuration can operate in modular fashion in order to increase drying capacity. Increased capacity can also be obtained by increasing the tray diameter to hold more desiccants or the number of beds stacked on top of one another is increased. The rotating horizontal bed can provide constant outlet moisture level. A disadvantage can be an uneven settlement and resulting distribution of the granulated desiccant material, which increases the possibility of uneven flow and leakage of air flow through the beds. A typical remedy is to direct the process and reactivation airflow in parallel, rather than generating a counter flow. With regard to energy efficiency, a parallel arrangement of process and reactivation airflows does not perform as well as a counter flow arrangement. As a result, reactivation energy consumption can be higher than compared to other designs. A significant benefit of rotating horizontal bed systems are the low first costs. The design is simple, expandable and easy to manufacture. Operating costs of the system can, however, be higher than with other designs, especially when energy costs are high. Figure 4.2.4: Rotating horizontal bed dehumidifier (Harriman, 2002) Horizontal solid desiccant material is slowly rotated between process and reactivation airstreams. Leakage between moist and dry airstreams can affect the performance. July 9, 2016 Page 38 of 171

47 SECTION 4 METHODS OF DEHUMIDIFICATION Multiple vertical bed: This dehumidification system has solid desiccant sitting in a vertical bed configuration. The air streams, either the process or activation air streams, pass vertically through the relatively high cells. This vertical bed configuration combines feature of a packed tower and a rotating horizontal bed design in an arrangement that is well suited to atmospheric pressure dehumidification applications. Figure illustrates the assembly. Figure 4.2.5: Multiple vertical bed dehumidifier (Harriman, 2002) Granular desiccant beds are installed vertically on a rotating assembly. Multiple cell like segments of the circular assemble pass through process and activation air streams. The system combines benefits static packed towers and rotating tray dehumidifiers. The design includes more complex parts due counter flowing process and reactivation air streams, which offer more energy efficient operation. The increased installation cost is offset by a lower operating cost than either packed tower or rotating horizontal bed type units Rather than single or double packed tower systems which operate batch wise, the multiple vertical beds are installed on a circular carousel with eight or more towers. These towers are exposed to alternative process and reactivation air streams. This design can achieve low dew points since leakage between process and reactivation air circuits is minimized. Due to separation between adjacent towers, process and reactivation airstreams can be arranged in the more efficient counter flow pattern for better heat and mass transfer. Different from the stationary packed tower with discrete process steps (e.g. swing operation), the rotating multiple vertical bed assembly operates more or less continuously and can provide a relatively constant outlet moisture condition on the process air. This can be a significant advantage to the undulating performance of the packed tower units. July 9, 2016 Page 39 of 171

48 SECTION 4 METHODS OF DEHUMIDIFICATION In comparison to the horizontal bed dehumidifier, the system is mechanically more complex, has a higher first cost and requires more complex operation. Generally, however, these are minor limitations compared to the large savings in energy and performance improvements at low dew points. Rotating Honeycomb or Solid Desiccant Dehumidification Wheel: The dehumidifier design uses a rotating wheel which hold structures of desiccant material about a horizontal axis. This assembly is often referred to as either desiccant wheel or rotating honeycomb. The wheel is a light weight structure with corrugated and desiccant impregnated material. The structure allows ready horizontal passes of the air streams through the slowly rotating wheel, resulting in low pressure losses. Figure shows a typical corrugated assembly, where the desiccant material adheres to the inner surfaces of the horizontal small cells. Figure 4.2.6: Honeycomb Structure of the Solid Desiccant Dehumidification Wheel, (Carnegie Mellon University, 2016) Desiccant material covers a lightweight, open structure through which air can flow with small pressure losses. Figure illustrates the working principle of the desiccant wheel. Process air flows through the horizontal cells of the corrugations in a major section of the desiccant wheel (or process) section. The desiccant adsorbs moisture from the moist air stream until it becomes saturated (attains process point (2)). As the wheel turns further, the desiccant material enters a section of the wheel that is exposed to the reactivation section of the wheel enclosure, where a heated reactivation air stream dries out the desiccant material in a counter flow process (attain process point (3)). After rotating through the reactivation section the desiccants in the corrugated wheel structure are cooled by small portion of the process air to lower their surface vapor pressure and bring the desiccant material to point (1). From here, a new process cycle continues. The rotating desiccant wheel design has several advantages over other dehumidifier configuration. The structure is lightweight and offers an effective moisture removal capacity to overall weight ratio. As a result, mechanical power to drive the system can be kept low. The corrugated wheel structure offers few air flow obstructions and therefore pressure losses and flow resistance remain low. All of these performance parameters result in good energy performance and relatively low maintenance efforts and costs. July 9, 2016 Page 40 of 171

49 SECTION 4 METHODS OF DEHUMIDIFICATION Figure 4.2.7: Rotating Honeycombe or Solid Desiccant Dehumidification Wheel (Harriman, 2002) The design combines high surface area with low total desiccant mass, making these units especially efficient. The small number of parts reduces maintenance to a minimum. 4.3 Comparing Desiccant to Cooling based Humidification The selection of the desiccant dehumidifier for a specific application requires a careful optimization of the entire system installation and operation. Harriman (2002) suggested that the following criteria have to be addressed. Installed cost: The first cost of the dehumidifier itself typically is only a small fraction of the cost of the entire installation, which includes heating and cooling systems and overall site infrastructure, including piping, electricity and other process components O&M cost: Operating and maintenance costs are usually significantly larger than the first costs, since dehumidification is an energy intensive process. The leading O&M cost items are heat for reactivation and cooling for the desiccant and process air. Harriman (2002) suggested that available low cost energy sources, such as solar or waste heat can often offset installed cost differences in a relatively short time period. Demonstrated operational reliability: Dehumidifier configurations differ between applications and weather conditions. The installation of the reliable equipment for the condition is of importance. July 9, 2016 Page 41 of 171

50 SECTION 4 METHODS OF DEHUMIDIFICATION Design assumptions: The selection of the type and size of dehumidification equipment configuration is dependent on the design assumption. Reasons for varying equipment selections could include incomplete or erroneous data available to the system designer. It impossible to state universally accurate comparisons because the working conditions of the desiccant dehumidification systems can differ between applications. With regard to selecting between desiccant and cooling based dehumidification, Harriman (2002) suggested the following additional design considerations: Cooling and desiccant based dehumidification technologies should complement each other and costs can be lowered by using integrated systems. The selection about using desiccant or cooling based dehumidification depends on cost of electrical power and thermal energy. When using solar or waste heat and high electricity costs, desiccant systems outperform cooling based systems. If electricity is inexpensive and activation thermal energy costly, cooling based dehumidification can outperform desiccant systems. Conventional design recommends cooling based dehumidification at high air temperatures and moisture levels. At low target dew point, e.g. below a 40F dew point, the cooling based dehumidification process becomes inoperative because condensate freezes on the coil. In these cases, only desiccant dehumidification is the viable option. Desiccants have advantages when treating ventilation air for building HVAC systems which also use ice storage. In these cases, desiccants can offer significant cost savings. Figure presents data that compares first and O&M cost of various desiccant and cooling based dehumidification technologies as a function of target dew point of dried air. July 9, 2016 Page 42 of 171

51 SECTION 4 METHODS OF DEHUMIDIFICATION Figure 4.3.1: First and O&M costs of various desiccant and coolingbased dehumidification technologies as compared to a Honeycombe desiccant wheel dehumidifier (Harriman,2002) July 9, 2016 Page 43 of 171

52 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS This section discusses important physical properties of desiccants, which affect the underlying processes and the performance of desiccant dehumidification technology. 5.1 Basic Considerations of Physical Properties in Desiccant Dehumidification Xing (2000) suggested that predicting the performance of desiccant material is difficult, because physical properties of desiccant material typically varies between vendors and is significant manufacturingprocess dependent. The author suggested that even for the same type of desiccant, different manufacturers have different property data, which are sometimes considered proprietary, since specific dehumidification applications call for different desiccant. Desiccant materials are hygroscopic substances that create significant affinity for water vapor. Several authors pointed out that technically speaking, any material would qualify as a desiccant since they can attract some amount of water from the air. In order to qualify for commercial use in space conditioning, however, a desiccant must be able to hold much larger amounts of water; some commercial solid desiccant materials can hold up to 50% of their weight in water Xing (2000). Collier et al (1981) suggested the following desirable physical properties for any desiccant material used in desiccant cooling applications are: 1. Mechanical and Chemical Stability: Solid desiccant material does not deliquesce (become liquid). Desiccants should not undergo hysteresis when cycled. 2. Large Moisture Capacity per Unit Weight: It is desirable to cycle as much water as possible for a given amount of desiccant. This reduces the amount of desiccant required and the size of the cooling system. 3. Large Adsorption/ Absorption Capacity at Low Water Vapor Pressures: The moisture capacity should not deteriorate at very low water vapor pressures. This increases the relative dryness achievable, which will have a strong effect on reducing fan power requirements. 4. Low Heat of Adsorption/Absorption: A low heat of adsorption increases cooling capacity and COP. 5. Ideal Isotherm Shape: The isotherm shape should be close to the Ideal function of the application. This lowers regeneration temperature and thus increases COP. July 9, 2016 Page 44 of 171

53 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS 5.2 Isotherms An isotherm is defined as the relation between relative humidity of the air in equilibrium with the desiccant particle and the water content of the particle at a specified temperature. Isotherms are considered important physical properties of desiccant materials, to assess heat and moisture transfer interactions during adsorption and desorption processes in various kinds of desiccant dehumidification systems. Using the example of silica gel Ramzy et al (2015) presented a procedure of developing isotherm equations of desiccant material that can be used in predicting heat and mass transfer interactions. The optimization of the desiccant bed design requires mathematical assessment of the heat and mass transfer interactions in the desiccant bed. Ramzy et al (2015) pointed out that several investigators have presented different procedures to identify the isotherm equations required for their theoretical simulations. All of the isotherm equations are based on experimental data. Two examples of isotherm expressions for silica gel are presented in the following: Ng et al. (2001) found that silica gel water vapor isotherms can be estimated by a detailed Henry's law's correlation for silica. The authors provide the following equation: Where: W = Gel water content [kg w /kg dry silica gel] T s = Temperature in o C Pv = Partial pressure of vapor in the air [Pa], R = Gas constant of air in [J/(kg*K)], K 1 0 and Q st are tabulated values for specific silica gel products. San et al.(2002) presented the isotherms for water vapor silica gel in terms of absolute humidity and particle temperature July 9, 2016 Page 45 of 171

54 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Where: W = Gel water content [kg w /kg dry silica gel ] T s = The temperature in o C C = The air absolute humidity [g/m 3 ] Ramzy et al (2015) suggested that obtaining experimental data which can be used in developing equations can represent a significant effort. The authors presented their own simple procedure to identify the water vapor silica gel isotherm, which requires less time effort and cost and provides acceptable predictions for the isotherm equation. In this procedure the water content of silica gel is identified by drying the silica gel sample with infrared heat. The authors pointed out that although silica gel has a very high melting temperature (1,600 ºC), it will lose its chemically bound water and hygroscopic properties if heated above 300 C. By determining the initial weight (before heating process) and the dry weight (after heating process) the initial water content of the silica gel is obtained. The representative isotherm curve was obtained by analyzing 20 samples with the same 10 % initial and different final water content. The samples remained in the fluidized bed for 48 hours so that water content changing adsorption and desorption processes could obtain final equilibrium. The results were curve fitted which resulted in the following isotherm relationship: [Ramzy et al (2015)] The graph of this isotherm relationship is depicted in Figure July 9, 2016 Page 46 of 171

55 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure 5.2.1: Relative humidity of inter particle air at different silica gel water content [Ramzy et al, 2015)] Figure shows a comparison between the simplified procedure proposed by Ramzy et al (2015) and several other researchers. The results reported by Ramzy et al are labeled (7) in Figure Figure 5.2.2: Isotherms of water vapor silica gel collected from literature along with the isotherm obtained from the present work (Ramzy et al,2015) July 9, 2016 Page 47 of 171

56 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure shows the results of a sensitivity analysis comparing the theoretical isotherm equations with the same experimental data. Figure suggests that the simple procedure for developing the equilibrium isotherm equation of water vapor and silica gel lies in the same range of the isotherms obtained in literature. Figure 5.2.3: Experimental and theoretical variations of exit air humidity ratio and temperature during adsorption process using different isotherm equations [Ramzy et al (2015)], Annotated 5.3 Types of Isotherm Models Generally speaking, the adsorption process creates a film of the adsorbate on the surface of the adsorbent. Adsorption is a surface phenomenon, and the adsorption process is controlled by surface active energy. The basic premise of adsorption is bonding capacities of atoms at the surface of the adsorbent. Atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the desiccant material. The adsorption process is generally classified as electrostatic, chemical or physical sorption. Foo and Hameed (2010) presented a comprehensive investigation into types of adsorption isotherm, which they characterized as an invaluable graph describing the phenomenon governing the retention (or release) or mobility of a substance to a solid phase at a constant temperature and ph. Adsorption equilibrium (the ratio between the adsorbed amount with the remaining in the solution) is established when an adsorbate containing phase has been contacted with the adsorbent for sufficient time to achieve dynamic balance. July 9, 2016 Page 48 of 171

57 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Foo and Hameed (2010) reported that over the years, a wide variety of equilibrium isotherm models have been formulated, included the often used isotherm models by Langmuir, Freundlich, Brunauer Emmett Teller, Redlich Peterson, Dubinin Radushkevich, Temkin, Toth, Koble Corrigan, Sips, Khan, Hill, Flory Huggins and Radke Prausnitz. According to Foo and Hameed (2010) these models can be grouped into three fundamental approaches, namely two parameter isotherm model, the three parameter isotherm model and multilayer physisorption isotherms. Each of the isotherm models describe different adsorption characteristics. In conjunction with desiccant adsorption processes the Brunauer Emmett Teller (BET) isotherm model is widely used. The BET model was developed to derive multilayer adsorption systems with relative pressure ranges from 0.05 to 0.30 MPa. Brunauer et al (1938) categorized sorption isotherms depending upon their sorption mechanisms. The sorption mechanisms are defined as types 1 through 5. Figure through show five types of Brunauer isotherms. Mei et al (1992) presented some commercially available materials (Figure 5.3.5). Generally speaking, molecular sieves are considered type 1 materials, silica gels are considered type 2 materials, and wool is considered a type 3 material. Hydratable salts vary between type 2 and type 3. Mei et al (1992) suggested an ideal isotherm for desiccant cooling applications (Figure 5.3.6). Figure 5.3.1: Brunauer isotherms of types Figure 1 and 3 compared to a linear sorption isotherm (Brunauer, 1938) reproduced Figure 5.3.2: Brunauer type 2 isotherm (Brunauer, 1938) reproduced July 9, 2016 Page 49 of 171

58 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure 5.3.3: Brunauer type 4 isotherm; (Brunauer, 1938) reproduced Figure 5.3.4: Brunauer type 5 isotherm; (Brunauer, 1938) reproduced Figure 5.3.5: Adsorption isotherms of typical desiccants (Mei, 1992) modified Figure 5.3.6: Ideal isotherm for desiccant cooling applications, (Mei, 1992) modified July 9, 2016 Page 50 of 171

59 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Pesaran (1993) reported on research in conjunction with a desiccant cooling ventilation cycle that identified the desiccant isotherm shape (characterized by separation factor) which would result in the optimum performance. The author applied different regeneration temperatures ranging from 65 C to 160 C to identify the corresponding optimum isotherm shape at each. The research used Brunauer Types 1 and 3 isotherms because previous studies had shown their potential over other desiccant types for the investigated application. The author described the relationship between moisture uptake and relative humidity for Type I and III desiccants by the equation: W* = RH* R + RH* R RH* where: R = Separation factor RH* = Temperature adjusted relative humidity W* = Moisture loading fraction, w /W max The isotherm shapes of various Type 1and 3 desiccants used by the author are shown in Figure The Type 1 isotherms correspond to desiccants with a separation factor of R < 1, and Type 3 isotherms correspond to desiccants with a separation factor of R > 1. Separation factors were varied between R = 0.05 (Type I moderate) and R = 1.5 (Type III moderate). Collier (1981) discussed the characteristics of an ideal desiccant material, which he contended are almost impossible to meet in practical applications. Collier suggested that the requirement of an "ideal" desiccant material would be that adsorption/absorption capacity are independent of water vapor pressure. The author points out that, "unfortunately", the heat of adsorption is not independent of the isotherm shape but is defined by the Clausius Clapeyron relationship. This relationship defines that a material with a weak pressure dependence must also have a weak temperature dependence in order to keep the heat of adsorption low. It is important to keep the heat of adsorption of the desiccant material reasonably low. Even though most of this energy can be recaptured by the sensible heat exchanger, a high heat of adsorption leads to the requirement of a high regeneration temperature. July 9, 2016 Page 51 of 171

60 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure 5.3.7: Normalized Moisture Capacity for Various Type I and III Isotherms used for the determination of adsorption in a desiccant cooling ventilation cycle Pesaran (1993) The figure shows isotherms for Type I and III desiccant material with different separation factors. Figures through show the adsorption isotherms for natural zeolite, molecular sieve, and silica gel. Silica gel shows considerable vapor pressure dependence, whereas natural zeolite and molecular sieve show much smaller water vapor pressure dependence. Natural zeolite and molecular sieve must undergo very large temperature variations in order to experience minimal changes in water content. Silica gel, on the other hand, can experience large variations in water uptake for much smaller temperature swings, because of the relatively low heat of adsorption of water. In actual practice, molecular sieve, natural zeolite, and silica gel produce about the same net cooling effect per unit weight of material with nearly the same adsorption energy. What silica gel loses in its pressure dependence, it seems to gain in its temperature dependence. The temperature insensitivity of molecular sieve and natural zeolites is advantageous during the dehumidification process; it is a disadvantage during the regeneration process. For silica gel, the temperature sensitivity is a disadvantage during dehumidification but an advantage during regeneration. July 9, 2016 Page 52 of 171

61 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure 5.3.8: Adsorption Isotherms for Natural Zeolite; Collier (1981) Figure 5.3.9: Adsorption Isotherms for Molecular Sieve; Collier (1981) July 9, 2016 Page 53 of 171

62 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure : Adsorption Isotherms for Silica Gel; Collier (1981) 5.4 Adsorption Energy The adsorption energy is referred to as the heat which is liberated or absorbed during the adsorption or desorption process, respectively. Mei et al (1992) suggested that it is very important to note that temperature dependence of the isotherm shape and the adsorption energy are related. Therefore, the shape of the isotherm will be uniquely determined by the heat of adsorption at that temperature and loading. The author pointed out that the physics that ties these properties together is defined by the Clausius Clapeyron relationship. During the adsorption processes the heat of adsorption is not constant over the entire range of desiccant loadings. The first molecules that attach themselves to the surface of the adsorbent are bonded by the strongest forces available to the system and they have the highest adsorption energies (strongest bonds) associated with them. As the process of molecules accumulating at the surface of the desiccant progresses, the available sites for bonding fill and the desiccant loading increases. This results in less energetic sites being occupied. As the desiccant approaches saturation conditions, the binding energies approach energy levels of condensation, or evaporation. Therefore, heat of adsorption will be highest when loading is zero and will be lowest at saturation loading. The rate at which this energy changes with water loading will vary depending upon the material. July 9, 2016 Page 54 of 171

63 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS 5.5 Chemical and Physical Stability As specified in the literature, desiccant material can suffer from performance losses if it is not stable under certain process conditions. Mei et al (1992) presented two important aspects of physical and chemical stability that deserve special attention. These are (1) the ability of the desiccant material to withstand high regeneration temperatures and (2) to be reliably regenerated by ambient source air. Many commercial desiccants rely on molecular sieve structures which are considers as not stable at high temperatures. Desiccant dehumidification cycles, however, require that desiccant sorption properties should be stable at temperatures between 300 F and 400 F. Some desiccants are "fouled" by the presence of certain molecules that have a greater affinity for the active sites than water vapor does. Others change chemical composition in the presence of certain compounds that change the desiccants water sorption characteristics. 5.6 Adsorption Rate and Performance The adsorption process is dependent on a number of properties, such as isotherm shape, heat of adsorption, heat capacity, moisture capacity, and on other operating conditions, including temperature and method of regeneration. Pesaran and Hoo (1993) investigated the characteristic shape of the desiccant isotherm as a function of regeneration temperature and process which affect the performance for a solar desiccant cooling cycle operating in ventilation mode. The authors used thermal coefficient of performance and cooling capacity of the system as metrics to describe the process. The authors suggested that solar heat derived from flat plate collectors was initially considered to be a good low temperature heat source available for regeneration. However, lower regeneration temperatures typically resulted in decreased cooling performance with several candidate systems investigated. As a consequence, size and cost of the solar cooling system had to be increased. As a remedy, high temperature regeneration desiccant materials and cycles were proposed to improved thermodynamic performance at higher regeneration temperatures. The main objective of the research reported by Pesaran and Hoo (1993) was the determination of how the performance of a solar desiccant cooling system would improve when the regeneration temperature was increased. The system used for the investigation resembled a desiccant cooling ventilation cycle with a rotary desiccant dehumidifier, a heat exchanger, two evaporative coolers, a desiccant regeneration heater, and ancillary equipment such as fans and pumps. Figure illustrates the system schematic where outside air was dried in the dehumidifier and then cooled by regenerative evaporative coolers and supplied to the conditioned space. The regeneration heater (powered by solar energy) heated the air, which reactivated the desiccant by driving the moisture from it. July 9, 2016 Page 55 of 171

64 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure 5.6.1: Schematic Diagram of a Desiccant Cooling System; Pesaran and Hoo (1993) Based on previous investigations, Pesaran and Hoo (1993) used Brunauer Types 1 and 3 desiccant isotherms because these desiccant characteristics had shown better performance than other desiccant types. In addition, in order to investigating the effects of regeneration temperatures and dynamics two regeneration methods were used, referred to as staged and no staged regeneration methods. The authors revealed that the differences of the staged and no staged methods had a significant effect on the mass and heat transfer performance. Figure shows a schematic of the staged and no staged regeneration schemes. The two methods differ in their regeneration process configuration. In the staged regeneration method, only a fraction of the regeneration airstream is heated to the design regeneration temperature. The remainder of the regeneration airstream is composed of warm air returning from the conditioned space. A portion of the heat of adsorption is added to the return air via heat exchange between the downstream dehumidifier flow and the upstream regeneration flow. Under the staged method, only a small portion of the regenerated area of the rotary desiccant bed (30% of the desiccant exposed to the high temperature airstream), at the end of the regeneration cycle, is exposed to high temperature airstream from the regeneration heater. Thus makes up about 30% of the total regeneration airstream. The 30% value was selected based on the previous experiments published by Collier (1989). Collier had suggested that the maximum thermal COP is achieved with a separation factor of R = 0. 1 and a regeneration temperature of 95 C. The results of the no staged regeneration are shown in Figure and Figure correlates the cooling capacity and coefficient of performance (COP) with the regeneration temperature. Figure indicates that thermal COP performance improves with lower regeneration temperature, with the maximum COP value encountered at the lowest temperature investigated. This performance is due to the fact that under the no staged method the entire regeneration area is subjected to the regeneration temperature and the desiccant material is exposed to the drying airstream for a much longer period July 9, 2016 Page 56 of 171

65 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS than necessary. This causes desiccant over regeneration and a drier and hotter desiccant bed at the start of the dehumidification cycle. While this increases the cooling capacity, it results in a lower thermal COP because of the inefficient use of the thermal energy in regeneration. This process especially applies to high regeneration temperatures as over regeneration becomes more pronounced and the COP decreases accordingly. On the opposite side, at low regeneration temperatures the over regeneration of the desiccant is less pronounced and one could would expect the thermal COP to improve. The Pesaran and Hoo (1993) pointed out that if cooling capacity and size of equipment is of higher concern than the cost of thermal energy the no staged regeneration is a good solution. The cost of thermal energy in a solar desiccant cooling system, however, is a major concern and has a significant impact on the overall size of the system. Under these circumstances a tradeoff between cooling capacity per unit size versus equipment size and cost of the solar collectors must be closely examined. Figure shows that the associated separation factors varies between Type I moderates (R=0.1) and nearly linear isotherms (R = 1). For no staged regeneration, the two performance criteria of thermal COP and system cooling capacity do not correspond to the same value of R. Maximizing cooling capacity favors a Type I moderate isotherm, while emphasizing COP performance favors a more linear isotherm. Figure 5.6.2: Schematic Diagram of Staged Regeneration and No staged Regeneration Schemes, (Pesaran & Hoo,1993) July 9, 2016 Page 57 of 171

66 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure 5.6.3: Maximum Cooling Capacity Performance with Corresponding COP, No staged Regeneration (Pesaran & Hoo,1993) Figure 5.6.4: Optimum Separation Factors Resulting in Maximum Thermal COP and Maximum Cooling Capacity Performance, No staged Regeneration (Pesaran & Hoo,1993) The results of the staged regeneration are shown in Figure and Figure correlates the cooling capacity and Coefficient of performance (COP) with the regeneration temperature. Figure suggests that the maximum thermal efficiency and maximum cooling capacity are reached at regeneration temperatures of 110 C and 160 C, respectively. Figure shows that for staged regeneration, optimum performance of the system in terms of thermal COP seems to favor two different values of R, one for low temperature applications and a second for high temperature applications. A similar trend can be observed for cooling capacity. Pesaran and Hoo (1993) concluded that No staged regeneration, while inherently simpler than a staged regeneration method, requires significant compromises in the performance of the overall system. Maximum thermal COP and maximum cooling capacity performance were achieved at completely different separation factors, especially at regeneration temperatures above 80 C. In addition, the regeneration temperature has opposing effects on thermal COP and cooling capacity, increasing one while decreasing the other. As a result, significant tradeoffs must generally be accepted when optimizing for either thermal COP or cooling capacity. On the other hand, Pesaran and Hoo (1993) concluded that staged regeneration, while adding complexity to the desiccant cooling system, provides an excellent combination of cooling capacity and thermal COP performance. At high regeneration temperatures, a low separation factor maximizes both COP and cooling capacity. At low regeneration temperatures, the value of R resulting in maximum COP performance. July 9, 2016 Page 58 of 171

67 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS The authors suggested that staged regeneration, when used in high temperature applications (T > 120 C), generally reduces the size and cost of a solar desiccant cooling systems. For most lowtemperature applications (T < 80 C), it would appear that no staged regeneration is preferred. Figure 5.6.5: Maximum Cooling Capacity Performance with Corresponding COP, Staged Regeneration (Pesaran & Hoo,1993) Figure 5.6.6: Optimum Separation Factors Resulting in Maximum Thermal COP and Maximum Cooling Capacity Performance, Staged Regeneration (Pesaran & Hoo,1993) 5.7 Basic Heat and Mass Transfer Modelling by Example of Desiccant Wheel Ruivo, C. et al (2011) suggested that desiccant dehumidification lowers O&M costs through a reduction in peak electricity demand and associated electricity infrastructure costs. The drawback is the higher initial cost compared with equivalent conventional systems. The authors identified opportunities of cost reduction at the design stage through careful cycle selection, flow optimization and size reduction. Designers typically can use only data given by the manufacturers of desiccants, which are usually restricted to particular sets of operating conditions. This design data is based on experimental or numerical approaches which by and large provide tools to assess steady state, but no dynamic heat and mass transfer performance of the desiccant applications. The authors pointed out the need to establish numerical models to describe the dynamic aspect of heat and mass transfer in desiccant applications. Ruivo, C. et al (2011) described the desiccant wheel as a compact and mechanically resistant heat and mass transfer system composed of a high number of channels with porous desiccant walls. The hygroscopic matrix is submitted to a cyclic sequence of adsorption and desorption of water molecules. July 9, 2016 Page 59 of 171

68 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS The regeneration process of the matrix (desorption) is imposed by a hot airflow. In each channel of the matrix, a set of physical phenomena occurs, which include heat and mass convection on the air side and heat and mass diffusion and water sorption processes in the desiccant lined wall. Figure shows the schematics of the desiccant wheel system that was used to define the dynamics of the combined heat and transfer processes. The airflow 1 (process air) and airflow 2 (regeneration air) cross the matrix in a counter flow configuration, with equal or different mass flow rates. The desorption zone is generally equal or smaller than the adsorption zone. (a) Desiccant wheel with porous structure of the matrix (b) Desiccant wheel, definition of adsorption and desorption zones Figure 5.7.1: Desiccant wheel system, (Ruivo, C. et al, 2011), annotated Ruivo, C. et al (2011) pointed out that approaching airflows in each zone can present instabilities and heterogeneities and are generally turbulent. The flow conditions inside the matrix, however, are laminar mainly because of low values of hydraulic diameter of the channels. In very short matrixes with larger hydraulic diameters of the channels, the entrance effects can be relevant. During the adsorption/ desorption processes there are non uniform distributions of adsorbed water content and temperature, inside the matrix. The authors provided examples of steady state and dynamic heat and mass transfer processes. Figure (a) and (b) illustrate the differences between steady state and dynamic performance of sorption observed in a desiccant wheel. The annotations depict the psychrometric evolutions in both air flows 1 and 2. Figure (a) describes the steady state conditions of the airflow and (b) the dynamic conditions. Figure (b) indicates the influence of the channel length and of the cycle duration on the psychrometric evolutions. The authors suggested that for each channel length there is an optimum July 9, 2016 Page 60 of 171

69 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS value of the cycle duration, i.e. the optimum rotational speed that maximizes the dehumidification rate. The effect of optimal rotation diminishes with increasing channel length. (a) Steady state conditions (b) Dynamic conditions, with Influence of the channel length and of the cycle duration Figure 5.7.2: Psychrometric evolutions of the airflows in a desiccant wheel (Ruivo, C. et al, 2011), annotated The authors utilized a numerical model of the channel flow with a set of conservation equations. Fig illustrates the physical domain of the channel model used in the investigations. Two phases coexist in equilibrium inside the desiccant porous medium, the equilibrium being characterized by sorption isotherms. Using a simplification, the authors used two mechanisms of mass transport, surface diffusion of adsorbed water and Knudsen (mean free path) diffusion of water vapor, with the wall considered to be a homogeneous desiccant porous material. Other boundaries of the domain were considered impermeable and adiabatic. July 9, 2016 Page 61 of 171

70 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure 5.7.3: Physical domain of the modelled channel (Ruivo, C. et al, 2011) Here H c is hydraulic radius of the channel, x = distance from channel entrance. The authors reported on the dynamic heat and mass transfer responses in the desiccant matrix. Figure (a) and (b) show heat and mass transfer characteristic of the bulk air flow inside a channel of the desiccant matrix. The data plotted in Figure (a) and (b) illustrate heat and mass transfer behavior of a desiccant wheel with a channel length of 0.3 m and a cycle time of 500 sec. The x values in the figure describe the distance from the channel entrance. The desiccant wheel used was divided into two equal parts, the adsorption and desorption zones with counter current airflows. The desiccant material used was silica gel. The inlet temperatures of the process and regeneration airflows were 30 C and 100 C, respectively. For the analysis the two airflows had the same inlet water vapor content (0.01 kg kg 1) and mass flow rate. Figure (a) and (b) show the dependencies of heat and mass flow on the time step in the process, respectively. Figure (a) and (b) suggest that the airflow and the desiccant lined wall were close to thermodynamic equilibrium throughout most of the wheel cycle in most of the rotor domain. This suggests that it is possible to optimize the dehumidification performance of the rotor through a selection of rotation speed and of the extent of the adsorption and desorption zones. Other cases with different cycle durations were simulated. The registered influence of cycle τ on the global heat and mass transfer rates is shown in Fig Ruivo, C. et al (2011) investigated the internal resistance to heat and mass transfer of the desiccant matrix. The authors pointed out the shortcomings of simplified methods describing air stream processes and related interaction with the desiccant medium by assuming a fictitious bulk flow pattern, as well as fictitious heat and mass convection coefficients for the gas side. July 9, 2016 Page 62 of 171

71 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure (a): Heat transfer characteristic inside a channel of the desiccant matrix. T i = Temperature at the matrix interface T f = Temperature of the bulk airflow (Ruivo, C. et al, 2011) Figure (b): Mass transfer characteristic inside a channel of the desiccant matrix. W i = Water ratio at the matrix interface W f = Water ratio of the bulk airflow (Ruivo, C. et al, 2011) Figure illustrates the physical model used to describe heat and mass transfer inside a desiccant wall element, which is reduced to an element of the channel wall consisting of a homogeneous desiccant medium. The heat and mass transfer phenomena inside the porous medium are considered July 9, 2016 Page 63 of 171

72 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS only in the lateral direction (e.g. y direction). For simplification, the bulk air flow was considered fully mixed, which means it was characterized by constant and uniform properties. Figure 5.7.5: Heat and mass transfer rates per unit of transfer area of the desiccant wheel as a function of the wheel cycle time (Ruivo, C. et al, 2011) Figure 5.7.6: Schematic representation of the channel wall element H P = Thickness of channel wall element. (Ruivo, C. et al, 2011) Figure (a) and (b) show time varying profiles of dependent variables along the desorption process in a desiccant layer of H P = 1 mm, for temperature and adsorbed water content, respectively. Figs (a) and (b) depict results obtained with the normal internal resistances. Figure (a) shows small temperature gradients, which indicate that the internal resistance to heat diffusion is almost insignificant. Figure (b), on the other hand shows significant mass diffusion gradients. The gradients of the adsorbed water content are significant during almost all the transient process, mainly near the convective surface. July 9, 2016 Page 64 of 171

73 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure (a): Time varying profiles of temperature in a desiccant layer of H p = 1 mm (Ruivo, C. et al, 2011) Figure (b): Time varying profiles of adsorbed water content X` in a desiccant layer of H p = 1 mm (Ruivo, C. et al, 2011) In order to account for varying assessments of heat and mass transfer coefficients Ruivo, C. et al (2011) introduced two additional scenarios. These are referred to as A = negligible internal resistances to the heat and mass diffusion (approach A null resistances ) and B = thermal lumped capacitance method (approach B null thermal resistance ). Figure (a) and (b) suggest that significant inaccuracies may result when the internal resistance to mass diffusion is neglected, leading to unrealistic estimation of the convection fluxes at the interface. July 9, 2016 Page 65 of 171

74 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure (a): Predicted temperature at the convective interface, H p = thickness of desiccant film (Ruivo, C. et al, 2011) Figure (b): Predicted adsorbed water content at the convective interface, H p = thickness of desiccant film (Ruivo, C. et al, 2011) 5.8 Heat and Mass Transfer Characteristics of Desiccant Polymers Staton (1998) discussed heat and mass transfer characteristics of advanced desiccant material in conjunction with air conditioning applications. The author described process characteristics of total energy exchangers, where the heat and mass transfer processes occur in a two phase process, and at different rates. July 9, 2016 Page 66 of 171

75 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS As an illustration, a limiting process factor could be the desiccant moisture uptake rate, if a desiccant material has a slow moisture uptake rate. As a consequence, an energy transfer wheel would have to rotate much slower to transfer moisture than heat. The author indicates that the difference in wheel rotation could be significant, with a desiccant wheel and a heat transfer wheel spinning at typical speeds of 6 10 rph and 60 rpm, respectively. As a consequence, slow adsorption and desorption transfer rates of the desiccant material used in an air conditioning systems could necessitate installing two phase processing wheels for latent and sensible load reduction. On the other hand, the use of a desiccant material with an increased rate of absorption and desorption and a low desorption temperature could enable the designer to incorporate the sensible heat exchanger component with the desiccant dehumidifier to produce a total enthalpy exchanger. The author pointed out that total enthalpy exchanger introduces possibilities for increased energy transfer efficiency, reduced material costs, a decrease in the number of components required to properly condition the air, and a decline in the demand for refrigerant based components within the conditioner. Figures (a) and (b) illustrate the one and two step conditioning process of the supply and discharge air in air conditioning systems, respectively. (a) One Step Air Conditioning Process (b) Two Step Air Conditioning Process Figure 5.8.1: Schematic of one and two step air conditioning process, Staton (1998) Staton (1998) suggests the use of polymer desiccants as an alternative to traditional desiccants within energy exchanger applications, since polymers have reported higher sorption rates and require lower regeneration temperatures. The author refers to five desirable characteristics for these new desiccant materials: 1. Mechanical and chemical stability 2. Large maximum moisture capacity 3. High sorption rates at low vapor pressures 4. Low heats of adsorption 5. Ideal Isotherm shape July 9, 2016 Page 67 of 171

76 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Polymers are large molecules consisting of repeating chemical units, or mers. One polymer molecule, or macro molecule, can consist of 50 to 3000 mers. Polymers are capable of being cross linked, which is the condition of the polymer chains being bonded together to form a network. The bonds responsible for cross linking are covalent bonds, which will neither dissolve when put in solution nor flow when heated. Super Absorbent Polymers (SAP) are hydrogels, water insoluble hydrophilic homopolymers or copolymers, which are able to swell and absorb 10 to 1000 times their own weight in water. Super Absorbent Polymers are crosslinked in order to avoid dissolution. There are three main classes of SAPs: 1. Cross linked polyacrylates and polyacrylamides 2. Cellulose or starch acrylonitrile graft copolymers 3. Cross linked maleic anhydride copolymers The most important property of hydrogels is their high affinity for water. The capability of hydrogels to attract water can be attributed to the presence of groups like hydroxyl groups which are highly watersoluble. Even when saturated, hydrogels can remain insoluble and structurally stable due to the threedimensional cross linked network. Staton (1998) identified five candidate polymer desiccant materials and conducted scoping tests to identify the best polymer desiccant for further studies. The five polymer desiccant materials and two traditional desiccant materials were as follows: Cross linked polyacrylamide copolymer: Typically used as a soil additive to keep moisture content more constant. Known to maintain strength when swollen and readily absorbs and desorbs water. Polyacrylate: Acrylic acid cross linked with sodium acetate; typically used in hygiene products. Known for selective absorbency as a result of the ph of the water. Polyvinyl alcohol: Porous open cell sheet foam capable of absorbing up to twelve times its weight in water. Also available in powder and liquid forms. Typically used as sponges in cosmetic and industrial applications. Polyethylene glycol: Copolymer exhibiting LCST characteristics with water in lower molecular weights. Lower molecular weight materials are in liquid form. These can be used as thickeners in cosmetics and food products. Polypropylene glycol: Copolymer exhibiting lower critical solution temperature (LCST) characteristics with water in lower molecular weights. Typically used in food, cosmetic, personal care and pharmaceutical applications. Silica Gel: Ceramic desiccant popular for use in dehumidification applications. Used for comparison with the polymer desiccants. Calcium sulfate: Ceramic desiccant popular for use in dehumidification applications. Used for comparison with the polymer desiccants. July 9, 2016 Page 68 of 171

77 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS For the above mentioned desiccant materials experiments were conducted (Staton, 1998) to determine each material s ability to absorb water and to determine each materials maximum sorption capacity. Similar experiments were also conducted to determine how quickly the material released the stored moisture (desorption rates). Figures and show the results of these scoping tests. Table indicates the equilibrium adsorption rates. Figure 5.8.2: Candidate polymer desiccant adsorption rates; (Staton, 1998) Table 5.8.1: Equilibrium moisture content values for the desiccant material candidates; (Staton, 1998) July 9, 2016 Page 69 of 171

78 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure 5.8.3: Candidate polymer desiccant desorption rates; (Staton, 1998) The results of the tests suggested that polymer desiccant polyvinyl alcohol had the best process performance, which included the fastest rate of adsorption and the fact that that desorption could be accomplished at low regenerative temperatures. In addition, film and foams, the desiccant material forms of interest, were proven safe to use in heat recovery ventilation application. Therefore, a more detailed analysis was conducted with this desiccant material only. Staton (1998) analyzed three different forms of polyvinyl alcohol coating applications for total energy exchanger in a desiccant wheel and fixed plate total enthalpy exchanger. Figures through summarize some main results and comparisons of the desiccant exchanger used in the tests. The three polyvinyl alcohol material coatings evaluated above were compared to a commercially available desiccant dehumidifier manufactured by LaRoche Air Systems. The LaRoche desiccant material is a ceramic coating on a fibrous paper substrate. Similar to the pure polyvinyl alcohol foam wheel structure, the LaRoche paper wheel needs no supporting substrate, which means that can be made entirely of the ceramic coated paper. July 9, 2016 Page 70 of 171

79 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Polyvinyl Alcohol Film Coated Rotary Wheel Total Enthalpy Exchanger Polyvinyl Alcohol Foam Coated Rotary Wheel Total Enthalpy Exchanger Polyvinyl Alcohol/Ceramic Composite Coated Rotary Wheel Total Enthalpy Exchanger Figure 5.8.4: Wheel Total Enthalpy Exchanger Efficiency vs. Channel Depth; (Staton (1998) July 9, 2016 Page 71 of 171

80 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Polyvinyl Alcohol Film Coated Rotary Wheel Total Enthalpy Exchanger Polyvinyl Alcohol Foam Coated Rotary Wheel Total Enthalpy Exchanger Polyvinyl Alcohol/Ceramic Composite Coated Rotary Wheel Total Enthalpy Exchanger Figure 5.8.5: Wheel Total Enthalpy Exchanger Efficiency vs. Desiccant Coating Thickness; (Staton (1998) Figure compares the efficiencies of the three polymer desiccant materials and the LaRoche coating option. The performance of a fixed counter flow plate exchanger was determined for polyvinyl alcohol foam coating and La Roche coating. The results are presented in Figure The results suggest a significantly higher efficiency of polyvinyl alcohol foam coating for the plat exchanger than for desiccant wheel applications. July 9, 2016 Page 72 of 171

81 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Polyvinyl Alcohol Film Coated Rotary Wheel Total Enthalpy Exchanger Polyvinyl Alcohol Foam Coated Rotary Wheel Total Enthalpy Exchanger Polyvinyl Alcohol/Ceramic Composite Coated Rotary Wheel Total Enthalpy Exchanger Figure 5.8.6: Wheel Total Enthalpy Exchanger Efficiency vs. air stream velocity; (Staton (1998) July 9, 2016 Page 73 of 171

82 SECTION 5 PHYSICAL PROPERTIES OF DESICCANTS Figure 5.8.7: Comparison between different rotary wheel total enthalpy exchanger coatings; (Staton,1998) Figure 5.8.8: Comparison between fixed plate total enthalpy exchanger desiccant materials; (Staton,1998) July 9, 2016 Page 74 of 171

83 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES Daou (2006) described the process of desiccant thermal regeneration as part of desiccant cooling. The regeneration heat source supplies the thermal energy necessary for driving out the moisture that the desiccant had taken up during the sorption phase. Several possible energy sources were considered. Those included solar energy, waste heat, and natural gas heating, and the possibility of energy recovery within the system. (Figure 6.1) In the case of solid desiccant, the heat of regeneration is furnished by blowing the scavenger air stream concurrently through the desiccant matrix. For a liquid desiccant system being used, the heat of regeneration is furnished to the desiccant solution inside the regenerator process vessel where a scavenger air stream is concurrently blown to carry away the moisture desorbed under the desiccant heating. The scavenger air can also be a hot air stream brought into contact with the dilute desiccant solution inside the regenerator thereby heating it. Figure 6.1: Process schematics of desiccant cooling, (Daou 2006) 6.1 Desiccant Regeneration with Heat from Natural Gas or other Fossil Fuels There are numerous desiccant applications where the regeneration heat is provided by fossil fuel. A typical application is natural gas dehydration systems using desiccants remove water vapor from natural gas during production, transmission, and distribution. Natural gas flowing from production wells or underground storage requires dehydration to protect the distribution system from corrosion and hydrate formation. In the commonly used solid desiccant absorption dehydration process, wet gas flows July 9, 2016 Page 75 of 171

84 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES through a pressure vessel tower filled with solid desiccant. Natural gas is used to gently heating the regeneration gas, and sending it to the wet desiccant tower to dry out the desiccant. Farang (2011) conducted research on a natural gas drying process using molecular sieves, which is considered as one of the most important desiccant materials in industrial natural gas dehydration. The authors stated that for the regeneration of weak desiccants, such as molecular sieve, the higher the regeneration temperature, the smaller are the required quantities of regeneration gas. In the experiments, the drying operation resulted in dew points of minus 30 C or lower (pressure 20 MPa). With a regeneration temperature equal or close to the adsorption temperature, the quantity of purge gas depended on its pressure. For example, when the regeneration pressure is increased from 0.1 to 0.5 MPa, the flow rate of purge gas is increased from 10% to 20%. Regeneration of the sorbent at a temperature above the adsorption temperature was leading to a reduction of the purge gas quantity. The authors identified a regeneration temperature as C. Netušil and Ditl (2012) described natural gas dehydration method water by a solid desiccant. They suggested molecular sieve, silica gel or alumina as the desiccant material of choice. The authors identified recommended regeneration temperatures of 230, 240 and 290 C for silica gel, alumina and molecular sieves. Figure shows a typical scheme of the temperature swing adsorption dehydration process used by the authors. The amount of adsorbed water increases with the pressure in the process vessel and decreases with its temperature. The swing adsorption dehydration columns always work batch wise. A minimum of two vessels are used, where one vessel dries the gas while the other is being regenerated. This method is known as temperature swing adsorption (TSA). The TSA heater was an ordinary burner or a shell and tube heat exchanger warmed by steam or other hot process fluid. The regeneration gas warms in the heater and flows into the column, where it passes through the adsorbent and the water desorbs into the regeneration gas. The water saturated regeneration gas then flowed into the cooler. The cooler used cold air to decrease the temperature of the regeneration gas. When the water saturated regeneration gas is cooled, partial condensation of the water occurs. Kozubal et al (2012) introduced the new concept of Desiccant Enhanced Evaporative Air Conditioning (DEVAP). The system is a compact AC system for residential and commercial applications. The regeneration heat for the liquid desiccant can be supplied by a variety of heat sources including natural gas, waste heat or even solar heat. Figure show illustrations of a DEVAP used in a commercial application. Person (1994) presented a two stage desiccant conditioner. Figure illustrates the process where the desiccant wheel is regenerated by a natural gas burner heating an air flow. July 9, 2016 Page 76 of 171

85 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES Figure 6.1.1: Typical scheme of the temperature swing adsorption dehydration process, (Netušil and Ditl, 2012) Figure 6.1.2: Process schematics of the DEVAP system, (Kozubal et al, 2012) Figure 6.1.3: All Air VAV System Retrofitted with a Desiccant Preconditioning System, (Pesaran, 1994) July 9, 2016 Page 77 of 171

86 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES 6.2 Desiccant Regeneration with Waste Heat Jalalzadeh Azar (2005) described desiccant cooling as an important part of the wide range of Thermally Activated Technologies (TAT) which use conversion of heat for the purpose of indoor air quality control. For regeneration of the desiccant, the vapor pressure differential is reversed in the regeneration process that drives the moisture from the desiccant. A burner or a thermally compatible source of waste heat can provide the required heat for regeneration. When implemented in the context of combined heat and power (CHP), also known as cogeneration, TAT can significantly improve fuel efficiency/utilization through heat recovery from the on site power generators and leads to a significant reduction in emissions. Desiccant materials with regeneration source temperatures typically ranging from about 160 F to 300 F are compatible with various types of on site generators, including reciprocating IC engines, micro turbines, and certain types of fuel cells. Cascading desiccant cooling with another compatible technology, such as absorption or adsorption cooling, further enhances the overall system fuel efficiency and utilization. Cascading processes allows sequential heat recovery from a single heat source for driving two or more thermally activated systems with different operating temperatures to achieve higher efficiencies. Jalalzadeh Azar (2005) described a CHP system which incorporated a 60 kw micro turbine for onsite power generation and a gas to liquid heat exchanger for heat recovery from the exhaust gas for space heating and desiccant dehumidification, depending on the season. A gas compressor was added to the systems to boost the natural gas pressure to the operating pressure of the micro turbine combustor. Figure illustrates the air handling unit (AHU) incorporating a desiccant wheel, a DX cooling coil, and a heating coil. A glycol solution leaving the heat exchanger at about 180 F preheats the regeneration air, which is further heated by a gas burner to about 275 F for regeneration of the desiccant. In the heating season, the hot liquid is circulated through the space heating coil instead (Figure 6.2.2). Monitoring of the system over a year revealed that the overall efficiency of the CHP system could exceed 50% on humid summer days, compared to the corresponding net electrical efficiency of about 21% to 23%. These results emphasized the importance of TAT, without which the CHP systems cannot attain high energy conversion efficiencies which usually justifies for higher first costs. July 9, 2016 Page 78 of 171

87 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES CHP system, roof top installation Schematic of air handling unit Figure 6.2.1: CHP system with desiccant unit (Jalalzadeh Azar, 2005) Kassem (2013) suggest that the use of heat to regenerate desiccant material in a drying system can lower the potential of energy saving in cooling desiccant cooling applications. However, the use of low energy or free available energy such as waste heat from industrial processes for regeneration of desiccant material will make the system more cost effective. Figure shows the operational concept and diagram of a desiccant based ventilation and airconditioning system. The temperature of the processed air from the desiccant dehumidifier increases due to the heat of condensation and heat of sorption. Heat recovery devices are used to recover this energy. Downstream of the heat recovery the air becomes warm and dry. Since the air is warmer than required for space comfort temperature, an evaporative cooling process is applied by either direct addition of air moisture or indirect addition of air moisture in secondary air stream. The July 9, 2016 Page 79 of 171

88 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES application of evaporative cooling process reduces the air temperature with either slight increase of air moisture content or constant air moisture content for the direct or indirect evaporative cooler, respectively. Figure 6.2.2: Operational concept and diagram of the desiccant based ventilation and air conditioning system (Kassem, 2013) Kassem (2013) suggests that liquid desiccants are better suited for the use of waste heat than solid desiccants. While the capacity to absorb moisture is generally greater in liquid than in solid desiccants, liquid desiccants require lower regenerating temperature, mostly in the range of C while it is in the range of C for a solid desiccant. Figure shows an example of a liquid desiccant cooling system which uses low grade regeneration heat, such as derived from waste heat. In the depicted system (in Figure 6.2.3) concentrated solution is sprayed at point A over the cooling coil at point B while supply air at point 1 is blown across the stream. The solution absorbs moisture from the air and is simultaneously cooled down by the cooling coil. The results of this process are the cool dry air at point 2 and the diluted solution at point C. July 9, 2016 Page 80 of 171

89 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES Figure 6.2.3: A liquid desiccant cooling system using low grade heat for desiccant regeneration, (Kassem, 2013), annotated An aftercooler reduces the air stream temperatures further down, if required. In the regenerator, the diluted solution from the dehumidifier is sprayed over the heating coil at point E which derives its heat supply from a waste heat source. Ambient air at point 4 is blown across the solution stream. Some water is taken away from the diluted solution by the air while the solution is being heated by the heating coil. The resulting concentrated solution is collected at point F and hot humid air is rejected to the ambient at point 5. A recuperative heat exchanger preheats the cool and diluted solution from the dehumidifier using the waste heat of the hot concentrated solution from the regenerator, resulting in a higher COP. Another process application of waste heat in desiccant regeneration is the use waste heat from condensers in vapor compression refrigeration systems. Misha (2012) suggested the system depicted in Figure 6.2.4, which is a hybrid system combining a vapor compression system with liquid desiccant dehumidification and regeneration. Such a system can obtain high thermal performance and energy saving. Zhang and Yin (2008) conducted simulations of the system depicted in Figure and concluded that the air and desiccant flow rates are the governing process parameters. Figure (a) and (b) show the regeneration efficiency as a function of air and desiccant flow rates. From these results the authors conclude that: For air side heating source regeneration models, there is an optimum solution flow rate to achieve maximum regeneration efficiency, which greatly depends on the air flow rate; The maximum regeneration efficiency decreases with the increase of the air flow rate; Usually, the maximum regeneration efficiency can achieve at very low flow rate of the desiccant solution. July 9, 2016 Page 81 of 171

90 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES Figure 6.2.4: An energyefficient air conditioner with a liquid desiccant regenerator using exhausted hot air from the condenser; (Misha, 2012) (a) Air flow rate (b) Desiccants flow rate Figure 6.2.5: Results of simulation of a liquid desiccant cooling system using low grade heat for desiccant regeneration; (Zhang and Yin, 2008) Process parameters indexed in Figure are as follows: G = mass flow rate, kg/s X = mass concentration of desiccant solution, % Ƞ reg = regeneration efficiency, dimensionless V air = Volume flow rate, m3/s July 9, 2016 Page 82 of 171

91 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES 6.3 Desiccant Regeneration with Solar Heat and other Renewable Heat Sources Solar heat is often mentioned as a main advantage of using desiccant dehumidification to remove latent loads in air conditioning systems. While energy and cost savings can be realized during the operation, solar heat from thermal collectors often also qualify for installation incentives (Chen et al, 1992). Many authors point out benefits of using heat energy instead of utility electricity to energize dehumidification. Heat sources required to regenerate/dry the desiccant for reuse can include various solar energy and recovered waste heat as well as gas produced heat (direct fired or cogenerated). Offpeak electric resistance heaters or heat pumps can also be used. Barlow (1982) performed a comprehensive analysis of solar heat used in desiccant dehumidification. The author suggested that both solid and liquid desiccants are suitable for solar heat regeneration. Barlow suggested that data and correlations of the properties of regular density silica gel appeared to be the most suitable available solid desiccant for solar cooling systems. Sahlot and Riffat (2016) suggested that the use of solar energy for desiccant applications in space conditioning has several overlapping benefits. Desiccant cooling systems require electricity to operate pumps and fans and heat energy to pre heat the desiccant solution for regeneration. The author stated that a solar PV system can be used to derive electricity to drive the pumps and fans. Secondly, solar heat can be used to regenerate weak desiccant solution. Figure shows the schematic diagram of a desiccant cooling system integrated with an evaporative cooler and a solar collector. Combined solar collector and regenerator is used directly to pre heat the desiccant solution before introducing it in the regenerator. Another type of solar collector uses water as a medium of heat transfer between collector and liquid desiccant. The combined type of solar collector device is more efficient as all the heat energy absorbed by the collector is directly transferred to the desiccant solution. However, as desiccant solution is often corrosive, the solar collector would need to be corrosion resistant. Sultan et al (2015) provided a comprehensive review of desiccant applications powered by solar heat. The authors suggested that zeolite prepared from fly ash has shown significant potential for solar cooling applications. While solid desiccants have advantages because of their relative operational ease, liquid desiccants are often aggressive agents and require suitable material and care when handled in operation. Common liquid desiccants are lithium chloride (LiCl), lithium bromide (LiBr), calcium chloride (CaCl2) and triethylene glycol [37,38,42]. Liquid desiccants have the advantage that they are requiring lower regeneration temperature, typically 60 to 75 C, which makes them very suitable to utilize solar or low grade waste heat. July 9, 2016 Page 83 of 171

92 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES Figure 6.3.1: Solar powered desiccant cooling system, (Sahlot and Riffat, 2016) Kassem (2013) highlights the cost effectiveness of the use of low energy or free solar energy for regeneration of desiccant material. The authors presented a review of solar energy as a free energy for the regeneration and pointed to the extensive research which had been conducted. The initial cost of solar energy system is considerable, but over time significant savings can be realized. Therefore, the payback period should be considered. A disadvantage is that solar radiation is weather dependent; therefore, back up energy or energy storage would be required to continue the drying process when solar energy is not available. Kassem (2013) pointed out that The use of evaporative coolers for mass exchange equipment can be applied in liquid desiccant systems. This application can reduce the installation cost of such systems because the evaporative coolers are available in the market with lower cost when compared with designing and manufacturing specified mass exchange equipment for such systems. Kassem (2013) mentioned advantages of the liquid desiccant over solid desiccants as the lower regenerating temperatures of liquid desiccants, typically in the range of C, while the higher values in the temperature range of solid desiccants is as high as 115 C. This allows the use of low grade heat sources such as solar energy or waste heat for liquid desiccant regeneration. In addition, liquid desiccants can be stored in form of concentrated solution for use during periods when solar energy is absent, and thus offer more flexible operational characteristics. The liquid desiccants are attractive because of their operational flexibility and their capability of absorbing pollutants and bacteria. Their disadvantages include entrainment and subsequent carryover of desiccant droplets into the process air stream during the dehumidification operation. July 9, 2016 Page 84 of 171

93 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES Technologically, the equipment providing air/solution contact surface (contactor) for liquid desiccant solutions include wetted wall/falling film absorber, a spray chamber or a packed tower. The packed towers are subdivided into regular (structured) or irregular (random) packing modes. Kassem (2013) concluded that liquid desiccant assisted air conditioning can achieve up to 40% of energy savings with regard to traditional air conditioning system and those savings become even greater when the calorific energy needed for regeneration is drawn from waste heat, solar energy or any other free energy sources. Figure shows a typical liquid desiccant cooling process, where liquid desiccant is regenerated by solar heat. Daho (2006) commented on feasibility studies of solar driven desiccant cooling in diverse European cities representing different climatic zones on the continent. The conclusion reached by the author was that primary energy savings were achieved in all climatic conditions. A decline in energy savings was noticed in highly humid zones. This decline was attributed to the high temperature required to regenerate the desiccant in the climates of high humidity. Sahlot and Riffat (2016) estimated that potential of solar energy uses in desiccant cooling systems and concluded that desiccant systems can save up to 50% of primary energy. Figure 6.3.2: A liquid desiccant cooling system with solar collector; (Kassem, 2013) Besides solar heat, other renewable heat sources can be used to regenerate desiccants. Jradi and Riffat (2014) present a tri generation system which was driven by biofuel. The process, as illustrated in Figure 6.3.3, used a biofuel gasifier to provide fuel for the generator, and the waste heat used to heat domestic July 9, 2016 Page 85 of 171

94 SECTION 6 DESICCANT REACTIVATION ENERGY SOURCES water supply as well as for use in absorption chiller. The authors pointed out that waste heat could drive the regeneration heat instead of the absorption chillers. Figure 6.3.3: An internal combustion based tri generation system with a biomass gasification unit ; (Jradi and Riffat, 2014) July 9, 2016 Page 86 of 171

95 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION The main focus of this literature review is the application of desiccant dehumidification to the conditioning of buildings. This section presents the technologies which are important to derive effective cooling applications combined with desiccant dehumidification. It must be noted that often the term desiccant cooling is used, which is somewhat inaccurate, since desiccants carry out only the latent heat removal while other cooling technologies contribute sensible heat removal. Figure 7.1 shows the types of AC systems discussed in this section. 1. Grid powered Conventional AC systems: A large percentage of all existing AC systems work with grid supplied power and conventional vapor compression cooling. The three functions of the AC system are carried out as follows: A. Sensible heat removal: Supply air passing cooling coils B. Latent heat removal: water vapor condenses in cooling oils C. Ventilation: Air supply that passes the cooling coils is provided to the spaces 2. Grid powered alternative AC systems: This alternative system works with a conventional vapor compression cooling systems which is augmented with a desiccant dehumidification system. The three functions of the AC system are carried out as follows: A. Sensible heat removal: Supply air passing cooling coils B. Latent heat removal: Water vapor is removed by desiccant sorption. The desiccant regeneration uses process (waste) heat derived from either combined heat and power (CHP) systems, other process heat or fossil fuel combustion. A combined dehumidification process of cooling based dehumidification (condensation on conventional cooling coils) and desiccant dehumidification is also possible, which can save energy. C. Ventilation: Air supply that passes the cooling coils is provided to the spaces 3. Solar powered Conventional AC systems: This AC system uses PV solar derived electric energy to drive a conventional AC system. The three functions of the AC system are carried out as follows: A. Sensible heat removal: Supply air passing cooling coils B. Latent heat removal: water vapor condenses on cooling oils C. Ventilation: Air supply that passes the cooling coils is provided to the spaces 4. Thermal solar powered Alternative AC systems: This alternative system obtains most of its energy through thermal energy conversion for sensible cooling and dehumidification. The three functions of the AC system are carried out as follows: A. Sensible heat removal: Sensible heat is removed by supply air passing over surfaces, cooled by closed systems, or be adiabatic evaporative cooling B. Latent heat removal: Water vapor is removed by desiccant sorption. The desiccant regeneration uses solar heat. C. Ventilation: Air supply that passes the cooling surfaces is provided to the spaces July 9, 2016 Page 87 of 171

96 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Figure 7.1: Types of building AC systems Note: Section 7 discusses only literature relative to closed and open thermal systems which supply sensible heat removal (shaded block in point 4 of above bulleted list) July 9, 2016 Page 88 of 171

97 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION 7.1 Thermal Cooling Technologies This section provides a brief review of several alternative cooling technologies which remove sensible heat from conditioned spaces. Table provides an overview of the technologies reviewed hereafter, where the main distinction is made between closed and open systems. Closed cycle systems are equipped with thermally driven chillers, which provide chilled water that is either used in air handling units to provide sensible and latent (cooling based) load removal or that is distributed via a chilled network to decentralized room installations such as fan coils or chilled ceilings. Available thermally driven chillers on the market are absorption chillers, which are most common, and adsorption chillers, offered currently by few manufacturers only. A component, necessary in all chilled water systems, is a heat rejection system. Open cycle systems are evaporative chillers which provide sensible heat removal. The refrigerant is always water, which is brought into direct contact with the process air and adiabatically reduce the dry bulb temperature of the moist air. Among the thermally driven cooling systems, closed cycle systems currently occupy a central position. According to a survey in 2003, approx. 71 % of the installations were equipped with chillers (63 % using absorption technology and 8 % using adsorption chillers) (SOLAIR (2009). 29 % of the installations are desiccant cooling systems SOLAIR (2009) suggested that solar cooling systems represent a fast growing market with several new international companies selling products. New interesting developments and concepts are appearing in the different technologies. In the future, the main barriers to overcome in order to increase the market share of thermal cooling technologies are the organization of the sector, cost reduction, and the adaptation of the machines and the main components to technical specifications of solar cooling systems. Table 7.1.1: Overview of main air conditioning technologies using thermal energy, SOLAIR (2009) Method Closed cycle Open cycle Principle / Process Name of cooling technology Providing chilled water for convective (supply air) or radiative cooling surfaces Adsorption chiller Absorption chiller Adiabatic heat removal through evaporation of water in air supply Direct evaporative cooling Indirect evaporative cooling Adds humidity to air No No Yes No Phase of sorbent Solid Liquid N/A N/A Typical material pairs water silica gel water/li Br water air water air July 9, 2016 Page 89 of 171

98 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION 7.2 Evaporative Cooling General Principle Evaporative cooling has been used for a long time to provide relief to humans exposes to high temperature climate. The concept of using water for air cooling has been around for centuries (Watt and Brown, 1997). While complex evaporative cooling systems have been invented and refined over the past decades, the basic concept has not changed. Egyptian frescoes portraying large, porous jars of water being fanned to force evaporation and subsequent cooling have shown the prevalence of evaporative cooling since ancient times. In the Arizona desert in the 1920s, people would often sleep outside on screened in sleeping porches during the summer. On hot nights, bed sheets or blankets soaked in water would be hung inside of the screens. Whirling electric fans would pull the night air through the moist cloth to cool the room. That concept, slightly more refined, became the evaporative coolers that to this day provide a low cost, low technology alternative to refrigerated air conditioning. An evaporative cooler produces effective cooling by combining a natural process water evaporation with a simple, reliable air moving system. Fresh outside air is pulled through moist pads where it is cooled by evaporation and circulated through a building by a fan system. With the evaporative process, the outside air temperature can be lowered as much as 30 F. (Palmer, 2002) The term swamp cooler is frequently used for evaporative coolers, because of the standard design of having a tray which collects the water that is not evaporating. Evaporative coolers work well, provided the outside air they are drawing in is dry and desert like. As the humidity increases, the ability for these systems to cool the air decreases. The dependency of the outside climate and humidity was one of the reasons that conventional air conditioning systems, basically all vapor compression systems, became popular since their ability to cool the air was not impeded by the outside humidity. The advantages of these conventional mechanical ACsystems were readily apparent as they were able to lower the temperature to a thermostatically controlled temperature, even on humid days. The disadvantage of conventional vapor compression ACsystems is their significant electricity demand, as much as four times the electricity needs of evaporative coolers. In addition, conventional AC systems are more expensive to install and maintain than evaporative coolers. And, they require ozone damaging refrigerants (Green Cooling Initiative, 2016). Palmer (2002) points out that in dry desert areas, evaporative coolers work the best, but they also perform reasonably well fine most of the time in California's more humid climates. Sacramento, for example, averages about 45 percent humidity on a typical hot summer afternoon, still dry enough for evaporative cooling to work effectively. Despite the potential, however, fewer than five percent of California homes and businesses use evaporative cooling. July 9, 2016 Page 90 of 171

99 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Several authors such as Kozubal et all (2011), Watt and Brown (1997), and Sahlot and Riffat (2016) summarize the benefits of evaporative cooling as follows: Evaporative coolers use significantly less electric energy use than energy intensive vapor compressor based cooling. The only electricity demands of evaporative chillers are supply fans and a sump pump. As a consequence, evaporative cooling requires 1/5 to 1/2 as much electricity as mechanical cooling to operate. Maintenance requirements are simpler for evaporative cooling systems than for refrigerated air conditioning equipment. Refrigeration compressors, evaporators and condensers must operate under high pressures, which require specialized tools and certified maintenance personnel. Evaporative cooler users can maintain their peak cooling effectiveness without the need for costly and sometimes unavailable specialized maintenance contracts. The compounded (life cycle cost) of using evaporative cooling is less than a comparable refrigerated air unit. This includes all dollar values such as first cost, energy, water, time value of money and maintenance costs. Evaporative cooling saves water at the power plant. During a typical summer in New Mexico, a coal fired power plant using evaporative cooling towers will typically need about 0.95 gallons per kwh. This quantity does not include the water needed to mine, process and deliver the coal used to generate the electricity. The amount of water used by an evaporative cooler is stated in terms of tons of cooling per gallon, which is on average one gallon of water per 0.6 ton of refrigeration. Therefore, the water requirement for evaporative cooling is about 50 to 60% of a typical water requirement for a thermal power plant. Evaporative cooling does not directly use any working fluids which are detrimental to the earth s ozone layer. This is unlike most of the pre 2000 commercial refrigerants whose use is regulated in order to reduce their harmful impact on the environment. Evaporative coolers do not operate under high pressure conditions and do not require any expensive controlled substances for their operation. An evaporative cooled building will always require less power to operate than a mechanically cooled building. As a rule of thumb 0.5 to 5 kw and 3 to 10 kw of electrical power is required by evaporative and mechanical cooling, respectively. The reduced power demand does not only lower operational costs through smaller demand charges, but also lowers first costs for electrical components will cost less. The energy savings of evaporative cooling translates directly into reduced carbon dioxide and other emissions from power plants, and decrease the peak electricity demand load that typically occurs during peak summer cooling hours. There are approximately 4 million evaporative air cooling units in operation in the United States which provide an estimated annual energy savings equivalent to 12 million barrels of oil, and annual reduction of 5.4 billion pounds of carbon dioxide emissions. They also avoid the need for 24 million pounds of refrigerant traditionally used in residential VAC (vapor compression air conditioning or refrigerated air) systems". Since evaporative cooling typically uses 100% outside air rather than recirculated air indoor air quality is improved. The outside air and humidity added to the room air by an evaporative July 9, 2016 Page 91 of 171

100 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION cooler can improve comfort conditions, flush out contaminants which are generated in the building and reduce the incidence of static electric shock which can be detrimental to microelectronics. The working principle of evaporative cooling systems is water evaporation. This change of phase requires latent heat to be absorbed from the surrounding air and the remaining liquid water. As a result, the air temperature decreases and the relative humidity of the air increases. The maximum cooling that can be achieved is a reduction in air temperature to the wet bulb temperature (WBT) at which point the air would be completely saturated, but most evaporative cooling devices attain less than the technically feasible temperature drop. When air below saturation moves over a surface of water evaporates. This evaporation results in a reduced temperature and an increased vapor content in the air. The bigger the area of contact between the air and water the more evaporation occurs, resulting in more cooling and the addition of moisture. In order for water to evaporate, heat is required. In order to evaporate one gallon of water approximately 8,700 BTU s of heat are required. For evaporative cooling, this heat is taken from the air, thereby cooling it. There are two basic types of evaporative air cooling (EAC) processes. Direct evaporative cooling occurs when process air has direct contact with the evaporating water. Indirect evaporative coolers utilize a heat exchanger; therefore, the process air never comes into direct contact with the cooling water. Direct EAC is commonly used for residential cooling. Developments in the evaporative cooling industry have reliably increased the efficiency or effectiveness of the cooling media. All direct EAC s use 100% outside air. Indirect evaporative air coolers (IEAC's) can be used in conjunction with direct EAC's and/or with refrigerated air coolers. IEAC systems use electricity for the supply fan motor, a sump pump, and a smaller secondary fan motor used for the heat exchanger s airflow. The combination evaporative and refrigerated system has a higher first cost, but offers a good mix of energy conservation and comfort. Additionally, these redundant cooling systems are more reliable. IEAC are rated from 60% to 78% thermal efficiency, depending on the configuration and the air speed past the heat exchanger. IEAC systems can be used in combination with direct evaporative cooling, in combination with refrigerated air systems or as a stand alone system. When combined with direct EAC systems the effectiveness is additive. It is essential that indirect evaporative chillers do not add moisture to the room air stream and therefore do not increase the room humidity level. Therefore, indirect evaporative coolers can recirculate the room air. The possible energy savings of evaporative coolers are significant when compared with conventional mechanical refrigeration cycles. Table shows a comparison of average energy savings through the use of evaporative cooling systems Palmer (2002). July 9, 2016 Page 92 of 171

101 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Table 7.2.1: comparison of evaporative and conventional split system AC; (Palmer, 2002) 7.3 Direct Evaporative Cooler Working Process The typical direct evaporative cooler unit consists of a metal, plastic or fiberglass housing and frame, a supply fan, water holding sump, water circulation pump, water distribution tubing, electric connections and a wetted pad. These pads are the surface from which the water evaporates, and are usually made of aspen shavings, paper or plastic media. Typical manufacturers stated evaporative effectiveness for this type of wetted media is 65 to 78%. All EACs use a small fractional horsepower pump to raise the water over the pads, then gravity and capillary action wet the entire area of the evaporative media. Advantages of direct evaporative cooling are its low life cycle cost, improved indoor air quality, reduced peak electrical demand, simple controls and low tech maintenance. Disadvantages include relatively short service life of aspen media aspen pad coolers (1 to 3 years, depending on media type and maintenance), the need for seasonal maintenance and reduced cooling performance during the wet season. There are many types of wetted media used in various configurations. Rigid media coolers are usually more effective than flexible aspen pad coolers because they have more surface area per cubic volume of media. A value of 100 to 130 square feet of surface area per cubic foot is common for rigid wetted media (Palmer, 2002). As an added advantage rigid wetted media does not sag and reduce cooling performance. It is available in various thicknesses between 2 and 24 inches, but 12 inch thick media is common for school and commercial building air handling units. As shown in the section on evaporative media, the manufacturers ratings of effectiveness of rigid media is 75% to 95%, depending on the thickness and air velocity through the media. Rigid media is washable, and with good regular maintenance will last 7 to 10 years. One significant factor is that direct EAC systems cannot recirculate the room air. All the air must be exhausted or otherwise relieved from the building. July 9, 2016 Page 93 of 171

102 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Figure and show a basic process diagram for a direct evaporative cooler and a typical Wetted Aspen Pad Cooler, respectively. Figure 7.3.1: Direct evaporative cooling AC (Palmer, 2002) Figure 7.3.2: Typical Wetted Aspen Pad Cooler (Palmer, 2002) 7.4 Indirect Evaporative Cooler Working Process Indirect EAC's (IEAC's) have been in used not as long as direct EACs, but they have gained increased acceptance because of more cost effective manufacturing and improved performance. Since IEAC s use July 9, 2016 Page 94 of 171

103 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION heat exchangers they do not add moisture to the room air stream, thus they only provide sensible cooling. Figure shows a schematic view of an indirect evaporative cooling section located upstream of a direct rigid media evaporative cooler. IEAC heat exchangers transfer heat across sheets of plastic or metal configured to keep the two air streams from mixing. A smaller fan pulls the wetter air through the secondary side of the heat exchanger. This type of air to air heat exchanger is used to transfer heat from the primary space supply air stream to the evaporative cooled secondary air stream. Figure 7.4.1: Schematic of indirect evaporative cooler; (Palmer, 2002) Palmer (2002) suggested that following system configurations of IEACs: Air to air heat exchangers (see Figure 7.4.2): Direct and indirect evaporative air cooling components are commonly packaged together in the same cooling unit by the manufacturer. The main advantages of using both types of evaporative cooling systems are the lower supply air temperature, increased reliability and more available comfort hours. The direct EAC section will usually cool the air more than the indirect section due to the losses of the IEAC heat exchanger Combination IEAC and refrigerated systems (Figure 7.4.3): IEAC's are used in combination with refrigerated air systems, because they do not add moisture to the air stream. The advantage of this combination is that the reduction in air temperature due to the indirect evaporative section comes at a significant cost savings, since cooling capacity by an IEAC is much cheaper than provided by a conventional mechanical refrigerated system alone. The return air from the room can be recirculated past both the IEAC and the refrigerated coil, or the IEAC can be located to pre cool only the required outside air. July 9, 2016 Page 95 of 171

104 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Cooling tower "free cooling" (Figure 7.4.4): Indirect evaporative cooling system can be combined with a waterside economizer. This is often the case with a chiller system that uses evaporative cooling tower(s) for condenser heat rejection. When the outside air humidity level is sufficiently low, the evaporative cooling tower system works so well that the chillers can be kept off much of the time. The indirect system uses a water to air cooling coil in the room air stream. This evaporative cooled water in the cooling tower sump can get very cool, especially when the air is dry. Depending on the design parameters of the system, this water can be within 3 to 6 degrees of the wet bulb temperature. Figure 7.4.2: Air to Air Heat Exchanger Indirect + Direct EAC; (Palmer, 2002) The figure shows a section through an IEAC combined with a direct EAC. Figure 7.4.3: Combination Evaporative and Refrigerated Cooling System Schematic; (Palmer, 2002) July 9, 2016 Page 96 of 171

105 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Figure 7.4.4: Free Cooling Evaporative and Refrigerated Combination System; (Palmer, 2002) 7.5 Maisotsenko (M) Cycle Enhanced Indirect Evaporative Cooling A recent development in evaporative cooling is M cycle evaporative cooling, which can be utilized as a special and highly effective process of indirect evaporative cooling. The M cycle operates on the principles of sub wet bulb cooling. These properties make the M Cycle ideal for many evaporative cooling applications, including cooling towers, where steady cooled water temperatures can be delivered as the higher inlet temperatures increase the cooling capacity. In conventional direct evaporative cooling technology, the temperature of the cold water produced is limited to the outside air wet bulb temperature. As a result, the typical cooling towers are most suitable for extremely dry regions. Unlike conventional evaporative cooling technologies, heat transfer within the M Cycle is driven by a dew point temperature gradient, not wet bulb temperature. As such, only the absolute outside ambient humidity is important, not the temperature of incoming air or water. Film type packing has gained prominence in the evaporative cooling process vessel design because of its ability to expose greater water surface within a given packed volume. Its efficient heat transfer can give greater cooling capacity, requiring less fan energy. Using the conventional evaporative cooling process with any existing type of fill for cooling towers, it is possible to cool down fluid (air or water) to near the wet bulb temperature. However, with the M cycle, using the existing film fill and a change of air and water distributions cooled water temperatures approaching the dew point temperature of the working gas is possible. Gas Technology Institute (GTI) (201) presented a comparison of a conventional and M cycle evaporative cooling towers. Figure and compares the performance of a conventional and M cycle evaporative cooling tower, respectively. July 9, 2016 Page 97 of 171

106 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Flow diagram packing Psychrometric chart Subscripts: DB= dry bulb, WB= wet bulb, DP= dew point Figure 7.5.1: Forced draft conventional cooling tower, (GTI, 2010) annotated packing Psychrometric chart Subscripts: DB= dry bulb, WB= wet bulb, DP= dew point Figure 7.5.2: M cycle open cooling tower, (GTI, 2010) annotated July 9, 2016 Page 98 of 171

107 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Table presents the results of the GTI investigation of the conventional and M cycle evaporative cooling tower performance. The result presented in Table highlights the significant process improvement of the M cycle of providing cooling water for indirect evaporative cooling systems. Table 7.5.1: Comparison of process effectiveness of conventional versus M cycle evaporative cooling tower Process flow unit conventional cycle M cycle Water to be cooled [F] process air intake DB [F] process air intake WB [F] process air intake DP [F] Cooled water temperature [F] Dew point evaporative cooling processes, such as the M cycle, can be used to remove undesired heat from a wide range of thermal applications, including building air conditioning systems. Dew point evaporative cooling processes take advantage of the fact that when a parcel of air is sensibly cooled, the saturated water vapor pressure decreases, reducing its wet bulb temperature, thus increasing its evaporative cooling potential. Consequently, as the working fluid is humidified, the temperature of the evaporative cooling liquid that it is in contact with is also cooled to theoretically as low as the incoming air dew point temperature GTI (2010) emphasized that the operating efficiency of the M cycle cooling tower can be enhanced by reducing the dew point temperature of the incoming air. For example, reduced dew point temperatures can be achieved by desiccants, which can take place before and/or after the air actually enters the dry channel. Additionally, heat sources can be utilized to increase the system cooling capacity, in preheating the incoming working gas (e.g. ambient air) which improves the thermal performance of the M Cycle. This process characteristics is especially advantageous in high absolute humidity climates where the dew point temperature of ambient air is in the range of 60 F to 70 F. For even higher dew point temperatures, the effect of changing absolute humidity, with changing temperature, will become even larger. Similarly, use of a warmer evaporative liquid (e.g. hot water) will have the same effect as pre heating the incoming gas (e.g. air). 7.6 Absorption Chillers The process of Absorption refrigeration is not a new process. In 1858 the absorption cooling process was invented by the French scientist Ferdinand Carré where a mixture of water and sulphuric acid was used July 9, 2016 Page 99 of 171

108 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION as the absorbent (Arsenault, 2016). The low coefficient of performance (COP) of the absorption cooling cycle was the development of the alternative means to provide cooling capacity and refrigeration, the vapor compression cycle, which was invented by Wills Carrier in The vapor compression refrigeration cycle became the cooling process of choice, whereas the absorption chiller remains as an alternative where suitable quality and quantity of process waste heat is available. The chillers can be classified by the type of thermal energy source, whether they are single or double effect processes and whether they are directly or indirectly fired. There are a number of absorption chillers available, including single effect indirect fired (steam, hot water); double effect indirect fired (external heat source); and double effect direct fired (gas and/or oil burner). Single effect absorption chillers have single set of process vessels. Double effect absorption chillers have two sets which make them more energy efficient since process heat is not rejected to the environment, as in the case of the single effect absorption chiller, but is recycled internally. Absorption chillers have substantially reduced internal pressures to take advantage of the lower water boiling temperatures. Absorption chiller internal pressures can range from 0.1 atmosphere (atm) to below 0.01 atm (Sakraida, 2016). Figure show a basic absorption chiller process diagram. Figure : Basic flow diagram of a single effect absorption chiller The diagram indicates the mass flow of the absorption and regeneration process. Pressures and energy flows (e.g. heat rejection) are not all indicated. (EERE, 2016) The absorption cooling process works on the basis of a solution of lithium bromide and water, which absorbs water vapor and drives the process. As illustrated in Figure water in tank A evaporates and water vapor flows towards the absorber (tank B) which contains the absorbing solution. Evaporation of July 9, 2016 Page 100 of 171

109 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION the water in in the evaporator (tank A) causes the temperature to decrease. The achievable temperature in the evaporator depends on the evaporator pressure and the effectiveness of the evaporative process. Since mass enters the absorber (tank B) and dilutes the solution, fluid from the absorber is continuously pumped from tank B to the generator (tank C). In the generator, the solution is heated either directly by a natural gas burner, or indirectly by means of a steam coil. While the solution is heated, water evaporates and passes into the condenser (tank D). The regenerated and concentrated solution accumulates in the sump of tank C, and is then returned to the absorber (tank B). Here, it again absorbs water vapor that comes from the evaporator. Water vapor in the condenser (tank D) is cooled by a separate coil of pipe through which cooling water passes. The condensed water is returned to the evaporator (A). Figure shows the absorption cooling cycle by adding cooling and chilled water loops as well as pumps. Figure 7.6.2: Process diagram of a single effect absorption chiller with major process components added, (EERE, 2016) July 9, 2016 Page 101 of 171

110 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Figure 7.5.3: Single effect absorption refrigeration cycle; (Sakraida, 2016); annotated A more detailed process illustration is provided in Figure The following describes the main components of the process shown in Figure Evaporator vessel (1): Liquid refrigerant (chilled water loop water) is pumped through the chilled water tube bundle. The chilled water cycle provides cooling capacity to conditioned spaces by transporting chilled water out of the evaporator. Water which cycled inside the evaporator is sprayed on the tube bundle and evaporates. At the low evaporator pressure (as low as less than 0.01 atm), the water evaporates at approximately 38 F, removing heat energy from the chilled water. Most lithium bromide absorption chillers can only produce chilled water supply temperatures down to about 40 F (Sakraida,2016). Process water which does not evaporate drops down into a well and is recirculated. The vapor generated by evaporation flows to the absorber through a low pressure loss conduit. Absorber vessel (2): The water vapor entering the absorber from the evaporator passes through a falling film or spray of liquid lithium bromide solution. The lithium bromide solution absorbs the vaporized refrigerant and becomes diluted. Cooling water absorbs the heat of vapor absorption and rejects it the heat to the outside. A portion of the liquid lithium bromide solution in the well is recirculated to the falling film or spray by the absorber spray pump (A). The remaining portion of the liquid lithium bromide solution is transported by pump (B) to the generator/concentrator vessel. On its way to the generator/ concentrator vessel the solution passes through a heat exchanger (E) where process heat is recovered. July 9, 2016 Page 102 of 171

111 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Generator/concentrator vessel (3): In the generator/concentrator vessel the dilute lithium bromide solution is heated by steam or hot water. The temperature of the lithium bromide solution increases to a point where the water which is absorbed in the solution evaporates. The water vapor travels to the condenser. The concentrated lithium bromide solution flows down to the absorber and passes through the heat exchanger (C). Condenser vessel (4): In the condenser, the water vapor is condensing on a tube bundle through which cooling water flows, which rejects the heat of condensation to the outside. The condensate collects in a well from where it overflows to the evaporator. Expansion piping (5): The condensate (water) flows from the condenser through expansion piping to the well in the evaporator vessel. Since the pressure in the condenser (about 0.1 atm) is significantly higher than in the evaporator (less than 0.01 atm) the water flow experiences a drop in pressure and temperature. Excess vapor, which is compromising the low process pressure in the evaporator is expelled from the system by the purge pump. This completes the refrigerant cycle. Figure and illustrate the difference between single cycle and multiple cycle absorption refrigeration cycles. The energy transfer, e.g. the process flows inside the system and across the systems boundaries is illustrated in Figure Double effect absorption cycles capture some internal heat to provide part of the energy required in the generator to create the high pressure refrigerant vapor. Using the heat of absorption reduces the steam or natural gas requirements and boosts system efficiency. Figure shows the reuse of internal process heat energy by the following effect in the double effect absorption cycle. July 9, 2016 Page 103 of 171

112 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Figure 7.6.4: Indirect heated single effect absorption refrigeration cycle; (Thermax, 2016) The double effect chiller requires significantly more system components and controls, but the efficiency of a double effect absorption cycle can be significantly increased of the single cycle process. Sakraida (2016) reports that the coefficient of performance (COP) of the double effect is near double that of the single effect system, with: Hot water or steam single effect chiller to 0.75 COP Hot water or steam double effect chiller to 1.35 COP July 9, 2016 Page 104 of 171

113 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Figure 7.6.5: Indirect heated double effect absorption refrigeration cycle; (Thermax, 2016) SOLAIR (2009), an European Solar Cooling Initiative of 13 countries, suggests that many absorption chiller products are available in the market, however typical chilling capacities of absorption chillers are several hundred kw. For several years, the smallest machine available was a Japanese product with 35 kw capacity. The absorption chillers are typically supplied with district heat, waste heat or heat from cogeneration. The required heat source temperature is usually above 80 C for single effect machines and the coefficient of performance (COP) is in the range from 0.6 to 0.8. Double effect machines with two generator stages require driving temperature of above 140 C, but the COP s may achieve values up to 1.2. July 9, 2016 Page 105 of 171

114 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION In a recent move to facilitate thermal solar energy conversion, several single effect absorption chillers with capacities below 50 kw are now available. Therefore, solar air conditioning (SAC) systems can now manufacture smaller units and be more flexible. A newly developed model for small capacity chiller enables part load operation with reduced chilling power at a heat source temperature of 65 C and a COP of still approximately 0.7, which is quite promising in combination with a solar heat source. This shows that there is a high potential for performance improvements of absorption chillers. The new medium size and small size absorption chiller will be suitable to cover the cooling loads for building areas from 200 m² to 500 m². The European manufacturers are located in Germany, Austria, Spain, Sweden, Italy and Portugal. Some of the new absorption chiller models are still being tested in pilot installations. 7.7 Adsorption Chillers When compared to absorption refrigeration, adsorption chillers are a relatively recent thermal technology development. Adsorption refrigeration builds on the older absorption chiller process, yet has important operational differences, especially in regard to usability in solar energy conversion (SOLAIR, 2009). Adsorption chillers use solid sorption materials instead of liquid solutions to drive the refrigeration process. Commercially available adsorption systems use water as refrigerant and silica gel or zeolite as the sorbent material. The currently available adsorption chillers consist of two process vessel compartments, one evaporator and one condenser, which are used batch wise. Figure illustrates the basic process scheme of adsorption cooling. While regeneration of the sorbent occurs in the compartment 1, using hot water from the external heat source, such as solar generated hot water, the sorbent in compartment 2 adsorbs the water vapor which is generated in the evaporator. Compartment 2 has to be cooled in order to enable a continuous adsorption of water vapor. Due to the low pressure conditions in the evaporator vessel, the refrigerant (water) vaporized by taking up heat of from the chilled water loop and thereby producing cooling capacity. When sorption material in the adsorption compartment is saturated with water vapor to a certain degree, the chambers are switched over in their function. The available adsorption chiller technology can work with external heat sources at temperature of 80 C, where a coefficient of performance (COP) of about 0.6 is obtained. It is possible, however, to operate the systems with heat source temperatures as low of approx. 60 C. The capacities of the commercially available chillers range from 5.5 kw to 500 kw chilling power. Greenchiller (2016) reports significant benefits of using adsorption chillers, such as long life of the system and very low operating costs. The chiller s electrical load of just 0.4 KW, for the 1.8 ton model, is for the controls and two small pumps. One is a vacuum pump for non condensable gases and the other is a refrigerant liquid (water) pump that runs only while unloading. Other operational costs such as maintenance are also very low. July 9, 2016 Page 106 of 171

115 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Figure 7.7.1: Basic process scheme of adsorption cooling; (SOLAIR, 2009), annotated The German company InvenSor GMBH, which is one the leading European manufacturer of adsorption chillers, offers systems with a range of chilling capacities between 10 and 30 kw. The typical specification of InvenSor adsorption chillers lists a thermal coefficient of performance (COP) of 0.72, as the efficiency of the use of heat for cooling, and an outstanding maximum energy efficiency ratio (EER) of 33, which included the use of electric energy. Figure illustrates the working principle of the InvenSor adsorption chiller. The compartments (1) and (2) interconnect the evaporator and condenser vessels inside the machine. An illustrated step wise description is illustrated in Figure Figure depicts the Adsorption Chiller InvenSor Model LTC 10, with a rated capacity of 10 kw. The machine operates with external heat sources as low as 65 C. July 9, 2016 Page 107 of 171

116 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Process Step A: Compartment (1) serves as the receiving process vessel for the water vapor that is generated in the evaporator. Compartment (2) serves as the regeneration of the sorbent. (InvenSor, 2016), annotated Process Step B: Compartment (1) and (2) switch their functions. Compartment (1) now serves as the regeneration of the sorbent Compartment (2) now serves as the receiving process vessel for the water vapor that is generated in the evaporator. (InvenSor, 2016), annotated Definition of working fluid flow Figure 7.7.2: Two phased working process of InvenSor adsorption cooling, (InvenSor, 2016), annotated July 9, 2016 Page 108 of 171

117 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Process step 1: Evaporation and adsorption in one compartment Water in the evaporator vaporizes and creates chilling capacity that is transferred to the chilled water flow. Water vapor flow to compartment (2) where vapor is adopted by the solid sorbent. The heat of adsorption is rejected to the cooling water Process step 2: Evaporation and adsorption in two compartments When the adsorption capacity of the solid sorbent is diminishing compartment (2), the regenerated sorbent in compartment (1) is operating parallel to Compartment (2) as the absorber vessel. July 9, 2016 Page 109 of 171

118 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Process step 3: Evaporation and Regeneration in one compartment As the sorbent in compartment (2) becomes saturated with water vapor, the process function of compartment (2) changes to desorption, where the solid sorbent is regenerated. External heat is supplied to compartment (2) and water vapor is liberated from the solid sorbent. The liberated water vapor flows from compartment (2) to the condenser where the vapor condenses on cooling coils. Compartment (1) is still in adsorption mode. Process step 3: Evaporation and Regeneration in two compartments As the sorbent in compartment (1) becomes saturated with water vapor, the process function of compartment (1) changes to desorption, where the solid sorbent is regenerated. Both compartments are now in desorption, e.g. regeneration mode, The water vapor condensate flows back to the evaporator Figure 7.7.3: Process steps 1 through 4 of the InvenSor adsorption cooling process, source (InvenSor, 2016), graphics created by the author July 9, 2016 Page 110 of 171

119 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Figure 7.7.4: Adsorption Chiller InvenSor LTC 10, rated capacity of 10 kw, (InvenSor, 2016) The machine operates with external heat sources as low as 65 C. 7.8 Comparison between Absorption and Adsorption Cooling Technology Absorption refrigeration process has been used for a long time, while adsorption refrigeration is a relatively recent development of thermal cooling technology. Bargman (2016) discusses the main differences of the two thermal cooling processes and points out main design and operational characteristics. Bargman (2016) suggested that since there is no use of CFC's or ammonia, the thermally driven cooling process is environment friendly. Adsorption chillers are known for their robust construction, ease of installation, and in many cases, considered more advantageous than absorption chillers. Adsorption chillers are not prone to problems such as crystallization, corrosion, hazardous leaks, and the electricity consumption is very low. The typical cooling capacity of absorption and adsorption chiller are ranges between 4.5 KW to 5 MW and 5.5 to 500 KW, respectively. Perhaps the most significant difference between absorption and adsorption chillers is the expected useful life of the equipment. Absorption chillers have an average lifetime of 7 to 9 years due to corrosion problems, and the systems are high in complexity and maintenance time. On the other hand, adsorption chillers have a high chiller life expectancy of about 30 years The main advantages of absorption and adsorption chillers are indicated in Table July 9, 2016 Page 111 of 171

120 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Table 7.8.1: Comparison of Adsorption Chillers vs Absorption Chillers; Bargman (2016) FACTORS ADSORPTION CHILLERS ABSORPTION CHILLERS Corrosion Protection Not Required High Corrosion Protection Required Crystallization No Crystallization Very High Inhibitor Not Required Heavy Metal Inhibitors Life Expectancy Greater than 30 Years 7 to 9 Years Complexity Simple, Easy Mechanical Operation Complex, Chemical Operation Replacement Requirements Not Required Heat Exchangers, Boilers, Absorbent Replacement Required Temperature Down to 122 F Shut down at 180 F, Needs Back up Heater Chilled Water Output F 48 F or More 7.9 Magnetic Refrigeration as an Alternative to Vapor Compression Refrigeration While the man objective of this chapter is to present thermally operated air conditioning technology, the recent cooling technology development of magnetic refrigeration is included in this literature and technology review. Magnetic refrigeration has the potential of being a disruptive cooling technology since it offers several significant benefits over the conventional vapor compression refrigeration technology. Early magnetic cooling products suggest that these cooling systems are more energy efficient, smaller and less noisy than vapor compression cooling and operate with much less maintenance. In regard to energy efficient cooling and desiccant dehumidification systems, magnetic refrigeration could be viable candidate to provide sensible cooling and waste heat for desiccant regeneration. July 9, 2016 Page 112 of 171

121 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION The magnetic refrigeration process has been known for over seventy years to achieve near 0 K temperatures for small gas volumes by a process known as adiabatic demagnetization refrigeration. Magnetic refrigeration is based on the magnetocaloric effect which was first observed in the early 20 th century by various researchers. Magnetic refrigeration has long been considered one of the simplest ways to retrieve extremely low temperatures. However, the application of focus in current research has shifted from providing extremely cold temperature to more practical uses, including consumer based refrigerators. In the 1950's, several magnetic refrigerators operated at temperatures between 1 and 30 K. However, these were too inefficient to be used commercially and could only run for a couple of days (O Handley, 2004). Magnetic refrigeration has only very recently been developed to commercial product status. The main advantage of magnetic refrigeration is the avoidance of refrigerant agents in providing chilling capacity, which is eliminating several environmentally harmful effects of refrigerants, including global warming and the depletion of the ozone layer. When a magnetic field is applied to a magnetic material which has a magnetocaloric effect. A spin of the magnets lowers the entropy of the system since disorder has decreased. In order to compensate for the aligned spins, the atoms of the material begin to vibrate, and lower the entropy of the system again. In doing so, the temperature of the magnetocaloric material increases. The warming and cooling process can be compared, in principle, to a standard refrigerator which implements compressing and expanding gases for variations in heat exchange and surrounding temperature. Figure illustrates the basic working scheme of the magnetic refrigeration cycle which is composed of the process of magnetization and demagnetization, in which heat is discharged or absorbed in four steps. While there have been early prototypes of magnetic refrigeration in ranges that can be used in consumer products, high costs have been a main barrier to gaining market share from conventional vapor compression refrigeration. One significant breakthrough on the way to making magnetic refrigeration competitive with conventional systems is the discovery of the so called "giant" Magnetic cooling effect which made previously considered implausible advancements possible. Some materials were observed to have unusually large magnetocaloric effects, and this discovery made it much more feasible to develop a machine at lower costs (Kuba and Ota, 2006). The French company Cooltech is offering the first commercial magnetic refrigeration product (Cooltech, 2016). Figure shows Cooltech s proposed cooling application. Figure shows a detail view of the core of Cooltech s magnetic cooling systems. July 9, 2016 Page 113 of 171

122 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Figure 7.9.1: Basic working scheme of magnetic refrigeration (Cooltech, 2016) Figure 7.9.2: Cooltech s proposed cooling application for magnetic refrigeration system (Cooltech, 2016) July 9, 2016 Page 114 of 171

123 SECTION 7 ALTERNATIVE COOLING SYSTEMS INTEGRATED WITH DESICCANT DEHUMIDIFICATION Figure 7.9.3: The Magnetic Refrigeration System (M.R.S) 2015, (Cooltech, 2016) This device is the first commercially available refrigeration device based entirely on magnetocaloric effect. Benefits of magnetic cooling are reported as flows (Cooltech, 2016): Energy savings: Magnets are permanent and do not require an energy source to operate. Achievable energy savings could be as high as 50%. Cost reduction: Magnetic cooling provides lower operational costs due to optimized level of maintenance (low rotational speed, low pressure, no leaks and no hazardous chemicals ) and lower energy consumption. No Refrigerant Gas: The magnetic refrigeration system does not use any refrigerant gas and is not concerned with the regulations of harmful refrigerants. Safety: Magnetic refrigeration provides safety due to the absence of refrigerant gas. This protects the users from hazardous leaks. Measurements were performed on all Cooltech machines and showed magnetic emissions that surrounded Cooltech s devices were in safe ranges. Reduced noise and vibrations: The magnetic refrigeration system operates with a low noise level (less than 35db) and reduced vibrations. July 9, 2016 Page 115 of 171

124 SECTION 8 SOLID DESICCANT COOLING SYSTEMS SECTION 8 SOLID DESICCANT COOLING SYSTEMS This section presents the review of literature that discusses the integrated use of desiccant dehumidification and cooling systems. Kolewar, A. et al (2014) suggested that desiccant cooling can be either a substitute to the traditional vapor compression air conditioning technology or can expand conventional AC systems and make them more cost efficient, accessible, and cleaner. The authors pointed out that when powered by free energy sources (renewable energy sources) such as waste heat and solar energy, desiccant cooling can considerably reduce the operating costs and increase noticeably the user friendliness to the air conditioning for the populations in remote areas. The emphasis of this literature and technology review is on alternative and energy efficient cooling systems which are augmented by desiccant dehumidification. This section presents several system combinations of solid desiccant and various cooling technologies. 8.1 Solid Desiccant Cooling Systems using Different Cooling Devices Daou et al (2005) discussed the use of different cooling units for desiccant cooling systems. A desiccant cooling system comprises three main components, namely the regeneration heat source, the dehumidifier (desiccant material), and the cooling unit. The author pointed out that the cooling unit can be the evaporator of a conventional air conditioner, an evaporative cooler or a cold coil. The function of the cooling unit is handling of the sensible load while the desiccant removes the latent load. When a desiccant wheel system is implemented, a heat exchanger is generally used in tandem with it to preliminarily cool the dry and warm air stream before its further cooling by an evaporative cooler or a cold coil, etc. In this case, the heat exchanger together with the evaporator cooler or the cold coil constitutes the cooling unit. Fig shows a psychrometric representation of the different cooling process combinations, with the following main process steps: Desiccant dehumidification lowers humidity of the air and at the same time increases the temperature of the air through the heat of adsorption of water vapor on the solid desiccants. (state 1 to state 2) A heat wheel lowers the air temperature by rejecting sensible heat (state 2 to state 3), while no latent load is removed. A cooling coil can act in concert with the heat wheel and further lower the air dry bulb temperature (state 3 to state 4 ). Alternatively, an evaporative cooler can adiabatically lower the air dry bulb temperature and simultaneously increasing the humidity. (state 3 to state 4). The evaporative cooling process depicted in Figure is direct evaporative cooling where water is sprayed directly into the air stream. The drawback of the direct evaporative cooling process is the added July 9, 2016 Page 116 of 171

125 SECTION 8 SOLID DESICCANT COOLING SYSTEMS humidity in the process air, which can be avoided with indirect evaporative cooling. Figure depicts the psychrometric properties of both direct and indirect evaporative cooling. Figure 8.1.1: Psychrometric chart illustrating the principle of desiccant cooling, (Daou, 2005) annotated Indirect evaporative cooling uses another air stream, which is cooled evaporatively (called second air stream), as the heat sink to cool the primary air stream, which is the process air. Typically, a plate heat exchanger is used to transfer heat between the primary and secondary air stream. Direct evaporative cooling works best in climates with low sensible heat ratio (SHR), since increasing the indoor humidity does not compromise thermal comfort due to high relative humidity levels. The indirect evaporative cooling is the better choice in humid climate since it provides sensible heat removal without adding moisture into the process air. In regard to cooling efficiency, direct evaporative cooling is more efficient because of the thermal losses of the secondary air cooling the primary air through the heat exchangers. Daou (2005) suggests that the effectiveness of direct and indirect evaporative cooling can be as high as 90% and 70 to 80%, respectively. Both processes are effective cooling technologies and can reach a COP of 5 in dry climates. In humid climates their effectiveness decreases as the maximum temperature drop is the difference between dry bulb and wet bulb temperatures. Therefore, in order to use evaporative cooling in humid climates, dehumidification of the supply air is required upstream of the evaporative cooler, which removes a portion of the humidity in the process air and thus creates the conditions of an effective evaporative cooling process. Sultan et al (2015) presented the basic working principles of desiccant cooling in Figure 8.1.3, where the desiccant dehumidifier is removing latent heat and a host of cooling technologies is rejecting sensible load. In Figure the desiccant dehumidifier is situated upstream of the cooling device. This is only necessary, however, for evaporative cooling since it requires July 9, 2016 Page 117 of 171

126 SECTION 8 SOLID DESICCANT COOLING SYSTEMS dry air to function efficiently. Therefore, especially in humid climate, evaporative cooling process has to lower the humidity upstream of the evaporative cooling device. Figure 8.1.2: Psychometric of direct and indirect evaporative cooling, (Daou, 2005) annotated Figure Basic working principle of desiccant cooling (Sultan et al, 2015) Sultan et al (2015) presented a generic desiccant cooling process diagram (Figure 8.1.4) using solid desiccant dehumidification and a range of cooling technologies. The numbered process steps in Figure are as follows: July 9, 2016 Page 118 of 171

127 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Figure 8.1.4: Generic schematic of desiccant cooling systems The generic system can accommodate various cooling devices and regeneration heat sources (Sultan et al, 2015), annotated Process step 1 to 2: The outside air is dehumidified be the desiccant wheel (e.g. rotor) and the air temperature is increased due to the heat of adsorption. Process step 2 to 3: The dehumidified air is cooled at constant humidity using sensible heat exchanger (heat wheel); the heat sink for the heat of adsorption is the return air from the conditioned spaces. Process step 3 to 4: The air is cooled by one of the following cooling techniques; the air attains the required temperature for suitable thermal comfort zone conditions. The candidate cooling technologies include: (A) direct or indirect evaporative cooling (B) cold coil, e.g. chilled water, (C) M cycle cooling, (D) Evaporator of the VAC. July 9, 2016 Page 119 of 171

128 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Notes: In case of DEAC cooling the water vapor is added as an isenthalpic process as shown in process 3 4 whereas the rest of cooling is performed at constant humidity that can be noticed from process 3 5. The M cycle, which is an extremely efficient evaporative cooling process can be applied for cooling towers, especially in humid climate. While an M Cycle based cooling tower design is not yet commercially available, the principles of sub wet bulb cooling are getting considerable attention. These favorable properties make the M cycle attractive for many evaporative cooling applications. Unlike many evaporative cooling technologies, heat transfer within the M cycle is driven by the dew point temperature gradient, not wet bulb temperature. As such, only the absolute outside ambient humidity is important, not the temperature of incoming air or water. (Gas Technology Institute, 2010) Process step 6 to 7: Sensible heat is transferred from the supply to the exhaust air stream. Therefore, the temperature of the airstream of the exhaust air stream is increased. Process step 7 to 8: Heat is added for the regeneration of the solid desiccant. Process step 8 to 9: Desiccant is regenerated (e.g. activated) by heating the desiccant. Sultan (2013) suggested that the available solid desiccant cooling systems can be categorized as follows: The type of the solid desiccant system adsorbent bed or rotating wheel type, Number of stages single stage or multi stage dehumidification type, Type of rotor single rotor or multi rotor type, Integration into conventional HVAC systems hybrid system assisted by VAC, and (Heat source used for regeneration electricity/gas/solar/waste energy. TRANE (2005) discussed different system configurations of desiccant wheel and conventional cooling devices. The three system configurations were wheel upstream of cooling coil, wheel downstream of cooling coil and series regeneration. Refer to Figure July 9, 2016 Page 120 of 171

129 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Figure 8.1.4: Typical desiccant isotherms (TRANE, 2005) The case wheel upstream of cooling coil is often used in parallel arrangements of desiccant dehumidification wheels with Type I or Type II desiccants. The desiccant wheel rotates between two discrete air streams. The regeneration air stream may be the building exhaust or a second outdoor air stream that is used exclusively to regenerate the desiccant material, where a suitable heat source provides the heat for regeneration. As a result, water vapor transfers from the higher RH process air (OA) to the lower RH regeneration air (RG ). Figure and illustrates this system configuration and the psychrometric state points, respectively. In the illustration, a second, dedicated, outdoor air stream regenerates the desiccant. Figure shows the desiccant wheel removing moisture from the process air stream but for every Btu of latent heat (moisture) removed, it adds more than one Btu of sensible heat. Therefore, air leaving the process side of the wheel (OA ) is dry (at a low dew point) but hot (145 F DB in the case depicted). Due to the costs of regeneration and re cooling, traditional desiccant wheels typically are used only when the required process air dew point can t be achieved with standard mechanical equipment. The cost penalty becomes larger if the price of energy for wheel regeneration rises.) The case wheel downstream of cooling coil is depicted in Figure This wheel configuration applies more favorably to the operating principles of cooling coils and desiccants. In this wheel configuration, the process air (OA) first passes through a DX or chilled water cooling coil, where it s cooled and dehumidified. The cool, saturated air (CA) passes through the desiccant wheel, which adsorbs moisture from the high RH air thereby lowering the dew point but raising the dry bulb temperature. The resulting conditioned air (CA ) is dry and warm but not as hot as in the wheel upstream configuration (Figure 8.1.5) described earlier. Water vapor transfers from the desiccant to the regeneration air (RG ) as the wheel rotates into the regeneration air stream. July 9, 2016 Page 121 of 171

130 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Compared with the wheel upstream arrangement, the wheel downstream configuration can dehumidify the process air to an equally low dew point, but requires less or perhaps non re cooling because the resulting dry bulb temperature isn t as hot. This system configuration still requires a separate regeneration air stream, and that air typically must be heated to dry out the desiccant. Figure 8.1.5: Desiccant dehumidification wheel upstream of cooling coil, parallel regeneration, (TRANE, 2005) Figure 8.1.6: Desiccant dehumidification wheel upstream of cooling coil, parallel regeneration, (TRANE,2005) The case desiccant wheel in series with the cooling coil is depicted in Figure This system configuration has the regeneration side of the wheel upstream of the cooling coil and the process side down stream of the coil. Moisture transfer occurs within a single air stream, where the desiccant wheel adsorbs water vapor from the process air downstream of the cooling coil and then releases the collected moisture upstream of that coil, allowing the cooling coil to remove it through condensation. A separate, regeneration air stream is not needed. This wheel placement requires a Type 3 desiccant selected specifically for this application, since desiccant must adsorb water vapor when the relative humidity of July 9, 2016 Page 122 of 171

131 SECTION 8 SOLID DESICCANT COOLING SYSTEMS the air is high. When the RH is below 80 %, moisture adsorption ability drops significantly. Therefore, adsorption is more driven by the Type 3 desiccant s ability to regenerate at low temperatures than by hot regeneration air, often without supplemental heat. A preheat coil can be added upstream of the regeneration side of the wheel for applications that require even drier air. Figure 8.1.7: Desiccant dehumidification wheel downstream of cooling coil, parallel regeneration, (TRANE, 2005) Figure 8.1.6: Desiccant dehumidification wheel downstream of cooling coil, parallel regeneration, (TRANE, 2005) Adding the series configuration of the desiccant wheel changes the dehumidification performance of the traditional cooling coil, trading sensible capacity for more latent capacity. The latent (dehumidification) capacity of the cooling coil increases while the total cooling capacity (enthalpy change across the coil) remains the same. July 9, 2016 Page 123 of 171

132 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Figure 8.1.9: Desiccant dehumidification wheel desiccant wheel in series with the cooling coil, (TRANE,2005) 8.2 Solid Desiccant Cooling with Evaporative Cooling In an early study Collier et al (1981) presented three basic process modes or cycles. These basic modes and cycles have been used in many desiccant cooling applications: Ventilation mode Recirculation mode Dunkle cycle The ventilation mode is illustrated in Figure In this mode, room air is used to regenerate the dehumidifier bed and outdoor air is cooled. The air leaving the room (6) is evaporatively cooled (7) and used as the cold sink for the dried room return air. The room exit air is heated during the heat exchange. (8). The return air is then further heated by an external source of energy (Q) for desiccant regeneration (9). Drying the desiccant cools and humidifies the air (10). Ambient air (1) is dried by the desiccant (2) to supply room make up air and cooled by heat exchanger (3) with room exit air. The dried and cooled ambient air is further chilled by evaporative cooling (4). Just before point (5), remix air is introduced. This scheme mixes evaporative cooled room dried room make up air in order to control the sensible heat factor. July 9, 2016 Page 124 of 171

133 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Figure 8.2.1: Ventilation mode (Collier et al, 1981), annotated The locations in the block diagram (a) correspond with state points in the psychrometric chart (b) The recirculation mode shown in Figure This mode uses the same components as the ventilation mode except that room air is constantly reconditioned in a closed loop and outdoor air is used for regeneration. Thermodynamically, this cycle has the advantage of processing air with greater cooling capacity. The disadvantage is having a warmer cold sink temperature than the ventilation mode. Selecting either mode is a trade off which depends upon room and ambient conditions. This trade off will be discussed in greater detail later in this literature review. Another important difference between this mode and the ventilation mode is that there is no direct fresh air supply, whereas the ventilation mode uses all fresh air. For the recirculation mode, as well as for most vapor compression cycles, fresh air to the building space is supplied by normal infiltration. In an era of tighter buildings, this fact is a disadvantage, since air intake has to be controlled. The Dunkle cycle shown in Figure This process cycle combines the thermodynamic advantages of both the ventilation and recirculation modes. The cycle uses the advantage of processing the higher cooling availability room air as in the recirculation mode, while retaining the lower cold sink temperature of the ventilation mode. This advantage in performance comes at the cost of increased complexity and an additional sensible heat exchanger. As with the recirculation mode, the lack of controlled fresh air to the building space is a disadvantage with tighter building envelopes. July 9, 2016 Page 125 of 171

134 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Figure 8.2.2: Recirculation mode; (Collier et al, 1981), annotated The locations in the block diagram (a) correspond with state points in the psychrometric chart (b) Pesaran and Hoo (1993) discussed desiccant cooling system where air is passed through a desiccant dehumidifier in order to increase the performance of the downstream evaporative cooling. Figure shows the desiccant cooling ventilation cycle which used a rotary desiccant dehumidifier, a heat exchanger, two direct evaporative coolers, a desiccant regeneration heater, and ancillary equipment such as fans and pumps. In this cycle outside air is dried in the dehumidifier and then cooled by evaporative coolers and supplied to the conditioned space. July 9, 2016 Page 126 of 171

135 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Figure 8.2.3: Dunkle cycle; (Collier et al, 1981), annotated The locations in the block diagram (a) correspond with state points in the psychrometric chart (b) Figure 8.2.4: Evaporative cooling system, (Pesaran and Hoo, 1993), annotated July 9, 2016 Page 127 of 171

136 SECTION 8 SOLID DESICCANT COOLING SYSTEMS The working process of the system depicted in Figure is described in the following with annotations referring to important process steps. Drying of the outside air (1): Outside air is dried by a desiccant wheel which is reactivated by the reheated exhaust air (process step (8)). Downstream of the desiccant wheel the air is lower in humidity but warmer than the outside air, due to the heat of adsorption of the solid desiccants. Cooling of the outside air (2): The conditioned outside air flows through an air to air heat exchanger, such as a heat wheel, where heat is transferred from the outside air flow to the return air. First stage of evaporative cooling (3): The outside air passes through a direct evaporative cooler where sensible heat is adiabatically removed from the air by the cooling effect of water droplets vaporizing in the air stream. The resulting colder and moister supply air is provided to the conditioned spaces. Conditioned space (4): In the conditioned space sensible and latent load is added to the air, which is vented out of the conditioned space. Second stage of evaporative cooling (5): The return air passes through a direct evaporative cooler where sensible heat is adiabatically removed from the air. This process results in a colder and moister air downstream of the second stage evaporative cooler. Warming of the return air (6): The air to air heat exchanger is cooled by the return air, thereby adding sensible heat to the outside air. Regeneration heat supply (7: Thermal heat is provided to a portion of the return air thereby heating up the air flow that passes through the desiccant wheel. A remaining portion of the return air bypasses the desiccant wheel which provides for a precise heating of the desiccant wheel and avoids over heating of return air and the desiccants. Drying of the desiccant wheel (8): The heated air flow through the desiccant wheel dries the desiccant matric of the wheel and provides adsorption capacity for the outside air (1) as the reactivated portion of the desiccant wheel is available for the outside air passing through the wheel matrix. The advantage of the desiccant cooling cycle depicted in Figure is that the system works in climate zones with high humidity levels. In humid higher climate zones evaporative cooling becomes less effective as a higher wet point of the outside air limits the removal of sensible heat. In addition, in humid climate zones humidity has to be removed from the outside air as it is supplied to the conditioned spaces. The drying of the outside air flow upstream of the evaporative cooler lowers the humidity level and therefore enables more effective evaporative cooling. July 9, 2016 Page 128 of 171

137 SECTION 8 SOLID DESICCANT COOLING SYSTEMS The level of dehumidification upstream of the evaporative cooling has to be higher in the case of indirect evaporative cooling than for direct evaporative cooling systems. The indirect evaporative cooling process does not add humidity to the air, which is an advantage. However, since the effectiveness of indirect evaporative cooling is lower than the direct process only detailed design analysis can ascertain whether direct or indirect evaporative cooling cycle is the best match for the desiccant system. 8.3 Solid Desiccant Cooling to Increase Conventional AC System Pesaran (1993) presented an application of an indirect evaporative coolers working in conjunction with two types of solid desiccant systems. The entire system was implemented to augment the cooling capacity of a conventional VAV system. The author presented a design strategy to add cooling capacity and enhance ventilation rates to the conditioned space. Rather than adding approximately 500 tons of conventional chiller capacity to the existing building. the author proposed to add a desiccant cooling system for pre conditioning the supply air. The proposed desiccant cooling pre conditioning process is described hereafter. Figure depicts the existing AC system which was undersized for the actual load. Figure Schematic of an Existing All Air VAV System of an Office Building; (Pesaran, 1993) The figure shows the existing AC system which was undersized to provide 15.0 and 2.5 BTUH/sqft of sensible and latent load. In addition, the existing 0.1 CFM/sqft ventilation rate was deemed as not sufficient. July 9, 2016 Page 129 of 171

138 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Figure shows the proposed addition of a pre conditioning outside air supply to the existing AC system. The added desiccant cooling system included two solid desiccant systems and an indirect evaporative cooler. Outside air enters the system and the pressure is increased in order to overcome pressure losses of the air as it passes through two desiccant wheels and one indirect evaporative cooler. The pressurized outside air supply passes through an enthalpy wheel where humidity and heat is transferred from the outside air to the return air (e.g. exhaust air). Downstream of the desiccant wheel the air flow is separated into two flow paths. A portion of the air flow passes through desiccant dehumidifier (e.g. desiccant wheel) while the remaining portion bypasses the desiccant wheel. The humidity of the air flowing through the desiccant wheel is transfers to the solid desiccants of the wheel. The solid desiccant material of the desiccant wheel is activated (e.g. regenerated) by a separate outside air flow that is heated by a natural gas furnace. An air to air heat exchanger pre heats the outside air upstream of the natural gas furnace. Downstream of the desiccant wheel an indirect evaporative cooler provide sensible heat removal to the air flow which serves as the preconditioned outdoor air supply. The system is depicted in Figure It provides an example of how desiccant cooling can enhance existing conventional AC systems in retrofit applications. In the early 1990s a desiccant cooling system that enhances conventional HVAC installations was first introduced and sold by ICC Technologies, Inc. The system used a desiccant drier, heat exchanger, and evaporative coolers in series to dry and then cool the air. The systems were marketed as alternatives or supplements to more traditional commercial air conditioning units, largely in light commercial and supermarket systems. The type of desiccant material used in the system was a so called Engelhard Titanium Silicate (ETS), which is suitable for lowtemperature applications. One form of ETS desiccant could be regenerated at less than 140 F, so waste heat from condensing units of electric chillers could be used. Figure shows ICC's DESI/AIR System which with a desiccant wheel, heat exchanger wheel, and evaporative cooling pads, and can be used as a preconditioner for ventilation air. (ICC Systems 1993). July 9, 2016 Page 130 of 171

139 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Figure 8.3.2: Schematic of an Existing All Air VAV System of an Office Building; (Pesaran, 1993), annotated July 9, 2016 Page 131 of 171

140 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Figure 8.3.3: ICC Technologies DESI/Air System (ICC Technology, 1993) July 9, 2016 Page 132 of 171

141 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Daou et al (2005) suggested that desiccant cooling in conjunction with conventional cooling coils can be successfully used in radiant cooling applications. The author pointed out several generic benefits of radiant systems. Such radiant cooling systems include metal ceiling panels, chilled beams, and tube embedded ceilings, wall and floors (e.g. also called core cooling). Benefits suggested by the author included energy savings in the range of 27 to 37%, due to a significantly reduced ventilation rate, the lower energy demand of pumping chiller water in lieu of air and the decoupling of ventilation to sensible heat rejection. The author also stressed benefits of radiant cooling on occupant comfort. The block diagram and configuration of the proposed system is depicted in Figure (a) and (b), respectively. The system included desiccant a wheel and a heat wheel which dehumidify and provide sensible cooling to the incoming air. The supply air had to be dried in order to prevent condensation on the radiant surfaces and a resulting reduction in comfort. Chilled water discharge from the cold coil was circulated through the chilled ceiling system. The incoming air was dehumidified by a desiccant wheel and pre cooled by heat wheel before been cooled further by the cold coil to the supply temperature. The author suggested that sensible load is entirely handled by the chilled ceiling radiant cooler while the latent load is extracted by the desiccant. The use of the desiccant wheel is of very importance for comfort point of view if this system is to be used in a hot and humid climatic zone. Daou (2005) stated several advantages of chilled ceiling itself and the integration of the decoupled desiccant assisted ventilation and sensible heat rejection as follows: The sensible and latent loads are handled independently, realizing the so called decoupled cooling approach. The absence of reheating allows great energy saving, the author suggested about 44% of primary energy savings in comparison with the vapor compression A single water chiller is used since water is circulated in series in the cold coil and the cooling panel. The temperature of chilled water can be as high as 17 C, which increases the COP of the chiller The system can be driven by low grade energy, especially the free energy such as solar energy and waste heat. The system delivers a draft and noise free conditioned air. July 9, 2016 Page 133 of 171

142 SECTION 8 SOLID DESICCANT COOLING SYSTEMS (a) Block diagram of the desiccant cooling system The desiccant cooling system provides latent heat removal and ventilation (b) Sensible heat rejection from the conditioned space through a chilled ceiling The discharge of the cooling coil is cold enough to serve as the chilled water supply of the chilled ceiling Figure 8.3.4: Desiccant cooling integrated into radiant cooling systems (Daou, 2005) July 9, 2016 Page 134 of 171

143 SECTION 8 SOLID DESICCANT COOLING SYSTEMS 8.4 Solid Desiccant Colling with Solar Heat Parmar and Hindoliya (2011) suggested that solar energy is a suitable option for regeneration of desiccant cooling system and thus saves the regeneration energy input. The authors suggest the use of evaporative cooling techniques in humid climate, with a solid desiccant as the dehumidifier. Figure illustrates the basic approach of using solar heat for the activation of the desiccant material. Figure 8.4.1: Basic approach of using solar heat for the activation of the desiccant material. (Parmar and Hindoliya, 2011) This literature search revealed an increasing interest in the use of solar heat for desiccant cooling. Three examples of solar supported desiccant cooling applications are presented hereafter. Sultan et al (2015) proposed hybrid solar thermal and electric driven desiccant cooling system that is shown in Figure During night time the system uses stored thermal energy that was harvested during the day. This electric back up system made the system cost effective by using relatively lower price off peak electricity and prevented uncertainty by preventing total reliance on solar energy. The system was tested and the performance was satisfactory. The outdoor and indoor conditions were set to 30 C and 60% RH and 26 C and 55% RH, respectively. Effect of desiccant regeneration temperature on the system performance, measured in COP is shown in Table The results suggest that the dehumidification performance by the system was improved with the increase in regeneration temperature, but this came with the penalty of increased supply air temperatures. The system COP decreased from 0.44 to 0.35 with the increase in regeneration temperature from 60 to 75 C. The authors concluded that the thermal heat supplied to the desiccant AC system is not exactly proportional to the actual cooling effect and that an optimum regeneration air temperature is required to obtain efficient performance of a desiccant AC system. July 9, 2016 Page 135 of 171

144 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Figure 8.4.2: Schematic diagram of solar thermal and electric driven desiccant AC system (Sultan, 2015) Table 8.4.1: Response of COP to regeneration temperature, for system depicted in Figure 8.4.2; (Sultan, 2015) July 9, 2016 Page 136 of 171

145 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Archibald (2001) presented a solar hybrid desiccant cooling system which is depicted in Figure The system includes a solar thermal system, a desiccant evaporative cooler, one additional direct evaporative cooler, one indirect evaporative cooler and one hot water heating system using the waste heat from the desiccant regeneration. The psychometrics of the integrated cycle are shown in Figure The cycle shown does not use recovered heat from the heat exchanger for desiccant regeneration. Instead, the excess solar heated air from the solar thermal tile roof is used as the primary heating source for desiccant regeneration : Solar roof integrated desiccant cooling system schematic, (Archibald, 2001) 8.4.4: Solar roof integrated desiccant cooling system psychometrics (Archibald, 2001) July 9, 2016 Page 137 of 171

146 SECTION 8 SOLID DESICCANT COOLING SYSTEMS Sultan (2015) discussed a solar driven two stage desiccant cooling system (TSDC), assisted by a VAC system to condition an office building (see Figure 8.4.5). The desiccant cooling unit had a design cooling capacity of 10kW, a VAC unit with nominal cooling capacity of 20 kw and a flat plate solar collector array of 90 m 2. Internal heat exchangers were used to reduce the adsorption heat in the process air flow. The system proved that it can provide sufficient cooling and dehumidification even at high temperatures and high humidity level, such as outside conditions of 35 C and a humidity ratio of g/kg. The two stage dehumidification provided higher air flow rate as compared to single stage at a comparable dehumidification level. Another benefit of the multi stage dehumidification is that the desiccant regeneration temperature can be decreased, which increases the overall COP. The fact that lower regeneration temperatures are used makes the system more suitable for utilizing low grade waste heat and solar energy for desiccant regeneration. Figure 8.4.5: Schematic diagram of solar driven two stage hybrid DAC system, (Sultan, 2015) July 9, 2016 Page 138 of 171

147 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Liquid desiccants have been in use in various industrial applications. Their use in conjunction with building cooling applications, however, has been very limited, since the solid desiccant dehumidifiers, such as the rotary desiccant devices, have become HVAC industry standards. ES Magazine (2010) suggested that various companies had proposed liquid desiccant dehumidification for HVAC applications in the past. Although these systems had produced very impressive EER and COP, these systems were small and few were installed, therefore these systems never overcame the critical mass for larger scale adoption. The authors stated, however, that there is a renewed interest in liquid desiccant technology for HVAC systems and these new liquids desiccant systems are showing very encouraging performances characteristics. This section presents several of recent liquid desiccant cooling applications. 9.1 General System Configuration of Liquid Desiccant Cooling Systems Sahlot and Riffat (2016) suggested advantages and disadvantages of liquid desiccant cooling systems as follows: Advantages (1) Low pressure drop across the liquid desiccant system makes them suitable to use with low regeneration temperatures. (2) The ability to pump liquid desiccants makes the entire unit small and compact. (3) Liquid desiccants can be stored and used when heat source is not available. This is advantageous when heat source is only intermittently available for desiccant regeneration. Disadvantages (1) Liquid desiccants like lithium chloride, lithium bromide and all other salts are corrosive and can damage the desiccant system. (2) Any carry over of liquid desiccant along with supply air stream can cause harm to the health of the occupants. (3) In order to handle large volume of liquid desiccant, large pumps are required, which draws a large amount of power. (4) Desiccants of aqueous salts also face the problem of crystallization. In the basic liquid desiccant dehumidification process moisture from the process air is removed in the dehumidification or absorber unit, where the desiccant absorbs the water vapor from the process air due to a difference in vapor pressure between the desiccant solution and process air. While water vapor is absorbed, the temperature of liquid desiccant rises due to heat of absorption and condensation being liberated. The diluted desiccant is pumped back to the regenerator for regeneration. Downstream of the July 9, 2016 Page 139 of 171

148 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS regeneration the diluted solution passes through a plate heat exchanger for heat recovery and a heating coil, where its temperature of the desiccant solution is raised. In the regenerator, the hot diluted solution is exposed to regenerative air stream, and moisture is transferred from the weak solution to air due to the difference in vapor pressure. Lowenstein (1998) discussed the evolution of industrial liquid desiccant dehumidifier and suggested that liquid desiccant devices used in conjunction with HVAC would have the same basic working scheme as industrial liquid desiccant systems. The author pointed out that the liquid desiccant would have to be adapted to the level of the target dew point and other issues, such as carry over and desiccant isotherms. Figure shows a simple industrial desiccant system configuration. Figure : Simple industrial desiccant system configuration (Lowenstein, 1998) Figure shows a generic process flow diagram of a liquid desiccant system comprised of dehumidification and regeneration units. Figure depicts the psychrometric performance of the liquid desiccant process, showing the state points (A) through (D) corresponding to the locations in process diagram in Figure Figure indicates that the vapor pressure of the liquid desiccant solution decreases with increasing concentration. July 9, 2016 Page 140 of 171

149 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Figure : Generic liquid desiccant process diagram (Sahlot and Riffat,2016) Figure : Psychrometric process (Sahlot and Riffat,2016) The process states (A) through (D) correspond to the locations A through D in the process diagram The process vessels for liquid desiccant dehumidification are classified with regard to internal geometry, such as finned tube surface, spray and packed tower and their heat extraction process, adiabatic and internally cooled dehumidifiers. Figure (a) and (b) shows an adiabatic and internally cooled dehumidifier, respectively. Sahlot and Riffat (2016) discuss advantages of multiple stage dehumidification and suggest that the effectiveness of the entire system can be significantly improved by adding successive effects. The authors suggest that the successive stages can be optimized not only in size but also be using different liquid desiccant systems. July 9, 2016 Page 141 of 171

150 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS (A) Adiabatic dehumidifier: Process heat removal occurs through air passing through the tower. Higher flow rates have disadvantages. (B) Internally cooled dehumidifier: Process heat removal occurs through cooling device inside the tower. Figure 9.1.4: Liquid desiccant dehumidification type of heat removal (Sahlot and Riffat,2016) Kassem (2013) investigated three common flow patterns in an adiabatic dehumidifier: parallel flow, cross flow and counter flow, (see Figure 9.1.5) through simulations and experimental verification. Flow patterns of liquid desiccant and contact air determine the contact area and the process of interaction between desiccant and airflow. The author concluded that the performance of counter flow is best followed by cross flow, while the performance of parallel flow is not optimum. Figure 9.1.5: Flow patterns of air and liquid desiccant in dehumidification unit, (Kassem, 2013) Figure illustrates basic process characteristic of liquid desiccant cooling. A liquid desiccant system used in cooling applications has several main components namely the dehumidifier (also absorber or conditioner), the regenerator (also desorber), the regeneration heat source and the cooling unit. Packed July 9, 2016 Page 142 of 171

151 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS columns or beds are the most frequently used technology for these components. The dehumidification process (sorption) is carried out by spraying the liquid desiccant into the process air to absorb the moisture out of the air. The liquid desiccant falls to a sump and is pumped back to the top of the process vessel where nozzles to be sprayed back to the air and onto the packing. As the desiccant solution becomes diluted as a result of absorbing moisture out of the air stream the solution must be regenerated. Liquid desiccant material continuously circulated through the regenerator where an external heat source drives desorption. Also, the desiccant solution is heated before any connection with air to raise the partial pressure of the desiccant above that of the of the air. Thus, the moisture content of desiccant is transferred to the regeneration air (desorption), which leaves the regenerator as hot and humid air. After the regeneration process, the liquid desiccant solution becomes concentrated and free from moisture content and must be cooled to lower the vapor pressure. Before being sprayed into the process air again to complete the cycle, the liquid desiccant solution is cooled by a chiller or cooling tower to adjust the temperature to the desired level (cooling). Sensible cooling is provided via employing traditional vapor compression and vapor absorption, direct or indirect evaporative coolers. (a) Schematic of a desiccant cooling air conditioning (b) Moisture removal process by desiccant Figure 9.1.6: Basic process characteristics of liquid desiccant cooling, (Abdulrahman, 2013) July 9, 2016 Page 143 of 171

152 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS 9.2 Liquid Desiccant Systems to Enhance Conventional AC System Unlike solid desiccants, liquid desiccant dehumidification systems have not become a standard design component to augment conventional AC systems using vapor compression cooling. The literature reviewed presented some liquid desiccant dehumidification applications, which will be presented in the following. Daou (2006) presented a vapor compression air conditioning which is aided by a liquid desiccant cooling system. Figure shows the system process diagram. Cool strong desiccant solution is sprayed onto the top of the dehumidifier. Since desiccant solution vapor pressure is less than that of the air vapor pressure, water vapor migrates from the air stream to the desiccant solution and condenses therein. Consequently, the process heat liberated increase in the temperature of the solution. The process air stream is slightly cooled down due to its contact with the cold desiccant solution. The dehumidified and warm process air stream then passes successively through the evaporative cooler and the evaporator of the traditional refrigerant vapor compression AC unit, before the air is delivered to the conditioned space. The diluted desiccant solution, which exits the dehumidifier, is circulated through the regenerator where it is heated. The moisture absorbed in the dehumidifier unit is now lost to the scavenger air stream. The hot and strong desiccant solution is thereafter cooled down in the pre cooler and then cooled further in the heat exchanger (HX) before being ready again to dehumidify the incoming process air. Because the latent load is handled independently by the desiccant dehumidifier, ventilation air does not have to be cooled below its dew point. The temperature of evaporation can thus be lifted up to 15 C from its generally level of 5 C for cooling based dehumidification in the traditional vapor compression system. This saves energy since reheating is not required and the increase in cooling coil temperature increases the system s coefficient of performance (COP). AIL (2016) presented an innovative and highly effective integration of liquid desiccant into a vapor compression cycle. This system is the Liquid Desiccant Direct Expansion air conditioner (LDDX), which is especially suited for hot and moist outdoor conditions. The system is described as an efficient, high latent cooling system that can adjust its Sensible Heat Ratio (SHR) between 0.35 to 0.75 and still retains a high overall efficiency with minimal losses in its rated total cooling capacity. The electric efficiency stays close to its projected value of 11.4 EER. AIL stated that the LDDX system is the first embodiment of a compressor based liquid desiccant air conditioner in which a solution of lithium chloride floods the surfaces of the air conditioner s evaporator and condenser providing direct contact between the desiccant and the air flowing through these two coils. July 9, 2016 Page 144 of 171

153 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Fig : Schematic of liquid desiccant aided vapor compression air conditioning, (Daou, 2006) Figure (a) and (b) shows a 3D depiction of the LDDX system and a liquid desiccant flow diagram, respectively. Figure (a) shows a strong desiccant applied to the top of a wicking fin evaporator and weak desiccant to the top of a wicking fin condenser. The strong desiccant absorbs water vapor from the process air flowing through the evaporator. The weak desiccant flowing off the evaporator is warmed in the interchange heat exchanger before it is delivered to the top of the condenser. The heat rejected in the condenser further warms the weak desiccant which then desorbs water to the cooling air that flows through the condenser. The warm, strong desiccant that flows off the condenser is cooled in the interchange heat exchanger before it is supplied to the evaporator. The essential characteristic of the LDDX is its ability to supply cool, unsaturated air without reheat. Whereas a conventional DX air conditioner without reheat might have a sensible heat ratio (SHR) of 0.75, the LDDX can have an SHR as low as The SHR adjustment is achieved by varying the amount of liquid desiccant that is recirculated over the evaporator. Figure (b) illustrates a diverting valve which controls the desiccant flow from the evaporator to the condenser. The essential characteristic of the LDDX is its ability to supply cool, unsaturated air without reheat and to avoid overcooling when the latent load is high, e.g. with low SHR. In applications where high dew point air is processed, for example Dedicated Outdoor Air Systems operating in humid locales, the most efficient cooling system would be a two stage system, with a first stage conventional DX air conditioner July 9, 2016 Page 145 of 171

154 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS that delivers saturated, partly cooled air to a second stage LDDX, which dehumidifies the air. Such a two stage system in shown in Figure (a) 3D rendering of the LDDX system Showing the refrigeration and liquid desiccant flow system (b) Flow system of the liquid desiccant loop shoeing the diverting valve for SHR adjustment Figure 9.2.2: Description of the single stage LDDX system (AIL, 2016) Figure 9.2.3: Description of a two stage LDDX system (AIL, 2016) July 9, 2016 Page 146 of 171

155 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Sahlot (2016) discussed a liquid desiccant system which was combined with a vapor compression and demonstrates that such an integrated system can be highly efficient in space cooling with possible savings in primary energy of up to 50%. The COP of the system tested was 3.8, with a total cooling capacity of 6.15 kw, using 2.6 kw VCS. Lithium chloride was the liquid desiccant solution. Figure shows the schematic diagram. Strong solution from the tank is pumped and sprayed uniformly over the evaporator surface area. Process air is passed through the evaporator. The evaporator and desiccant are engaged in simultaneously cooling and dehumidifying the process air while the diluted desiccant solution is collected in the weak solution tank. The diluted solution is then pumped to absorb heat from the heat exchanger that uses the waste heat rejected from the condenser of the vapor compression system to preheat the diluted solution. A heating coil in the regenerator tank provides additional heat for regeneration. Figure 9.2.4: A schematic of hybrid desiccant assisted air conditioner, (Sahlot, 2016) Kassel (2013) demonstrated a simple integration of a liquid desiccant dehumidifier into a vapor compression air conditioning cycle. The hybrid system is depicted in Figure July 9, 2016 Page 147 of 171

156 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Such a system can succeed in obtaining high thermal performance and energy savings since waste heat is recovered internally and used for the desiccant regenerator to recover waste heat. Although the performance of the regenerator is not as good as the regenerator using the heating desiccant solution, this system uses free heat from waste hot air. In order to overcome intrinsic disadvantages of using waste heat, the author discusses the importance of identifying the optimum inlet parameters that drive the rate of evaporation of water vapor from the regenerated liquid desiccant, a was solution of CaCl 2. The main inlet parameters considered are the inlet air temperature, solution flow rate, solution inlet concentration, airflow rate and humidity of inlet air. The author reported that the regeneration process is highly dependent on the air inlet temperature, humidity, and flow rate. An increases in air velocity from 0.5 to 1.5 m/s resulted in the overall masstransfer coefficient in the structured packing dehumidifier and regenerator from 4.0 to 8.5 g/m 2 s and from 2.0 to 4.5 g/m 2 s, respectively. Higher solution temperature resulted in lower overall mass transfer coefficients. Figure 9.2.5: An energyefficient air conditioner with a desiccant solution regenerator using exhausted hot air from the condenser (Kassel, 2013) Jradi and Riffat (2014) demonstrated the feasibility of desiccant supported air conditioning in a trigeneration system powered by biomass gasification. The tri generation system used was a combined heating, cooling and power system, which was operated using gas turbine, internal combustion unit (Figure 9.2.6). Regeneration heat for the liquid desiccant was obtained from the waste heat of the gas turbine that ran with gasified biomass. July 9, 2016 Page 148 of 171

157 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Figure 9.2.6: An internal combustion based tri generation system with a biomass gasification unit, (Jradi and Riffat, 2014) 9.3 Liquid Desiccant Cooling with Evaporative Cooling and Solar Energy Liquid desiccants perform the same basic function as their solid desiccant counterpart in integrated cooling applications with evaporative cooling. The main function is to regulate humidity levels of the supply air to the conditioned spaces. In climates with higher humidity levels, the effectiveness of evaporative cooling is impeded and, in the case of direct evaporative coolers, adding more moisture to the already humid outdoor air would create uncomfortable, or even unhealthy indoor conditions. This section presents several desiccant cooling systems where sensible load is removed by means of evaporative cooling. The systems are examples of basic systems designs. A very innovative and ground breaking approach to evaporative air conditioning is presented in the Chapter 9.4. The use of evaporative cooling in conjunction with liquid desiccant dehumidification will be illustrated with examples of solar and other heat sources for regeneration. The basic concept of liquid desiccant based evaporative cooling, either direct or indirect, is illustrated in Figure and Figure illustrates the concept of liquid desiccant cooling, which is two stage process of removing sensible and latent loads. A mixture of outside and return air first passes through July 9, 2016 Page 149 of 171

158 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS first stage which is the liquid desiccant dehumidifier where humidity is reduced. Then, in the second stage evaporative cooling reduces the air temperature, which means it reduces the outside air dry bulb temperature and the heat of absorption from the first stage desiccant dehumidification. Figure 9.3.1: Concept of liquid desiccant based evaporative cooling system), (Buker, 2015) Legend: Figure illustrates the difference between direct and indirect evaporative cooling. The main difference being that direct evaporation increases the humidity in the air whereas the indirect process does not, since the heat transfer occurs through an impermeable membrane, such as a metal plate. For both types of evaporative cooling, however, the importance of the first stage dehumidification is that humidity levels are reduced so that evaporative cooling can be effective, especially in humid climate. The use of solar heat for regeneration is of significant importance and many system developments of liquid desiccants cooling use of solar or some other low grade waste heat source for desiccant regeneration. Advantages of liquid desiccant applications using solar heat is pointed out by a number of researches and HVAC practitioners. For example, Enteria and Mizutani (2011) discussed that liquid desiccant regeneration can be accomplished with at a temperature as low as C, thus at temperatures that can be easily obtained by solar thermal devices, such as flat plate collector. That fact that regeneration temperatures can be below 80 C is one of the most favorable features of the liquid desiccant systems. The authors state that carry over of desiccant droplets to the supply air is one of the main concerns of the liquid desiccant cooling systems. Liquid desiccant solutions are usually corrosive and simple handling of the working media is difficult. These design problems, however, are currently being solved thanks to fast technology advancement. The authors conclude that the potential of the liquid desiccant is remarkable. July 9, 2016 Page 150 of 171

159 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS (a) (a) Indirect evaporative cooler has two air streams: the primary air stream is cooled by heat transfer over metal plates of an air to air heat exchanger and rejects heat to the secondary air stream, e.g. there is only heat transfer and no mass transfer (b) direct evaporative cooling has only one air stream: the secondary air stream has an adiabatically evaporation of water which results in significant temperature drop but also increase in humidity in the air stream; (b) direct evaporative cooler is cooled by water vaporizing and lowering the temperature, but the humidity level of the primary air stream increases. Figure 9.3.2: Concept of liquid desiccant based evaporative cooling system and illustration of direct and indirect evaporative cooling; (Buker, 2015) (b) Buker and Riffat (2015) discussed a solar liquid desiccant air conditioner (LDAC) coupled with a direct evaporative cooler for the climate in Queensland, Australia. The system is shown in Figure The system used a high effectiveness cross flow polymer plate heat exchanger as an absorber. The liquid desiccant agent was a lithium chloride solution which was regenerated by using hot water derived from flat plate solar collectors. The process air was dehumidified by strong desiccant solution and then the warm and dry air was cooled and humidified through the direct evaporative cooler. The supply air was then provided to the conditioned space. The test results revealed that electrical COP of the system was around 6 with 3.5 kw electrical energy consumption and the effectiveness of the conditioner with the desiccant solution was about 82%. Buker and Riffat (2015) discussed a prototype of a solar driven liquid desiccant systems which was operated in the hot and humid tropical climate of Thailand. The system (Figure 9.3.4) used only solar energy for the regeneration of liquid desiccant. The authors pointed out that the system was capable of reducing the temperature by 1.2 C and relative humidity by 11.1%. The reason for the limited performance was the performance of the conventional cooling tower. Since the ambient air was humid, the cooling of process water in the cooling tower was only limited. July 9, 2016 Page 151 of 171

160 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Figure 9.3.3: Schematic diagram of the solar LDAC system in Queensland, Australia, (Buker and Riffat, 2013) Figure 9.3.4: Schematic diagram of the solar regenerated liquid desiccant system, (Buker and Riffat, 2015) Das et al (2013) presented a small capacity solar liquid desiccant cooling prototype system which was tested in the laboratory (See Figure 9.3.5). One of the main differences to other liquid desiccant cooling July 9, 2016 Page 152 of 171

161 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS systems was the intent to avoid carry over of liquid desiccant solutions to the supply air. The system provided 100% outdoor air and consisted of a dehumidifier or absorber (b), a regenerator or desorber (i), an indirect evaporative cooler (d), a cooling tower (f), two water solution heat exchangers (g, j), and one solution to solution heat exchanger (h). Hot and humid ambient air (a) entered the unit and passed through a dehumidifier (b). In the dehumidifier (also called an absorber), concentrated liquid desiccant absorbed water vapor from the process air. The air and the liquid channels were separated by semipermeable microporous membranes. Therefore, only the water vapor passed through the membranes into the liquid desiccant when the vapor pressure of the air exceeds that of the liquid desiccant. The authors emphasized that indirect contact between desiccant and air stream in the dehumidifier eliminates carryover of the liquid desiccant into the supply air. Downstream, the dehumidified air was first cooled by a fin tube air water heat exchanger and an indirect evaporative cooler (d) and then supplied to the room via a fan (e). The indirect evaporative cooler used part of the dehumidified process, while the recirculated air got humidified and exhausted to the atmosphere. Moisture laden dilute desiccant from the dehumidifier was preheated in the solution heat exchanger (h) by recovering heat from the hot concentrated desiccant leaving the regenerator and stored in a tank (o). The preheated dilute desiccant was then pumped to the plate heat exchanger (j) where it was further heated by hot water. Water was heated in solar collectors (k) (Evacuated tube Collectors, Heat Pipe Collectors) and stored in a hot water storage tank (l). The hot dilute desiccant flowed through the regenerator (i), where it released water vapor to the air stream owing to the vapor pressure difference. The concentrated desiccant from the regenerator was stored in a tank (n) and then precooled in the solution solution heat exchanger (h). The solution was then pumped to the plate heat exchanger (g) where it was cooled by water from the cooling tower. Cooling increased the vapor pressure difference between air and concentrated desiccant, resulting in higher dehumidification efficiency. Das et al (2013) stated that the system performances need some further optimization to lower thermal and pressure losses. Since the liquid desiccant cooling is well suited for air at higher humidity level, air at lower humidity levels does not necessarily need dehumidification upstream of the evaporative cooler to function effectively. The level of humidity in the air can either by a characteristic of region or of season. For instance, a region with constantly high humidity levels in the summer or during periods in the year could have dryer air conditions for the rest of the year. These changes would require that the liquid desiccant systems are suitable to adjust for different outdoor air conditions. Kim et al (2014) discussed an air conditioning system which offered flexibility by engaging the different main systems components. July 9, 2016 Page 153 of 171

162 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Figure 9.3.5: Prototype of solar liquid desiccant cooling prototype system (Das, 2013) Kim et al (2014) discussed the performance enhancement of an indirect and direct evaporative coolingassisted 100% outdoor air system (IDECOAS) by a liquid desiccant dehumidification unit. The authors called the liquid desiccant enhanced IDECOAS the LD IDECOAS. The authors state that prior studies had indicated that an IDECOAS could save as much as 50% of the energy requirement for a conventional variable air volume (VAV) system. However, during a hot and humid summer, the energy savings potential of the IDECOAS was significantly reduced by the low process effectiveness of the evaporative cooling units. A desiccant system to precondition the incoming outdoor air upstream of the IDECOAS was added to mitigate high humidity effects on the evaporative coolers. Figures through show the system configurations that were used in comparing the annual performance of the conventional VAV, the IDECOAS and the LD IDECOAS, respectively. July 9, 2016 Page 154 of 171

163 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Figure 9.3.6: Schematics of a conventional VAV system used in system comparison (Kim et al, 2014) Figure 9.3.7: Schematics of IDECOAS used in system comparison (Kim et al, 2014) Figure 9.3.8: Schematics of LD IDECOAS used in system comparison (Kim et al, 2014); colored system components refer to four regions in psychrometric chart shown in Figure July 9, 2016 Page 155 of 171

164 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Kim et al (2014) stated that both solid and liquid desiccant systems could be used to dehumidify the outdoor air flow. The authors, however, suggested that liquid desiccant system can provide deeper dehumidification with less regeneration energy consumption compared to solid desiccants. Additional benefits of the liquid desiccant included less fan energy due to lower pressure drops in the airside of the system, and the ability to use solar derived regeneration heat for the desiccant. Figure 9.3.9: Liquid desiccant sub system of the LD IDECOAS (Kim et al, 2014) The LD IDECOAS shown in Figure consists of an indirect (IEC) and direct (DEC) evaporative cooler and a liquid desiccant system for dehumidification of the process air. The liquid desiccant loop is depicted in Figure The heat of regeneration of the liquid desiccant units can be derived from a range of heat sources, including solar heat or low grade waste heat. In order to achieve an enhanced cooling effect in the IEC, the exhaust air can be supplied into the secondary side of the IEC when the exhaust air has a lower wet bulb temperature than the outdoor air. The heating coil (HC) and sensible heat exchanger (SHE) are located at the exhaust air side to maintain adequate temperature set points during periods when no cooling was needed. The supply air flow rate was modulated on the basis of the air conditioning load of the conditioned zones, in the same way as in a conventional VAV systems. Figure shows a psychrometric chart which the authors divided into four regions. These four regions represented either different climate zones or different phases during the annual climate cycle at a particular location. Within the four regions the components of the system would operate at different levels or would be turned off or on or they would be bypassed. Table summarizes how the main system components of direct and indirect evaporative coolers and the liquid desiccant unit would operate. The authors concluded from their system analysis that all different system operating settings (e.g. Regions A through D) performed well with different components on or off and that the required supply air temperature and humidity could be continuously maintained. The results indicated that on an annual level the LD IDECOAS resulted in a 64% energy savings relative to the conventional VAV system. By using solar heat for the desiccant regeneration would enhance the energy savings further. The July 9, 2016 Page 156 of 171

165 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS authors point out that the use of the liquid desiccant systems resulted in a 23% annual energy reduction over the IDECOAS system (e.g. without dehumidification) due to the increased effectiveness of the evaporative cooling process. While the energy savings are higher in the LD IDECOAS than in the IDECOAS, in very humid conditions the LD IDECOAS would be required to provide suitable indoor humidity levels, since the IDECOAS would not be able to provide such humidity levels. Figure suggest additional system improvements of the LD IDECOAS could include the use of solar heat and cooling tower for the liquid desiccant system. Figure : Four regions of performance of the LD IDECOAS (Kim et al, 2014) Table 9.3.1: Modes of operation of LD, DEC and IEC in four regions of psychrometric chart, (Kim et al, 2014) July 9, 2016 Page 157 of 171

166 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Figure : LD IDECOAS with thermal components added (Kim et al, 2014) 9.4 Desiccant Enhanced Evaporative Air Conditioning This section is dedicated to the discussion of a recent desiccant cooling systems development by National Renewable Energy Laboratory (NREL) which AC system developers called nothing short of revolutionary AC technology (MIT Technology Review, 2016). The system is referred to as DEVAP, from Desiccant Enhanced Evaporative Air Conditioning. As discussed in Section 9.3 the thermodynamic performance of evaporative cooling is impeded in humid weather and adding desiccants to dehumidify the supply air to the evaporative coolers can rectify this shortcoming. The DEVAP technology solves the combined desiccant dehumidification and evaporative cooling by using membrane technology that separates desiccant from air travelling through the process vessel. The polymer membrane used in the process has pores about 1 micrometer to 3 micrometers in diameter, big enough that water vapor passes through easily while the salty liquid cannot pass through it. The membrane material is also coated with a Teflon like substance to repel liquid water. The desiccant pulls moisture from the airstream, leaving dry, warm air. This dry and warm air stream enters a second channel, where water evaporates indirectly to cool a secondary airstream, which in turn cools the first airstream. The result is cool and dry air. July 9, 2016 Page 158 of 171

167 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Kozubal et al (2011), who are the inventor of the DEVAP process, discussed that simply combining desiccant based dehumidification and indirect evaporative cooling technologies is feasible, but has not shown promise because the equipment is too large and complex. The DEVAP technology provides a single cooling core, evaporative and desiccant cooling, which makes the system easier to install and operate and renders dehumidification many times more efficient. The authors claimed that due to the DEVAP technology of using membranes, the system solves the important issues of eliminating desiccant entrainment into the airstream. When an additional water containment is used wet surfaces are eliminated, which prevents bacterial growth and mineral buildup, therefore avoiding cooling core degradation. DEVAP decouples cooling and dehumidification performance, which results in independent temperature and humidity control. The energy input is largely switched away from electricity to low grade thermal energy including waste heat, solar, or biofuels. Thermal energy consumption correlates directly to the humidity level in the operating environment. The authors suggested that yearly combined source energy savings can be as high as 90% for dry climate and about 30% for very humid climate. The authors further pointed out that liquid desiccant technology is a new science with unpracticed technology improvements that can reduce energy consumption an additional 50%. The DEVAP technology builds on the previously developed liquid desiccant based A/C (LDAC) technology by an industry partner of NREL. In this particular LDAC process liquid desiccant is absorbed into the conditioner (absorber) where the inlet ambient air is dehumidified. The liquid desiccant is regenerated in the regenerator (desorber) where the water vapor desorbs into the exhaust air stream. This process is called low flow liquid desiccant A/C, because the desiccant flow is minimized in both heat and mass exchangers to the flow rate needed to absorb the necessary moisture from the air stream. The heat and mass exchangers must therefore have integral heating and cooling sources (55 F 85 F cooling tower water is supplied to the conditioner). The regenerator uses hot water or hot steam at F. The DEVAP enhancements to the LDAC system is illustrated in Figure The figure depicts how the DEVAP cooling core enhances the already developed LDAC technology and converts it from a dedicated outdoor air system to an air conditioner that performs space temperature and humidity control and provides all the necessary ventilation air. DEVAP can be configured to provide 30% 100% outdoor air. As a further significant advantage the DEVAP system does not require a cooling tower, which reduces maintenance requirements. Figure illustrates the three basic ways to regenerate the desiccant system with a thermal source: solar, water heater, and a double effect. The water heater or boiler can be fueled by many sources, including natural gas, combined heat and power (CHP), or even biofuels. July 9, 2016 Page 159 of 171

168 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Figure 9.4.1: DEVAP enhancement for LDAC, (Kozubal et al, 2011) July 9, 2016 Page 160 of 171

169 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Figure illustrates, and the following steps describe the physical DEVAP process: 1. Ventilation air [1] and warm indoor air [2] are mixed into a single air stream. 2. This mixed air stream (now the product air) is drawn through the top channel in the heat exchange pair. 3. The product air stream is brought into intimate contact with the drying potential of the liquid desiccant [d] through a vapor permeable membrane [e]. 4. Dehumidification [ii] occurs as the desiccant absorbs water vapor from the product air. 5. The product air stream is cooled and dehumidified, then supplied to the building space [3]. 6. A portion of the product air, which has had its dew point reduced (dehumidified), is drawn through the bottom channel of the heat exchange pair and acts as the secondary air stream. 7. The secondary air stream is brought into intimate contact with the water layer [c] through a vapor permeable membrane [b]. 8. The two air streams are structurally separated by thin plastic sheets [a] through which thermal energy flows, including the heat of absorption [i]. 9. Water evaporates through the membranes and is transferred to the air stream [iii]. 10. The secondary air stream is exhausted [4] to the outside as hot humid air. Figure 9.4.2: Physical DEVAP concept description, (Kozubal et al, 2011) Figure indicates the use of two types of membranes, the water side and the desiccant side membranes. The desiccant side membrane is necessary to guarantee complete containment of the desiccant droplets, which thus prevents desiccant leaks and entrainment into the air stream. The waterside membrane is part of the original DEVAP concept, but is not a necessary component. Its advantages are complete water containment and dry surfaces, which are made completely of plastic and resist biological growth. July 9, 2016 Page 161 of 171

170 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Kozubal (2013) provided an illustrated DEVAP heat and mass flow process which is depicted in Figure From the psychrometric performance the process has the significant advantage that the achievable dry bub temperature is not limited by the wet bulb temperature at saturation but rather by the dry bulb temperature of the air. This characteristic is comparable with the M cycle evaporative cooling which was discussed in Section 7. (a) Physical heat and mass flow diagram for DEVAP process The locations in the schematics correspond to the state point in the psychrometric chart (b) Psychrometric chart with state point for DEVAP process Figure 9.4.3: Thermal performance of the DEVAP process, (Kozubal, 2013) July 9, 2016 Page 162 of 171

171 SECTION 9 LIQUID DESICCANT COOLING SYSTEMS Kozubal et al (2011) presented possible DEVAP installations in residential and commercial buildings. Figure illustrates proposed generic DEVAP implementations in residential and commercial building implementations. In both cases solar heat is used for all or a major part of the desiccant regeneration. In a residential applications DEVAP performs the following functions: Air conditioner with independent temperature and humidity control Dedicated dehumidifier Mechanical ventilator In commercial applications DEVAP units would be installed as roof top units (RTU). There are several options desiccant regeneration heat sources, such as: Natural gas only CHP with or without natural gas backup Solar heat with or without natural gas backup. The figure above shows DEVAP system with a solar and CHP options. Figure 9.4.4: Example of commercial and residential installation of DEVAP A/C, (Kozubal et al, 2011) Kozubal et al (2011) performed a simplified cost analysis for a typical commercial DEVAP installation, which assumed monthly electricity and natural gas costs, but did not include time of use electricity costs, such peak power. Because electric power draw is a main concern of commercial peak consumption, inclusion of time of use costing would increase electricity costs and therefore savings, thus improve the economics of DEVAP applications. A 15 year present value analysis suggested a 28% and 39% internal rate of return (IRR), for the humid Houston and dry Phoenix climate, respectively. July 9, 2016 Page 163 of 171