Master Thesis CHARACTERIZATION OF HEAT TRANSFER AND EVAPORATIVE COOLING OF HEAT EXCHANGERS FOR SORPTION BASED SOLAR COOLING APPLICATIONS"

Transcription

1 Master Thesis CHARACTERIZATION OF HEAT TRANSFER AND EVAPORATIVE COOLING OF HEAT EXCHANGERS FOR SORPTION BASED SOLAR COOLING APPLICATIONS" César Augusto González Morales Academic Supervisor: Phd. Candidate Amir Vadiee Industrial Supervisor Dipl. Phys Gunther Munz

2 Abstract Master of Science Thesis EGI MSC EKV965 CHARACTERIZATION OF HEAT TRANSFER AND EVAPORATIVE COOLING OF HEAT EXCHANGERS FOR SORPTION BASED SOLAR COOLING APPLICATIONS César Augusto González Morales Approved Examiner Supervisor Prof. Torsten Fransson Commissioner Amir Vadiei Contact person Abstract The content of this Master thesis is the characterization of three different cross unmixed flow heat exchangers. All of the heat exchangers have different inner geometries and dimensions. In order to perform the characterization of these heat exchangers, measurements of heat transfer were done under different conditions: five different temperatures at the inlet of the sorption side, different mass flow for both inlet sides of the heat exchangers. The heat transfer measurements were done with and without applying indirect evaporative cooling in order to find out the influence of indirect evaporative cooling. This research was done with the objective to find out which heat exchanger presents the best performance. The purpose is to install the heat exchanger in the novel solar driven open air SorLuKo system. This system was developed in Fraunhofer ISE and works under the same principe as the ECOS system. The main objective of the SorLuKo system is to dehumidify and cool a dwelling or small office. ii

3 Acknowledgment Acknowledgment For the realization of this master thesis I would like to thank Dypl Phys Gunther Munz for giving the opportunity of doing my master thesis at Fraunhofer ISE and for being such a helpful supervisor. I thank him for all the support he provided during the realization of this Master thesis in academic as well as in professional matters. I want to thank as well Dr. Constanze Bongs as well as Dr. Alexander Morgenstern for assisting me anytime I needed help or had questions regarding the operation of the ECOS system. Their help was key factor for achieving all of the goals set for this master thesis. I also thank them for all the feedback provided for improving the thesis. I also want to acknowledge the Phd candidate Amir Vadiee for being my academic supervisor. I want to express my gratitude to all my friends. Their company, friendship and support always kept going further. I also want to thank for their unconditional help during the good and bad times. Their company had made this experience one of the best experiences of my life. Finally I want to thank to my parents and family for being always there for me since the beginning of this journey. They have always encouraged me to keep going further and thanks to them I have always achieved all the goals I set to myself. For that I will be forever grateful. César Augusto González Morales August 2013 iii

4 Index Index Abstract... ii Acknowledgment... iii List of Figures... v List of Tables... vi Nomenclature... vii 1. Introduction a) Background b) Objectives Theory ) Heat transfer ) Heat exchangers a) Types of heat exchangers b) Heat exchanger analysis ) Psychrometry a) Molliere Diagram ) Adsorption principle ) Description of the ECOS system ) Description of the SorLuKo Project ) Description of the Test Rig a) Test rig operation sequence b) Test Rig & Weather c) Uncertainty calculaltions Heat exchangers a) Description of the Klingenburg heat exchanger b) Description of the Haugg I heat exchanger c) Description of the Haugg II heat exchanger Measurements & Results a. Dry heat transfer measurements b. Evaporative heat rejection Conclusions Literaturverzeichnis Appendix Appendix 1 ECOS Test rig Scheme Appendix 2 Installed sensors in ECOS measurement Unit Appendix 3 Heat transfer uncertainty calculation Appendix 4 Results a. Dry heat transfer b. Heat transfer with indirect evaporative cooling iv

5 List of Figures List of Figures Total final energy consumption... 1 Physical ways to convert solar radiation into cooling... 2 Heat transfer by conduction... 4 Velocity and thermal boundary layers... 5 Parallel flow (top), Counter flow (bottom) HE... 6 Cross flow heat exchanger... 6 Heat exchangers construction classification... 6 Shell and Tube Heat Exchanger... 7 Working principle of a plate heat exchanger... 8 Compact heat exchangers... 8 Heat exchanger work principle... 8 Parallel flow heat exchanger temperature profile Differential elements for energy balance Counterflow HE temperature profile Hot End Pinch (left) Cold End Pinch (right) Effectiveness of a parallel (top left) counterflow (top right) and a crossflow (bottom) HE Molliere Diagram Terms of adsorption Desorption/Adsorption Cooling/Adsorption Adsorption/Desorption Adsorption/Cooling SorLuKo operating phases Pre-conditioning Unit Simple mounting/demounting of the HE Pipelines connecting the Measurement Unit Schematic of the Measurement Unit Working principle dew point mirror sensor Working principle of the Capacitive dew point sensor Water pump & sprayer Touch and Mappit Interfaces Sequencer Interface Access based RControl file R Console (GUI) Typical result graphs Embossed plates (left) [21] & Klingenburg HE (right) Silica gel spheres (left) & Embossed plate (right) Bar-plate compact heat exchanger Front view of the cooling side showing the offset View of the slotted cooling fins Heat transfer of each of the 3 HE with 3 different mass flows Heat transfer losses from the HE with 3 different mass flows v

6 List of Tables Calculated Overall Heat transfer coefficient (left) Effectiveness (right) Heat extracted from Sorption side Heat transfer comparison Effectiveness with indirect evaporative cooling Effectiveness comparison Wet bulb temperature reading List of Tables Specs of the Klingenburg Heat Exchanger Specs of the Haugg I Heat Exchanger Specs of the Haugg II Heat Exchanger Characterization Scenarios Comparison between calculated and measured values Percentual reductions Characterization scenarios with Evaporative Cooling vi

7 Nomenclature Nomenclature Symbols Symbols Name Unit Mass flow kg/s C Heat capacity rate J/K s C p Specific heat capacity J/kg K h Specific enthalpy J/kg H Enthalpy J h Convection heat transfer coefficient W/m 2 K k Thermal conductivity W/m K L Length m m Mass kg NTU Number of transfer Units p Pressure Pa, q Heat rate W q Heat flux W/m 2 R Universal Gas Constant J/mol K T Temperature K U Overall heat transfer coefficient W/m 2 K V Volume m 3 x Absolute humidity g/kg ΔT Temperature difference K ΔTm Mean value of the temperature K difference ε Effectiveness θ Averaged temperature difference ρ Density kg/m 3 τ Dew point temperature K φ Relative humidity % Sub indexes Sub index c d h h i max min o s v w Name cold dry hot humid inlet max value min value outlet saturation vapor water vii

8 1. Introduction 1. Introduction 1a) Background On the recent years, global warming has been a topic thoroughly discussed due to its negative impact to the environment. The main cause of global warming is the CO 2 emissions to the atmosphere. These CO 2 emissions are the product of fossil fuels combustion for electrical and mechanical energy production. This energy produced is mainly consumed by 5 main sectors. [1] Industry Household Services Transportation Fishing, agriculture & Forestry Fig. 1 Total final energy consumption [1] As it can be appreciated in Fig. 1 the percentage of energy consumed in households is worth paying attention to. From the total energy consumed in the households sector, more than 50% of the consumed energy serves the purpose of keeping the proper level of comfortableness inside the premises [1]. This comfortableness involves mainly, heating, air conditioning and ventilation of the premises. In recent years, plans as the 2020 Energy Initiative or the Energy Roadmap 2050 have driven to creation of thermal regulations for new dwellings in the entire EU countries. These regulations have aided in energy saving and reduction in energy consumption in the household sector of countries with moderate climate. The main comfortableness factor to take into consideration for this kind of climates is space heating. Energy reductions are mainly achieved by improving the insulation properties of the dwellings. On the other hand, air conditioning is another comfortableness aspect that acquires more relevance during warm/hot weather season. The relevance of air conditioning and ventilation is increased as well by the insulation properties of the houses.

9 1. Introduction Nowadays the most common system for air conditioning is the vapor compressor system, which is electrically driven. Yet other technologies for cooling purposes are already available. These technologies are heat driven and work along with an ad/absorption process. The heat required to drive this system can come from different sources, such as waste heat of an industrial processes. In the spirit of increasing the use of renewable energies, solar radiation has been implemented as a driver for air conditioning systems. This can be done by converting solar radiation into electricity or into thermal energy. Fig. 2 shows how solar radiation plays a role in air conditioning systems. The processes marked in dark gray are currently available in the market. Fig. 2 Physical ways to convert solar radiation into cooling [2] At the Fraunhofer Institute for Solar Energy System one new concept known as ECOS (Evaporative Cooled Sorptive Heat Exchanger) was developed. This development belongs in the same category of the well know DEC (Desiccant Evaporative Cooling) concept which in Fig. 2 is denominated as Dehumidifier Rotor. The ECOS concept involves as main component a heat exchanger coated with sorptive material. The SorLuKo Project, which is the development of a solar sorptive air conditioning system driven by solar air collectors, involves the integration of an ECOS heat exchanger and evacuated tube solar thermal air collectors in order to develop an air cooling and dehumidifying device. Both, the ECOS concept as well as the SorLuKo project will be explained further in Chapter 2. 2

10 1. Introduction 1b) Objectives As mentioned in the previous section, the main component of the ECOS concept is a coated heat exchanger. In Fraunhofer ISE several air-air heat exchanger geometries and configurations have been constructed for testing purposes in order to find out the one with the optimal performance. At Fraunhofer ISE a novel coating process has been developed for the application of the sorptive material to the heat exchanger. The main objectives of this master thesis work, is to perform measurements of the different heat exchangers on the test rig. For pure/absolute/straight heat transfer: Comparison of different built heat exchangers: o Plate heat exchanger, coating by epoxy resin/silica gel o Bar plate heat exchanger cooling fins Type 1 o Bar plate heat exchanger cooling fins Type 2 For the realization of this master thesis work the following outline will be followed. Chapter 2. Theory and some basic concepts for a better understanding of the project are explained. Following these ground concepts a detailed description of the ECOS concept as well of the SorLuKo project is provided Chapter 3. Detailed description and data for each one of the different heat exchangers configurations is given Chapter 4. Measurements done on each one of these different heat exchangers and the multiple results of the heat transfer measurements are shown. Comparison between the performance of the 3 heat exchangers is shown in this chapter Chapter 5. Conclusions for this Master thesis work 3

11 2. Theory 2. Theory 2.1) Heat transfer In order to understand the basic principles of how a heat exchanger works a basic explanation of certain concepts is required. The main purpose of a heat exchanger is to perform heat transfer between two fluids at different temperatures. According to Incropera [3] heat transfer is thermal energy in transit due to a spatial temperature difference This heat transfer can take place in three different modes: Conduction, convection and radiation. Due to the scope of this master thesis, the explanation of radiation will be excluded. The concept of conduction has to do with the molecular activity of the substance or body in question. Conduction is the transfer of energy from the molecules with higher energy to the less energetic molecules. These molecular energies are directly related to the temperature, meaning that the heat transfer takes place from the side of higher temperature to the lower temperature side. It can be said then that there is a diffusion of thermal energy. In any mode of heat transfer, heat transfer can be calculated with rate equations [3]. These equations allow calculating the amount of energy transferred per unit of time. In case of heat transfer by conduction, this rate equation is called Fourier s Law: Eq. (2.1.1) q (heat flux) [W/m 2 ] is the heat transfer rate per unit of area normal to the direction of the heat transfer. dt/dx is the temperature gradient in the x direction k [W/m K] is the thermal conductivity which is characteristic for each material Fig. 3 Heat transfer by conduction [3] As it can be observed in Figure 3, the temperature gradient in steady state shows a linear behavior. In this case dt/dx can be then expressed as: And then Eq. (2.1.2) The heat flux is the amount of heat transferred per unit area. In order to calculate the heat rate q through a plane surface is the product of the heat flux and the area. 4

12 2. Theory Eq. (2.1.3) The second mode of heat transfer is convection. In this mode, the medium in which the heat transfer is performed is a fluid in motion. This movement in the presence of a temperature gradient contributes to the heat transfer [3]. As in the same case of conduction, diffusion of energy is also present. A good example to explain better how these two mechanisms work is with a fluid in motion above a plane surface. The fluid and the surface are at different temperatures. Due to the dynamics of this interaction there is a region in the fluid where the velocity varies from zero (closest to the surface) until a finite value at which the fluid is moving. This region in the fluid is known as the velocity boundary layer [4]. Due to the temperature difference there will also be a region in the fluid in which its temperature will vary from the temperature the surface is at (closest to the surface) until a finite temperature value proper of the fluid. This region is called thermal boundary layer [4]. Fig 4 Velocity and thermal boundary layers [3] At the point on the velocity boundary layer, where the velocity is zero, the only heat transfer mechanism taking place is diffusion. The heat transfer contribution of the fluids motion takes place when the heat conducted to the boundary layer is swept in the stream and eventually it is transferred to the outer part of the layer. The boundary layer will then grow along with positive direction of x [4]. The rate equation that describes the convection heat transfer mode is known as the Newton s law of cooling. Where: Eq. (2.1.4) q is the convective heat flux [W/m 2 ] T s is the temperature of the surface T is the temperature of the fluid h is the convection heat transfer coefficient [W/m 2 K] which is influenced by the boundary layer conditions such as geometry, nature of the fluid in motion, fluid thermodynamics and transport properties [3]. 5

13 2. Theory 2.2) Heat exchangers After explaining the two main heat transfer modes taking place in a heat exchanger, the operation of heat exchangers will be provided in this section 2.2a) Types of heat exchangers A heat exchanger is a device in which two fluids at different temperatures and separated by a wall experience a heat transfer [3]. The main ways to classify heat exchangers is by its flow arrangement as well as by the way they are built. The simplest heat exchangers are the parallel flow and the counter flow heat exchangers. The construction of both of these heat exchangers is done with a couple of concentric tubes through which the cold and hot fluids flow on each one of these tubes. Fig 5 Parallel flow (top), Counter flow (bottom) HE [5] Another type is the cross flow heat exchanger, where the fluids are perpendicular to each other as shown in Fig 6 Fig 6 Cross flow heat exchanger [5] Regarding the type of construction of a heat exchanger, the following classifications are done Fig. 7 Heat exchangers construction classification [5] 6

14 2. Theory As shown in Fig. 7 two main types of classification can be done regarding the construction of the heat exchanger, recuperative and regenerative. The recuperative type of heat exchanger has separate flow paths and the fluids flow at the same time thus performing the heat transfer using the wall that separates these two fluids. The regenerative type of heat exchanger is also known as a capacitive heat exchanger [5]. In this kind of heat exchanger a matrix is heated up with the hot fluid flowing through it. Then this heat is removed by the flow of the cold fluid through this same matrix. The recuperative type of heat exchangers has 3 main sub-classifications as shown in Fig. 7. Due to the scope of this thesis only the indirect classification will be further explained. The indirect recuperative heat exchangers have as a main characteristic the separation of both fluids by a solid wall mainly made out of metal. Through this wall the heat transfer is made without any direct contact of both fluids. Hence the name of indirect heat exchangers. The shell and tube heat exchanger is the most common of the tubular indirect heat exchangers. It consists of tubes inside a shell. Through the tubes one of the fluids will flow while the other fluid flows freely inside the shell, having the heat transfer through the walls of the tubes. Normally baffles are installed inside the shell section in order to promote turbulence and also a cross flow velocity component [3]. Fig. 8 Shell and Tube Heat Exchanger [3] In the case of plate indirect heat exchangers the two most common kinds are the frame and plates heat exchanger and the Plate fin heat exchanger. The frame and plates heat exchanger consists of a frame holding 2 end members. In the middle of these members are rectangular plates stacked together. These plates are embossed in the corners and are separated by gaskets. The arrangement of the plates stack is such that the two fluids flow between the plates but without having any contact. This is achieved by the alternate position of the gaskets in the stack. Fig. 9 shows how the gaskets in the plates stack allow alternatively the cold and hot fluid to flow up and down respectively in order to have the heat transfer through the plates. 7

15 2. Theory Fig. 9 Working principle of a plate heat exchanger [6] Another important type of plate heat exchanger is the plate fin heat exchanger, also known as compact heat exchanger. The purpose of this type of heat exchanger is to achieve a high surface transfer area in determined volume [3]. This is achieved by having a dense arrangement of tubes and finned plates. This type of heat exchangers is used normally when one of the fluids possesses a low convection coefficient. Fig. 10 Compact heat exchangers [3] 2.2b) Heat exchanger analysis As mentioned in the previous section, the main purpose of a heat exchanger is to perform heat transfer of two fluids at different temperatures through a solid wall. Fig. 11 Heat exchanger work principle [3] As it can be seen in Fig. 11 the two heat transfer modes explained in Section 2a) are shown: Convection in each side of the wall where the two fluids at different temperatures are in motion and conduction through the wall separating these two fluids. 8

16 2. Theory For each convection and conduction section Eq. (2.1.2) and Eq. (2.1.4) are used to find out the heat flux on each of these sections. For analysis purposes an analogy with electrical circuits is made, where the resistance is defined as the ratio of a driving potential to the corresponding transfer rate [3] which for conduction is defined: And for convection it is defined as: Eq. (2.2.1) Eq. (2.2.2) Since the heat rate is constant through the whole system it can be then expressed: Eq. (2.2.3) Eq. (2.2.3) can then be expressed in terms of the overall temperature difference: Eq. (2.2.4) R tot is the total resistance, which in this particular case are considered to be all in series, thus: Eq. (2.2.5) In the case of a system with different type of fluids and materials, which have different heat transfer coefficients, it is more convenient to work with an overall heat transfer coefficient U which U= 1/R tot and can be used in the expression: Eq. (2.2.6) In which ΔT is the overall temperature difference of the system being analyzed. In order to design a heat exchanger or to find out about its performance, relations between the inlet and outlet temperatures of the fluids, the area for heat transfer and the previously explained heat transfer coefficient shall be done with the total heat transfer rate. Some of these relations can be done by performing energy balances for the cold and hot fluids and assuming that there is no heat exchange with the surrounding of the heat exchanger. Eq. (2.2.7) Eq. (2.2.8) In the previous equations C ph and C pc are the specific heat capacities for each of the fluids, h stands for the hot fluid and c for cold as well as i stands for inlet and o for outlet. Another relation of interest is the amount of heat transferred between the cold and hot fluid through the solid wall. For this relation Eq. (2.2.6) come in handy. It is of extreme importance to consider, that the ΔT in this equation will depend on the position in the heat exchanger, thus a mean value of this ΔT is required. 9

17 2. Theory The calculation of the mean value of the temperature difference (ΔT m) will be done for a parallel flow heat exchanger. Fig. 12 Parallel flow heat exchanger temperature profile [3] Fig. 12 shows the temperature profile of a parallel flow heat exchanger. As it can be appreciated, at the entrance of the heat exchanger (left side) the max temperature difference is shown. At the exit of the heat exchanger the fluids have a tendency to asymptotically approach one temperature value. This is due to the fact that heat transfer was taking place, thus increasing the temperature of the cold fluid and reducing it from the hot fluid. In order to perform the calculation energy balances on differential elements will be implemented. Fig. 13 Differential elements for energy balance [3] So from Fig. 13, the energy balances for the cold and hot fluid are: Eq. (2.2.9) Eq. (2.2.10) Where C c and C h are the heat capacity rates and For the local heat transfer between both fluids Eq. (2.2.11) In this particular equation ΔT refers to the local temperature difference which, as shown in Fig. 12 can be then expressed as: Eq. (2.2.12) By substituting Eq. (2.2.9) and Eq. (2.2.10) in Eq. (2.2.12), then substituting Eq. (2.2.11) in the previous substitution, and integrating, the obtained equation is: ( ) Eq. (2.2.13) 10

18 2. Theory Doing the substitution of Eq. (2.2.7) and Eq. (2.2.8) in Eq. (2.2.13) the following is obtained: ( ) Eq. (2.2.14) With the proper manipulation the following equation is obtained: [ ] Eq. (2.2.15) Looking at Fig. 12 Eq. (2.2.15) can be written as Eq. (2.2.16) In which the term can be performed for any type of heat exchanger. is known as the log mean temperature difference ΔTlm. This type of analysis Other type of analysis that is commonly used to analyze heat exchangers is called the effectiveness method. This method mainly focuses on the concept of the maximum possible heat transfer rate. Ideally this would mean that one of the fluids would achieve the largest possible temperature difference. This would mean that one of the fluids would experience a ΔT T hi - T ci. This max temperature difference can be reached in an infinite length Counterflow heat exchanger [7]. The reason of this is that on the side where the hot fluid goes in with its highest temperature, the cold fluid comes out with a higher temperature than the inlet. On the other side the opposite happens; the cold fluid goes in at its lowest temperature and the hot one goes out with lower temperature than the inlet. Fig. 14 Counterflow HE temperature profile [3] In an infinite length heat exchanger either the cold water on the outlet could reach the inlet temperature of the hot water or the hot water going out could decrease its temperature to the cold water inlet temperature. These points are known as pinch points [7]. Fig. 15 Hot End Pinch (left) Cold End Pinch (right) 11

19 2. Theory Having this ΔT as a guideline it is possible to do the following analysis making use of Eq. (2.2.7) and Eq. (2.2.8) and using the heat capacity rates. maximum temperature difference, expressed as Now it is assumed that the cold fluid is the one, who experiences the and this equation may be Eq. (2.2.17) Eq. (2.2.17) shows that since temperature ratio is a value lower than one, due to the fact that the max temperature difference is in the denominator. This also indicates that C c < C h. This analysis can be performed then for the hot fluid as well: Eq. (2.2.18) The same behavior is found. The heat rate of the hot fluid, which is now experiencing the max temperature difference, has now the lowest heat transfer rate. It can then be expressed: Then the effectiveness can be expressed as: Eq. (2.2.19) Eq. (2.2.20) And: Eq. (2.2.21) Two new parameters used to characterize a heat exchanger are introduced; Dimensionless averaged temperature difference θ and Number of transfer units (NTU): Eq. (2.2.22) Eq. (2.2.23) With the previous equations, it can be then stated: Eq. (2.2.24) This dimensionless parameter is a measure of thermal length of the heat exchanger [7]. Recalling Eq. (2.2.16) it can be then established that ( ) [8] 12

20 2. Theory Algebraic expressions for relating the effectiveness with the NTU parameter for different types of heat exchangers have been developed. Fig. 16 Effectiveness of a parallel (top left) counterflow (top right) and a crossflow (bottom) HE [3] 2.3) Psychrometry In ventilation and air conditioning, the state variables of humid air, play quite an important role. In a ventilation or air conditioning system, no chemical reaction takes place, thus the humid air can be treated just as a mixture of air and water [9]. In this case, air will always have the same composition. The water will be dependent of the state variables. It can be in ice, liquid or vapor state. Another important consideration is that in most of the ventilation and air conditioning equipment work at conventional pressures and temperatures. So the mixture of dry air and water might be treated as an ideal gas mixture [10]. The main parameters to consider in a humid air system are: V volume of the mixture of dry air and water vapor m d mass of the dry air m w mass of water m v mass of water vapor T absolute temperature of the humid air P Total pressure of the humid air So the total mass of humid air is expressed as: Eq. (2.3.1) 13

21 2. Theory As mentioned before, since this mixture is treated as an ideal gas mixture it can then be expressed for the dry air: As well as for the water vapor Eq. (2.3.2) Eq. (2.3.3) The pressure terms stated in these two previous equations are the partial pressures of each of the components. This partial pressure is defined as the pressure each of these gases would exert if the total volume would be just filled with that specific gas. Dalton s law states, that the total pressure of an ideal gas mixture, is the sum of the partial pressure of each of its components: Eq. (2.3.4) Another concept of great importance is the saturation pressure. Saturation is achieved when in the gas mixture (air-water vapor) above the water; the amount of molecules leaving the water surface becomes equal to the amount of molecules being captured in the water surface [9]. The pressure at which this condition is achieved is known as saturation pressure.-.p s which is dependent of the temperature. Holmgren developed a formula based on the standards of the International Association for Properties of Water and Steam (IAPWS97). This particular formula is valid for a temperature interval K T K [11] Where: ( ) Eq. (2.3.5) Then another concept for describing the saturation condition of the humid air is introduced. The relative humidity is then defined as the proportion of the water vapor pressure p v over the saturation pressure. Eq. (2.3.6) Since the water vapor pressure cannot exceed the saturation pressure, then the values of the relative humidity can only range between 0-1 (0-100%). If humid air is cooled down to a certain temperature, the water vapor contained in it will condense and become water. This temperature is known as dew point temperature τ. This means that at this temperature saturation is reached. For air is not possible anymore to increase the water vapor content. It is also valid to state, that if with Eq. (2.3.5) the pressure is calculated at the dew point temperature τ the obtained value is the water vapor partial pressure. Eq. (2.3.7) 14

22 2. Theory Besides the relative humidity another important concept is the absolute humidity, which is defined as the ratio of mass of water over the mass of dry air Eq (2.3.1) can now be expressed as: Eq. (2.3.8) Eq. (2.3.9) Also when the humid air is just composed of water vapor and dry air Eq. (2.3.2) and Eq. (2.3.3) can be introduced in Eq. (2.3.6) Eq. (2.3.10) The previous equation can be written as well using Dalton s law stated in Eq. (2.3.4) where and p v can be expressed as from Eq. (2.3.5) The final equation is expressed then: Eq. (2.3.11) The gas constants for dry air as well as for water vapor are known. The values are 287 J/kg K and J/kg K respectively. Then Eq. (2.3.9) can be written as: Eq. (2.3.12) can also be expressed as: If the humid air is completely saturated: Eq. (2.3.12) Eq. (2.3.13) Eq. (2.3.14) For calculations of the mass flow with the regarding the volumetric flow it is required to calculate the density of the mixture of dried air and water vapor. Eq. (2.3.15) The constants for both components are known. Certain manipulation can be done taking into account the use of mbar instead of bars. Eq. (2.3.16) Applying Dalton s law then the previous equations is defined as: Eq. (2.3.17) 15

23 2. Theory Calculations about treatment of humid air can also be made on the energetic side. On a similar fashion as Dalton s law, it can be stated that: Making use of the specific enthalpies: Eq. (2.3.18) Eq. (2.3.19) By de and humidification of the air, the mass of the water changes, but the mass of the dry air remains constant. In that sense it is better to write the equation relating the specific enthalpies to the mass of dry air: Eq. (2.3.20) In practice, what is important for doing energetic calculation is the enthalpy difference. It is then possible to establish the enthalpy origin freely. So in this case h, referring to the simplified variable of the expression in parenthesis in Eq. (2.3.15) For dry air h = 0 at 0 C For boiling water h = 0 at 0 C One of the advantages of doing this particular selection is that the temperature difference may remain in C. Another simplification is that the specific heat of dry air as well as the specific heat of water in its different phases can be used almost as constants For the calculation of the water s enthalpy, first it is calculated the change of phase enthalpy at 0 C and then the enthalpy required to warm water vapor at a desire temperature. This way of proceeding gives as a result for the evaporation enthalpy r o constant values [9]. With the previous explanation the following relations for different cases of humid air can be expressed. These relations are for 1 Kg. of air an x kg of water. Dry air: Dry air with unsaturated water vapor: Eq. (2.3.21) Dry air with saturated vapor: Eq. (2.3.22) Where c pd is the specific heat of the dry air KJ/kg K, r o is the vaporization heat at 0 C and has a value of KJ/kg and c pw is the specific heat of water vapor equal to 1.86 KJ/Kg K [9]. In order to simplify the calculation of enthalpies for different possible combinations of air and water vapor at different temperatures, the use of the Molliere Diagram is implemented. 2.3a) Molliere Diagram The Molliere Diagram is a graphic representation of the relation between moisture content of the air, the air s temperature and the enthalpy. Normally the Molliere Diagram is applicable for a fixed or constant pressure, usually 1 bar. 16

24 2. Theory The diagram shows on the horizontal axis, the absolute humidity x (content of water). On the y axis the air temperature is indicated. The curve line indicating a value of 1 is known as τ dew point line. The curve lines quasi parallel to this dew point line refer to the relative humidity of the air. Finally, the diagonal lines across the diagram are the enthalpy lines. These lines indicate the enthalpy values for each particular case of air humidity, temperature and water content. A small example will be explained in order to show the use of the Molliere Diagram On any given day, the temperature measured is about 27 C with a relative humidity of 50%. This point is indicated with green at the diagram. Having this data it is possible to read from the x-axis a water content of approximately 11.2 g/kg. In the same fashion it is possible to find out the dew point temperature, by moving downwards until the dew point line and then reading the temperature, as indicated in color orange. The dew point temperature is 15.5 C. Standing once again in the original point (green) if adiabatic cooling is experienced, this mean decreasing the temperature but without any heat transfer, then this point should slide through the same enthalpy line which in this case h = 56 KJ/kg until reaching the dew point line. Then reading once more on the y-axis as shown in yellow, it is possible to read the minimum temperature achievable by adiabatic cooling, which in this case is around 19.8 C. Finally the final amount of absolute humidity can be read on the x-axis with an approximate value of 14.5 g/kg. 17

25 2. Theory Enthalpy line Dew point line Fig. 17 Molliere Diagram [27] 18

26 2. Theory 2.4) Adsorption principle Adsorption refers to the physical process where the addition of particles from a liquid or gas phase to an inner surface of a solid or liquid material takes place. This addition of particles from the gaseous or liquid material brings as a consequence, accumulation in the inner surface of the solid material, thus having a change of concentration in the interphase. Normally this adsorption process occurs between a gaseous and a solid material [12]. Adsorptive Adsorption Desorption Adsorbent Adsorbate Fig. 18 Terms of adsorption [12] Fig. 18 shows the main terms occupied in the adsorption. Adsorbent: the solid material which gathers or collect particles of the liquid of gaseous material Adsorptive: The free particles of the gaseous or liquid material that can be adsorbed by the adsorbent Adsorbate: The particles of the gaseous or liquid material that have been already adsorbed by the adsorbent The speed of this phenomenon depends on speed at which the adsorbent is able to assimilate the energy of the particles of the gaseous material. If this happens too slowly, the gas particles will be repulsed and not all the gaseous particles will be adsorbed. In a gas solid interface two types of adsorption might take place: 1) Physical adsorption (physisorption) a. No structure change in the solid surface b. No activation energy is required c. More than one layer of adsorption can occur d. It is a reversible process e. Adsorbates can move freely over the surface The physisorption depends on the effect of the Van der Waals forces between the adsorbate and the adsorbent. These constant changing forces are extremely weak but abundant in long distances. Due to these forces, adsorption is also possible at the adsorbate layer thus, having more than one adsorption layer. 19

27 2. Theory When one particle approaches the solid material surface, this particle experience two opposite forces. One repulsion force as well as an attraction force. Both forces are dependent on the distance r between the solid material surface and the particle. The repulsion force is proportional to r -12 while the attraction force is proportional to r -6.. The interaction of both forces can be expressed in the following equation: Eq. (2.3.15) This equation expresses the potential energy of the particle and is known as Lennard-Jones potential [12]. The value of enthalpy of physisorption is normally about 20 KJ/mol 2) Chemical adsorption (chemisorption) a. Chemical bonds are formed b. Activation energy is associated c. Only one layer of adsorption d. Most of the time is an irreversible process e. Site specific (no free movement of the adsorbates) In case of the chemisorption, due to the chemical bond, the bonding forces are higher than in physisorption. The adsorption taking place is not at a molecular level but at an atomic level. The normal value of enthalpy of chemisorption is about 200 KJ/mol Desorption is the opposite process of adsorption. In this case the particles that have been adsorbed by the adsorbent are released. In order for this process to happen, it is required to input energy. This amount of energy must overcome the minimum potential energy reached by the particles as they were being adsorbed. Due to the week binding force that is created in the physisorption, the required energy to perform desorption is also low, thus small temperature increments would suffice to eject the particles from the surface. On the contrary, the bonds created in the chemisorption must be broken by means of activation energy due to their higher bonding strength. In the case of the SorLuKo project, the type of adsorption taking place is physisorption since SorLuKo requires a cyclic operation. Chemisorption if irreversible, only allows to perform adsorption once and desorption is not possible anymore once the adsorption was performed. 2.5) Description of the ECOS system As mentioned in the introduction of this master thesis, the ECOS system is an open cycle system which makes use of a solid adsorbent to dehumidify the air. The ECOS system works with two compact heat exchangers. These heat exchangers are divided into adsorptive channels as well as cooling channels. The adsorptive channels are coated with the adsorptive material, as air flows through this adsorptive channels, it gets dehumidified. At the same time this air is being cooled down due to indirect evaporative cooling effect performed by return air being injected with water and then flowing through the cooling channels, which are in direct contact with the adsorptive channels. The air flowing through the cooling channels also cools down the adsorptive material, thus enhancing its adsorption capacity [13]. After some time of operation, the adsorbent will become saturated and will no longer be able to adsorb humidity from the ambient air. This is the main reason why two heat exchangers are involved in the ECOS system. Once one of the heat exchangers is completely saturated the other heat exchanger will start its operation. The heat exchanger will now be heated up in order to experience desorption process until it is 20

28 2. Theory able to remove humidity from the air once again. It is important to mention that after desorption, the heat exchanger temperature has increased, due to the high temperature air used to perform the desorption process. Before starting adsorption operation, the heat exchanger needs to be pre-cooled [13]. The following diagrams explain the working sequence of the ECOS system. Ambient air Room return air Exhaust air Delivery air Fig. 19 Desorption/Adsorption [14] In Fig. 19 the top Heat Exchanger is in desorption mode, while the bottom on one is working in adsorption mode. For the desorption mode, ambient air is heated up and then circulated through the heat exchanger. Then this air is thrown back to the atmosphere. In the meantime the second heat exchanger gets as an input ambient air at ambient temperature, then the dehumidified and cooled air is delivered to the room i question. At the same time the return room air is being injected with water and then circulated through the cooling channels to perform the indirect evaporative cooling on the ambient air flowing on the second heat exchanger. Ambient air Room return air Exhaust air Delivery air Fig. 20 Cooling/Adsorption [14] 21

29 2. Theory In Fig. 20 the top heat exchanger now is under pre-cooling conditions. This is done by circulating ambient air with injected water through the cooling channels and then thrown back to the ambient. The bottom heat exchanger keeps its normal dehumidifying and cooling operation. Ambient air Room return air Exhaust air Delivery air Fig. 21 Adsorption/Desorption [14] Fig. 21 shows the top heat exchanger under adsorption conditions, while the bottom heat exchanger is now under desorption mode in the same way. Ambient air is taken in and then heated up until reaching the desorption temperature. Ambient air Room return air Exhaust air Delivery air Fig. 22 Adsorption/Cooling [14] Fig. 22 shows now top heat exchanger under adsorption operation while the bottom heat exchanger is being pre-cooled. Then the whole operating cycle restarts. With the alternating operation of both heat exchangers, the ECOS System can therefore dehumidify and cool down delivery air continuously. 22

30 2. Theory 2.6) Description of the SorLuKo Project The SorLuKo project is a collaborative project between Fraunhofer ISE and industry partners to develop an air conditioning system aimed to small spaces such as dwellings. The SorLuKo device is basically the integration of one heat exchanger with the ECOS system configuration and evacuated tube solar air collectors. One of the main differences with the ECOS system is that the SorLuKo device only has one heat exchanger. The main reason for this reduction is to minimize the use of space and cost. This means that SorLuKo operates in time intervals. This means that during desorption and cooling there is no input or delivery of cool dehumidified air in the premises. The air inside the house acts as a buffer [15].The heat exchanger is composed of adsorptive channels and cooling channels and has the same phases as the heat exchangers in the ECOS system. Fig. 23 SorLuKo operating phases In Fig. 23 [15] the different phases of the pilot SorLuKo device are shown. Top left shows the desorption phase, top right shows the pre-cooling of the heat exchanger after desorption. Bottom left shows the normal adsorption operation of the system. On the bottom right it s shown another possible application of the SorLuKo system; heating up the dwelling in winter. This is achieved by heating up air and mixing it with ambient air. Afterwards the humidity of this air can be removed adiabatically if required and then delivered into the premises. In this case no room air is sent back to the heat exchanger in order to avoid cooling of the incoming air. 23

31 2. Theory 2.7) Description of the Test Rig The ECOS test rig consists of two main sections; the air pre-conditioning unit and the measurement unit. As defined by its name the pre-conditioning unit is responsible to condition the air to the required conditions and deliver it to the measurement unit in order to perform the measurement of the heat exchangers. The pre-conditioning unit is capable to increase the temperature of the air, when required, it can also increase the humidity of the air and is able to deliver air at different mass flows. In order to increase the temperature, the pre-conditioning unit is equipped with electric resistances and with humidifiers for increasing the humidity to the desired value. The air used by the pre-conditioning unit comes directly from the ambient and the unit is not equipped with a proper cooling system or an air dehumidifying system. This means, that whenever the ambient air temperature and/or humidity are higher than required, it is not possible to perform measurements. The pre-conditioning unit is responsible to deliver conditioned air for 3 different scenarios. One scenario is the one for the air coming from the simulated environment which goes inside the heat exchanger through the Sorption side, in order to be dehumidified and cooled down, to be delivered into the dwelling. The second set of conditions for the simulated air comes from the air that is being extracted from the room. This is air is the one that goes inside the heat exchanger through the wet side of the heat exchanger. Finally the third set of conditions is the one for performing the desorption of the heat exchanger. The temperature required for performing the desorption is high. The air is reheated with the electric resistances until the desired temperature is reached and then delivered to the measurement unit. Fig. 24 Pre-conditioning Unit The second section of the ECOS test rig is the measurement unit. In this unit is where the heat exchanger is mounted as well as the sensors in charge of doing the for the heat exchanger characterization. The measurement section of the test rig is designed in such a way that it works as a plug & play device. This means that mounting and dismounting a heat exchanger in order to perform the measurements is quite easy and it is not time consuming. One heat exchanger is removed and the other heat exchanger is installed and ready to perform measurements without the need of disassembling the test rig. 24

32 2. Theory Fig. 25 Simple mounting/demounting of the HE The pre-conditioning unit is connected with the measurement unit by pipelines as shown in Fig. 26. Fig. 26 Pipelines connecting the Measurement Unit Since the heat exchangers are cross flow heat exchangers, they have two inlet sides. From the right side of the heat exchangers the ambient air that is going to be dehumidified and cooled down as well as the air performing the desorption flow inside the heat exchanger. This side is the sorption side. From the top the exhaust air flows inside the heat exchanger. This side is the wet side. On upper side of the pipe a water sprayer is installed in order to humidify the air and perform the evaporative cooling. The measurement section is equipped with different sort of sensors. The main parameters to be measured in the test rig are: Temperature Relative humidity Differential and absolute pressure Dew point temperature 25

33 2. Theory Fig. 27 Schematic of the Measurement Unit [16] Fig. 27 is a schematic of the measurement section of the ECOS test rig. Fig. 27 also shows the location of some of the most relevant sensors. For measuring the inlet and outlet temperatures on both sides PT100 thermocouples are used. Other sensors of remarkable importance are the dew point temperature sensors. The value measured by these sensors is necessary in order to calculate pressure values as it will be further explained. The measurement section is equipped with two different types of dew point temperature sensors. The ones installed in the inlet and outlet side of the Sorption side are capacitive dew point sensors. The dew point temperature sensor from the wet side inlet is a dew point mirror sensor. The working principle for both types of sensors is the same. The sensors lower their temperature until the water vapor contained in the air is condensed. The difference between the mirror sensor and the capacitive sensor lies on how this condensation is detected. The mirror dew point sensor works with a light transmitter, light receiver and a mirror. The receiver will always get certain amount of light reflected on the mirror while it is clean and clear. As the mirror lowers its temperature until reaching the dew point temperature, water vapor will condense on it. The light intensity in the receiver will change. This is when the sensor indicates the dew point temperature of the air. 26

34 2. Theory Fig. 28 Working principle dew point mirror sensor [17] In the case of the capacitive dew point sensors; as the temperature decreases and the water vapor is condensed, the electrode capacitance is affected. When this capacitance change the dew point temperature is registered. Fig. 29 Working principle of the Capacitive dew point sensor [18] In Appendix 2 Table with all the specs of the Sensors mounted in the Measurement Unit is included. In order to perform the evaporative cooling on the Wet side, a water sprayer was installed Water is being pumped all the way up to the inlet of the Wet side and then sprayed into the air. By doing this the incoming air reduces its temperature and it is possible to extract a larger amount of heat from the sorption side. Fig. 30 Water pump & sprayer 2.7a) Test rig operation sequence The ECOS test rig needs to condition air at the desired conditions as well to perform the measurements of different parameters by the use of sensor installed in the rig. As any other equipment that needs to have constant operation conditions, a control system needs to be implemented as well as an interface to interact with the user. 27

35 2. Theory In the case of the ECOS test rig. There are two possible ways to interact with the ECOS system. One is directly with a touch screen and the other one is by doing a remote connection to the touchscreen with the Mappit computer. The software installed in the ECOS test rig is called Remus and is divided in 3 main blocks. One block is in charge of acquiring the data from the sensors, other block is in charge of the control of the rig and the third section is in charge to bind the previous sections together. This configuration is needed, because the acquisition block can only read values at a speed of 5 seconds, while the control block can work perfectly at a speed of 1.25 seconds. The function of the binding block is which can also work at a speed of 1.25 seconds, is to do 4 readings and then obtain the sensor values from the data acquisition block. Fig 31 Touch and Mappit Interfaces Remus is also used as an interface to implement Control software. In this case the software used is Sequencer. Sequencer is used with the Mappit PC. The main objective of Sequencer is to control valves, fans as well as to perform PID control to reach the desired values for air conditioning Sequencer allows to automate a whole measurement sequence where sorption, desorption and cooling are performed. Fig. 32 Sequencer Interface Fig. 32 shows the interface of the Sequencer software. Sequencer runs in the Mappit PC and it has 6 main windows. From left to right and from top to bottom, the first window is the sensor window. This window shows all the values being measured at real time. The second window is the actor list. In this window is possible for the user to control the opening or closing of valves, as well as speed of fans. The PID list shows 28

36 2. Theory the controllers for every of the input parameters. The user can input values for the PID controllers. The module list shows if which controllers and alarms are on and off. The user parameter window is the most important. In this window the user inputs the desired values of temperature, humidity and mass flow. It is also possible to run a complete cycle sequence of absorption, desorption and cooling with time periods of time defined by the user as well. The main sensors that come into play are the temperature, pressure and the dew point temperature sensors. These types of sensors are the ones that will provide inputs for all the functions used to make the calculations. As the test rig is on, one of the first inputs required is the dew point temperature provided by the different dew point temperature sensors. This temperature is an input for Eq. (2.3.5). By using these temperatures, as stated in Eq. (2.3.7) the value obtained is the partial pressure of water vapor. Once this value is obtained, the following step is to calculate the density of the humid air. For this calculation Eq. (2.3.17) is required. This function is dependent of 3 parameters: Absolute pressure, temperature and partial water vapor pressure. Absolute pressure and temperature are input values from sensors installed in the rig and the water vapor pressure was the value previously calculated. The density of the humid air allows now to calculate the mass flow. The test rig is equipped with two different types of mass flow sensors: One is manufactured by Schmitz & Partner GmbH and is an orifice plate flow meter. The second type is manufactured by HALTON KLIMATECHNIK GmbH. For the orifice plate flow meter the following equation is used [19]: Eq. (2.7.1) Where C is the discharge coefficient and is the ratio between the actual volumetric flow over the ideal volumetric flow. In this particular meter C= , which is the orifice diameter over the pipe diameter Y is the expansion factor which for this particular case Y= d 2 = which is the orifice diameter. Finally Δp is measured by a sensor in the test rig and ρ h was calculated in the previous step. The second type of mass flow sensor converts the differential pressure by using a deduction of Bernoulli equation where 2 variables A and B are unknown Eq. (2.7.2) For both sensors the A and B values were obtained after calibrating the sensor using the orifice plate flow meter as a reference Once the mass flow is calculated, the following step is to calculate the absolute humidity. This is done using Eq. (2.3.12) or Eq. (2.3.13) depending if the saturation pressure is calculated with a temperature sensor or a dew point sensor respectively. The absolutes pressures are sensed and the saturation pressures were previously calculated. The test rig also counts with a relative humidity sensor for using Eq. (2.3.12). The next step is to calculate the mass flow of dry air using Eq. (2.3.9) where the value of the m h was calculated with the mass flow meters and the absolute humidity has been already calculated as well. 29

37 2. Theory The following step is to obtain the specific enthalpy of the inlet and outlet side of both sides of the heat exchanger. For doing this Eq. (2.3.21) is applied. In this equation the temperature values come from the sensors installed in the inlets and outlets and the absolute humidity has been already calculated. The rest of the values are constant values. Finally the heat transfer can be calculated by for both sides of the heat exchanger which theoretically should be equal assuming an ideal heat transfer. Once the measurements are done, it is also required to perform a post processing of these measurements. Once the connection between the Mappit and the Touch interfaces is done, a file with all the raw data is created in Mappit. This file includes all the measured values since the moment the test rig was on. Since only a specific period of time is of interest, it is required to make the selection of the specific period of time and post process the values for the calculation of the final values. In order to perform the time period selection an access based file with name Rcontrol is used. Fig. 33 Access based RControl file The function of these file is to specify the post processing to be performed in the a desired period of time. The inputs required from the user, are the date at which the file to be post processed was created. The type of the post processing to be perform; in this thesis work only the options of Heat transfer (Wärmeübertragung) and Indirect evaporative cooling (Verdunstungskühlung), the name to be given to the file with the post processed data and finally the time interval to be analyzed. Once all the required information is input, the execute button is pressed. By doing this a script file written in R programming language is created. The post processing is performed by several scripts written in R programming language. There is a script for each of the different post processes, the condition script created with the access based file and the main R script which calls all of the different scripts. The condition script has the function to specify what type of script will be called, in which folder the results will be saved, which period of time will be processed and name of the file. Once the condition file is created the final step is to run the main script. This is done with a R Graphic User Interface. 30

38 2. Theory Fig. 34 R Console (GUI) Once the main script was run succesfully the tables and graphs containing the post processing results are saved in the specificied folder. Fig. 35 Typical result graphs 2.7b) Test Rig Remarks Weather plays an important role for performing the measurements. The temperature from the Wet side is always constant and it is of 26 C. During the summer, ambient temperatures above 30 C are reached. The air used in the ECOS system is taken from the ambient. Unfortunately the ECOS unit is not equipped with a proper cooling system, so when the ambient temperature is too high it is not possible to decrease the air temperature to 26 C and eventually no proper characterization measurements can be done. In order to perform measurements with evaporative cooling, the humidity becomes a parameter to be considered as explained previously. The desired absolute humidity in any case is 10 g/kg. The preconditioning air unit is able to increase the humidity of the air but the unit is not equipped with any sort of dehumidifier. As a consequence, when the relative humidity of the ambient air is higher as the one desired, it is also not possible to perform any proper measurements. These two factors should be considered at all time in order to avoid delays. 31

39 3. Heat exchangers The ECOS test rig as any other data acquisition rig, is equipped with sensors and a control system. In order to perform valid measurements the sensors need to work accordingly and need to be properly calibrated. It is of high importance to monitor at all time, that there are no leakages in any joint or sensor mounting of the ECOS system. Having a leakage would increase the losses and the obtained values would not be valid. When dismounting and mounting a heat exchanger, it is imperative to make sure that the joints between the heat exchanger and the ECOS systems are completely shut to avoid any leakage. 2.7c) Uncertainty calculations In any experiment where measurements are involved, a validation of the data must be made. Errors while performing the measurements will always exist, regardless how carefully these measurements are made. Sometimes these errors can be of random nature and sometimes are product of a human error [19]. Certain type of errors is the experimental errors. This type of error cannot be corrected and there will be always experimental errors. Having these errors will always cause uncertainty in the measurements. It is then required to determine how uncertain the measurements can be. Experimental uncertainties might be defined as the possible value the error may have [19]. As calculations are done with measured values, the result from these calculations will also present an uncertainty, consequence of the uncertainties of the measured values with which the calculations were made. The Kline and McClintock method calculates the uncertainty of experimental results when the uncertainties on the primary measurements are known [19]. The result of a given function of several independent variables is given: Eq. (2.7.3) When the uncertainties of each of the independent variables are known, the total uncertainty can be calculated as follow: [( ) ( ) ( ) ] Eq. (2.7.4) Where is the uncertainty in the result and,, are the uncertainties of each independent variable. Appendix 3 shows the total uncertainty of a heat transfer calculation. 3. Heat exchangers 3a) Description of the Klingenburg heat exchanger The Klingenburg heat exchanger is a commercial cross flow plate heat exchanger produced by Klingenburg. This particular heat exchanger was coated with sorptive material by Sortech AG. The heat exchanger is built with 0.15 mm thick aluminum. In this particular model of heat exchanger, the gap between the plates is achieved by piling up embossed aluminum plates and placing them in alternating order. 32

40 3. Heat exchangers This heat exchanger consists of channels with a gap of 4.5 mm before coating in each one of them. The length of the embossed plates is 400 mm. The channels of the heat exchanger are done by folds at the edges of the plates. Approximately 1 cm is used to make the folds. The active contact area of the heat exchanger after the folds is of m 2 [20]. Fig. 36 Embossed plates (left) [21] & Klingenburg HE (right) [20] The coating of the Klingenburg heat exchanger was done all at once. First, all of the surfaces of the heat exchanger were prepared for the epoxy resin powder coating, by means of removing the grease from the surfaces then a chromate free treatment and finally a drying process. This chromate free treatment, done with an acid solution, reflects an improvement for the adhesion between the aluminum substrate and the epoxy resin powder. Once the plates have been coated with the epoxy resin powder, silica gel in form of spheres is added to the coating. This bonding takes place at a temperature of 150 C Fig. 37 Silica gel spheres (left) & Embossed plate (right) [20] Material Mass [Kg] c p [KJ/KgK] C [KJ/K] Aluminum Heat Exchanger Silica gel Total weight Table 1 Specs of the Klingenburg Heat Exchanger [22] 33

41 3. Heat exchangers 3b) Description of the Haugg I heat exchanger The Haugg heat exchanger is a bar-plate compact cross flow heat exchanger built by Haugg Industriekühler GmbH. The bar-plate configuration consists on an arrangement of alternate wavy fins stacked together. In order to separate the wavy fins that correspond to the adsorption side and the water side, bars at the sides of the heat exchanger are installed and then a plate is welded on top of it in order to separate the fins from the adsorption side and the cooling side. Wavy fins Plate Bar Fig. 38 Bar-plate compact heat exchanger [23] This particular heat exchanger is made out of aluminum and has a total weight of 28.1 Kg. The external measurements for the Haugg heat exchanger are 400x400x400 mm. The measurements of the channels network reduce to 330x380x388 mm due to the space used by the frame and bars. On the ambient air side the fins are corrugated, have a thickness of 0.2 mm and a height of 10.2 mm. The distance between each fin is relatively high, due to the fact that this side of the heat exchanger will be coated with sorptive material. The total transfer area of the ambient air side is about m 2 On the side of the exhaust air where the evaporative cooling takes place the fins are smaller. The fins have a height of 6.3 mm and a thickness of 0.3 mm. These fins are also corrugated and have a distance between each other of about 7.5 mm. The total transfer area of the exhaust side is about m 2. In [24] the heat transfer coefficient of the heat exchanger, the transfer capacity, the efficiency and the cooling capacity are calculated with an inlet temperature difference of 6 K. Parameter Value Mass 28,1 Kg. External Measurements 400x400x400 mm Channels network measurements 330x380x388 mm Ambient air transfer area m 2 Exhaust air transfer area m 2 Heat transfer coefficient W/m 2 K Efficiency 56.7% Cooling capacity (ΔT 6K) W Table 2 Specs of the Haugg I Heat Exchanger [24] 34

42 3. Heat exchangers 3c) Description of the Haugg II heat exchanger The Haugg II heat exchanger is built under the same concept as the Haugg I heat exchanger. It is as well a bar plate compact cross-flow heat exchanger. The materials used for its constructions are as well the same, which in this case is aluminum. The Haugg II also has the same dimension as the Haugg I heat exchanger. These similarities in dimensions and materials show that the weight and the areas are the same for the Haugg II. The only difference with the Haugg II heat exchanger is that the cooling fins are not continuous but slotted and have an offset of approximately 2mm from each other. Fig. 39 Front view of the cooling side showing the offset The main purpose of this configuration is to create a more turbulent flow which might improve the mixture between the room air coming out and the water with which this air is being humidified for performing the indirect evaporative cooling. One of the purposes of the measurements realized to the Haugg II heat exchanger is to find out if this slotted cooling fins will increase the cooling capacity or not. Parameter Fig. 40 View of the slotted cooling fins Value Mass 28,1 Kg. External Measurements 400x400x400 mm Channels network measurements 330x380x388 mm Ambient air transfer area 22.9 m 2 Exhaust air transfer area m 2 Heat transfer coefficient W/m 2 K Efficiency 67.5% Cooling capacity (ΔT 6K) W Table 3 Specs of the Haugg II Heat Exchanger [24] 35

43 4. Measurements & Results 4. Measurements & Results 4a. Dry heat transfer measurements In this chapter, the results of the measurements done to each of the heat exchangers will be displayed. In order to be able to perform a valid comparison, general conditions under which all of the heat exchangers will be measured need to be established. The heat exchangers will be characterized in two modes of operation. The first mode of operation will be dry heat rejection. The heat transfer is calculated with the regular heat transfer equation: Eq. (4.1) For performing the characterization of dry heat rejection of the heat exchangers Table 4 shows the scenarios under which the measurements were done. Measuring Scenarios Inlet Temperatures Mass flows Ambient air side (Sorption side) Exhaust air side (Wet side) Ambient air side (Sorption side) Exhaust air side (Wet side) 450 kg/h 450 kg/h 71 C 26 C 350 kg/h 350 kg/h 250 kg/h 250 kg/h 450 kg/h 450 kg/h 65 C 26 C 350 kg/h 350 kg/h 250 kg/h 250 kg/h 450 kg/h 450 kg/h 53 C 26 C 350 kg/h 350 kg/h 250 kg/h 250 kg/h 450 kg/h 450 kg/h 38 C 26 C 350 kg/h 350 kg/h 250 kg/h 250 kg/h 450 kg/h 450 kg/h 32 C 26 C 350 kg/h 350 kg/h 250 kg/h 250 kg/h Table 4 Characterization Scenarios In order to obtain values from the measurements, the data that was registered during the measurement needs to be post processed. This is done, by selecting an interval of time, in which the desired scenario temperatures as well as the mass flows reach a constant stage. Once this period of time is selected, with the use of scripts written in R language the calculations can be done. The main function of these scripts is to perform linear regressions for the whole values registered in the selected period of time and then perform the calculations of heat transfer, efficiency as well as NTU. 36

44 4. Measurements & Results Fig. 41 Heat transfer of each of the 3 HE with 3 different mass flows Fig. 41 shows the performance of the three different types of heat exchangers with 3 different mass flows and different temperatures. At plain sight it can be seen, that the Haugg I heat exchanger is the one that has a higher heat transfer for every case. It can be also observed, that at the lowest inlet temperatures difference (32-26 C) the heat transfer of all the heat exchangers do not vary that much in comparison with the other cases. In [24] the calculation of the cooling capacity of the Haugg I heat exchanger by the ε-ntu method as performed. Table 3 shows the result of both, the calculated cooling capacity as well as the measured one. 37

45 4. Measurements & Results Cooling Capacity (32-26 C & mass flow 450 kg/hr) Calculated cooling capacity W Measured cooling capacity W Table 5 Comparison between calculated and measured values The similarity in the heat transfer for the 3 heat exchangers at the lowest Sorption side inlet temperature is based on the fact, that the temperature differences from the inlet and the outlet sides of the heat exchangers are quite low. This means, that the losses are as well diminished as it will be shown in the following graphs. Fig. 42 Heat transfer losses from the HE with 3 different mass flows 38

46 Haugg II Haugg I Klingenburg 4. Measurements & Results In Fig. 42 the heat losses from the three Heat exchangers can be appreciated. Heat exchangers Haugg I and Haugg II have quite a similar behavior, regardless the mass flows. An odd behavior can be appreciated with the Klingenburg heat exchanger. Both Haugg heat exchangers have always a decrease in the heat losses as the mass flow is being reduced. In the case of the Klingenburg heat exchanger, it is not the case. In most of the cases the heat losses increase and in some others decrease. Mass flow 450 kg/hr--->350 kg/hr Mass flow 350 kg/hr--->250 kg/hr Q SoS Q WS Q loss Q SoS Q WS Q loss C 19,38 22,50 15,28 24,24 27,61-138, C 18,80 22,50-7,87 24,14 28,08-120, C 18,99 22,87 56,22 21,07 27,91-366, C 19,33 22,27-237,50 24,62 27,86-40, C 23,07 26,09 77,87 21,63 27,42-172, c 19,42 22,18 58,36 24,69 29,08 59, C 19,05 21,73 57,10 25,31 29,16 75, C 18,72 21,71 58,16 25,09 29,11 75, C 19,46 21,51 57,71 24,77 28,69 39, C 19,26 21,17 58,15 27,86 31,44 34, c 17,72 20,51 35,74 22,14 27,59 65, C 17,49 20,79 41,85 23,51 27,70 65, C 18,17 20,98 40,78 24,06 27,67 62, C 17,81 20,63 41,99 23,72 27,71 70, C 23,40 19,92-8,15 18,19 28,13 84,97 Table 6 Percentual reductions Table 6 shows the percentual reductions of the heat transfer for both, sorption and wet side as well as for the losses. In most cases the values are extremely similar with the different temperature intervals for all of the heat exchangers. Haugg I and II heat exchangers have reductions on the losses as the mass flows are reduced as shown in the table. In the case of the Klingenburg the losses increase in almost every case. Looking at the values and the similarity between them, it can be stated that the measurements were done properly with no external heat losses like leakages. The odd behavior of the Klingenburg heat exchanger might be consequence of its design. It is important to mention that the Klingenburg heat exchanger is coated with silica gel beads on the ambient air side, (Sorption side) which of course have an influence in the heat transfer property, while the Haugg I and II have no coating at all in the surface. It is possible to do the calculation of the heat transfer coefficient U by making use of Eq. (2.2.23). 39

47 4. Measurements & Results Fig. 43 Calculated Overall Heat transfer coefficient (left) Effectiveness (right) Fig. 43 shows how the overall heat transfer coefficient for the Sorption side behaves, depending on the inlet temperature. This heat transfer coefficient has a direct influence in the effectiveness of the heat exchangers as it is shown in the right side of Fig. 43 From Fig. 43 can be appreciated, regardless the scenario, the overall heat transfer curve for all heat exchangers is the same, having just an offset depending of the mass flow. The Haugg I heat exchanger has the highest overall heat transfer coefficient. Heat exchangers Haugg II and Klingenburg have a similar behavior except when the inlet temperature of sorption side is 53 C. In this particular case, the Klingenburg heat exchanger shows its highest overall heat transfer coefficient. The Haugg I heat exchangers shows its highest overall heat transfer coefficient at a sorption side inlet temperature of 71 C at any given mass flow while the Haugg II has its highest overall heat transfer coefficient at an inlet temperature of 65 C on the sorption side on any given mass flow as well. In relation to the effectiveness, in general can be appreciated; that the curves of each of the heat exchangers differ depending on the mass flow. It is worth noticing that for a mass flow of 350 kg/hr, the effectiveness for the Haugg I and the Klingenburg heat exchangers is constant except for an inlet temperature on the sorption side of 32 C which for both heat exchangers decreases. The Haugg I heat exchanger has the highest effectiveness in all cases, followed by the Haugg II heat exchanger and the one with the lowest effectiveness is the Klingenburg heat exchanger. The highest effectiveness for the 3 heat exchangers is appreciated at a mass flow of 250 kg/hr. The reason of this behavior is that with a mass flow of 250 kg/hr all the heat exchangers experience a higher temperature difference on the sorption side which in accordance to Eq. (2.2.20) makes complete sense to get a higher effectiveness value. 40

48 4. Measurements & Results 4b. Evaporative heat rejection The second mode of operation for the measurements includes the indirect evaporative cooling. This mode is achieved by spraying water into the air entering the heat exchanger on the Exhaust air side (Wet side). The purpose of spraying water into the air, is that this water will evaporate, thus absorbing heat from the air and reducing the dry air temperature. It is important to mention that the amount of heat content in the air does not reduce; it just changes its nature into latent heat (evaporation) and since no heat is being transferred this process is considered an adiabatic process. As shown in section 2.3a in the Molliere Diagram, dry air at certain temperature with certain humidity can be cooled down adiabatically until certain temperature, where it reaches a 100% of relative humidity, this concept previously defined as wet bulb temperature. Due to the fact that the temperature going in the heat exchanger on the Exhaust side (wet side) the calculations differ. Mainly for calculating the efficiency; Eq. (2.2.20) considering that since its air flowing on both side of the heat exchanger and consequently the heat capacity rates are the same, turns into the following equation: Eq. (4.2) In Eq. (4.2) the denominator represents the largest difference between temperatures and when water is being sprayed to the incoming air from the cold side the temperature to consider will be the wet bulb temperature, turning Eq. (4.2) into: Eq. (4.3) For the measurements with indirect evaporative cooling, it is also important to take into account the humidity of the incoming air. The wet bulb temperature that can be achieved depends also from the humidity of the air, thus in the scenarios for these measurements, humidity on the Exhaust air side (wet side) is also kept under control. Table 7 shows the scenarios under which the measurements with evaporative cooling were done. Measuring Scenarios Inlet Temperatures Mass flows Inlet absolute humidity Ambient air side (Sorption side) Exhaust air side (Wet side) Ambient air side (Sorption side) Exhaust air side (Wet side) Ambient air side (Sorption side) Exhaust air side (Wet side) 53 C 26 C 450 kg/h 450 kg/h Ambient 10 g/kg 250 kg/h 250 kg/h Ambient 10 g/kg 32 C 26 C Table 7 Characterization scenarios with Evaporative Cooling 450 kg/h 450 kg/h Ambient 10 g/kg 250 kg/h 250 kg/h Ambient 10 g/kg As done with the dry heat transfer measurements, in order to make the calculations for the indirect evaporative cooling, the measured data needs to be post processed with a particular script. This script calculates heat transfer and effectiveness. 41

49 4. Measurements & Results Fig. 44 Heat extracted from Sorption side Fig. 44 shows the amount of heat extracted from the Sorption side. In comparison with the dry heat transfer, the behavior is almost the same. The Haugg I heat exchanger has the highest heat transfer followed by the Haugg II and at last the Klingenburg heat exchanger. Fig. 45 Heat transfer comparison 42

50 4. Measurements & Results Fig. 45 is a comparison between heat transfer with and without indirect evaporative cooling. It can be appreciated clearly, that indirect evaporative cooling increases the heat transfer remarkably. For both mass flows, with an inlet temperature of 53 C on the sorption side, the heat transfer increases in approximately 60% in all of the heat exchangers. The most noticeable increase for all the heat exchangers takes place when the inlet temperature on the sorption side is 32 C for both mass flows. The increase in heat transfer is about 190%. This increase in the heat transfer for all cases is the effect of the larger temperature difference on the sorption side, which directly increases the amount of heat transferred. As a matter of fact, the percent increase in the temperature difference is directly the same as in the heat transfer. The second parameter of interest in the effectiveness reached when indirect evaporative cooling is applied. Fig. 46 Effectiveness with indirect evaporative cooling In Fig. 46 the effectiveness for both mass flows is shown. The Haugg I heat exchanger has the highest effectiveness for any case. The Haugg II has a higher performance than the Klingenburg heat exchanger, except for the case of an inlet temperature on the sorption side of 32 C at a mass flow of 450 kg/hr. The effectiveness, as well as in the dry heat transfer measurements, increases as the mass flow decreases. 43

51 4. Measurements & Results Fig. 47 Effectiveness comparison Fig. 47 shows the comparison between the effectiveness of all heat exchangers in dry heat transfer as well with indirect evaporative cooling. As expected, when indirect evaporative cooling is applied the effectiveness in all cases increase. For the case of the evaporative cooling the proper equation to be used to calculate the effectiveness is Eq. (4.3) where the denominator change; instead of having the temperature difference between the inlet temperatures of both sides, now the temperature of the Wet Side is substituted by the Wet bulb temperature. In order to find out the wet bulb temperature, a Molliere Diagram can be used. The values input for the measurements are an inlet temperature of 26 C and an absolute humidity of 10 g/kg. In the Molliere Diagram the absolute humidity is represented in the horizontal axis. Knowing the temperature and the absolute humidity is possible to find the relative humidity, which is represented with the curve lines. In this case the relative humidity is about 57%. By spraying water into the air, the temperature of the air drops adiabatically, this means no heat transfer, thus no change in the enthalpy. Enthalpies are represented on the Molliere diagram by diagonal lines. In order to find out the Wet bulb temperature, it is necessary to move from the crossing point between temperature and x in parallel with the enthalpy lines all the way to the line of a 100% relative humidity. Then it is possible to read a new temperature on the vertical axis. This temperature is the wet bulb temperature. Fig. 48 Wet bulb temperature reading 44

52 4. Measurements & Results Fig. 48 shows the wet bulb temperature that was found with the known input values. In this particular case, the wet bulb temperature is 18 C. This means that in Eq. (4.3) the denominator will increase in value by 8 C, which is the number of C that are reduced by spraying water into the air. This would mean that the effectiveness values would decrease if the temperature in the nominator would stay the same as in the dry operation mode. Since the cooled air is extracting way more heat as well, the temperature difference in the nominator also increases thus having an overall increase in the effectiveness of the heat exchangers when indirect evaporative cooling is applied. 45

53 5. Conclusions 5. Conclusions In this master thesis work, the characterization of 3 different heat exchangers was performed. The characterization was done for dry heat transfer and with indirect evaporative cooling. The heat exchangers that were analyzed were a Klingenburg heat exchanger and the Haugg I and Haugg II heat exchangers. The measurements done in order to characterize the heat exchangers were done on the ECOS test rig. To perform the measurements in this rig takes a considerable amount of time due to the fact that stable conditions need to be reached. In the case of the Klingenburg heat exchanger, more time is required to perform the measurements because of the sorptive coating. It is required first to remove all the humidity from the sorptive material in order to have valid values when the heat transfer is measured. The ECOS test rig as any other data acquisition rig, is equipped with sensors and a control system. In order to perform valid measurements the sensors need to work accordingly and need to be properly calibrated. The results obtained from the dry heat transfer measurements show that the heat exchanger with the best performance is the Haugg I heat exchanger. This performance involves the amount of heat transferred, the losses as well as the effectiveness of the heat exchanger. Regarding the amount of heat transferred, the Haugg II heat exchanger seems to perform better than the Klingenburg heat exchanger, on the other hand the losses of the Haugg II heat exchanger were remarkably higher than in the case of the Klingenburg and the Haugg II heat exchanger. Only in the particular scenario of 250kg/hr mass flow, the losses of the Klingenburg heat exchangers are higher than the losses form the Haugg II heat exchanger. This behavior does not follow the behavior from the other 2 scenarios, but as it was shown in the result section, the Klingenburg heat exchanger has a really odd behavior regarding the losses. This behavior is assumed to be a consequence of its own design as well as the coating from the sorption side. The results of the measurements done to the Haugg I heat exchanger were compared to the results obtained from calculations done with the ε-ntu method. The conditions taken into consideration for the ε-ntu method were an inlet temperature difference of 6 C (32 C-26 C) and a mass flow of 250 kg/hr. The results of total cooling capacity in both cases were extremely similar. On the other hand, the calculations done for the Haugg II differed completely from the acquired values from the measurements. In the theoretical calculations, the Haugg II was supposed to have a better performance as the Haugg I. The results obtained from the measurements showed that ε-ntu method might not be as precise, when the behavior of the fluid is not entirely known. With the offset in the cooling thins it was intended to create certain turbulence on the Wet Side and improve the mixture between the incoming air on the wet side and the water that is being sprayed. The overall heat transfer coefficient was calculated for all of the heat exchangers and for all of the different mass flows. The overall heat transfer coefficient curve of each heat exchanger behaves exactly the same. There is only a displacement on the Y axis, which is consequence of the variation in mass flow. Regarding effectiveness the Haugg I heat exchanger is the one with the highest effectiveness for every mass flow. It is also the heat exchanger with the highest overall heat transfer coefficient. The behavior of the effectiveness curves from the heat exchangers is not the same. The maximum efficiency for every heat exchanger is reached with the lowest mass flow due to the larger temperature difference achieve in the sorption side. 46

54 5. Conclusions To perform heat transfer measurements with indirect evaporative cooling the absolute humidity of the incoming air on the wet side needs to be monitored and controlled. The same absolute humidity is required for all of the heat exchanger in order to do a valid performance comparison. The amount of heat removed by applying indirect evaporative cooling increased remarkably. In the cases where the inlet sorption side temperature is 32 C the heat removed is almost twice as the one without indirect evaporative cooling. The effectiveness for all heat exchangers increased as well. Haugg I heat exchanger showed the highest effectiveness followed by the Haugg II and at last the Klingenburg heat exchanger. The effectiveness behavior for indirect evaporative cooling is the same as the dry heat transfer measurements; the effectiveness of all heat exchangers increase as the mass flow decreases. The measurements done to the Haugg I and Haugg II heat exchangers show that the offset in the cooling fins of the Haugg II heat exchanger do not increase the performance of the heat exchanger. The wavy fins proved to perform better in terms of heat removal. In general from all the measurements done, Haugg I heat exchanger is the one with the best performance. The Haugg I would be ideal for operation in the SorLuko Project. It is important to mention that Haugg I does not have any sort of sorptive material on the Sorption side. In order to be suitable for operation in the SorLuko project, the Haugg I heat exchangers need to be coated with a sorptive material for dehumidification. Applying a coating would require the heat exchanger to be characterized once more in order to find out how its performance is affected by the sorptive coating. 47

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57 Appendix Appendix Appendix 1 ECOS Test rig Scheme 50