A SOLAR LIQUID-DESICCANT AIR CONDITIONER

Transcription

1 A SOLAR LIQUID-DESICCANT AIR CONDITIONER Andrew Lowenstein AIL Research, Inc. P.O. Box 3662 Princeton, NJ ABSTRACT A new technology for cooling and dehumidifying buildings that uses liquid desiccants is now entering the U.S. market. This technology, which can efficiently serve large latent loads, will greatly improve indoor air quality by both allowing more ventilation as well as more tightly controlling humidity. Furthermore, since a liquid-desiccant air conditioner can be packaged as a roof-top air-handler, it will compete directly with the most popular cooling system now being used in the U.S. electric DX roof-top systems. Early applications for the liquid-desiccant air conditioner will be as a thermally activated cooling system for processing ventilation air to buildings in humid climates. For systems that are gas-fired, the total ownership costs can be reduced by adding solar thermal collectors. In roof-top installations the collectors can be located near the air conditioner to simplify installation and reduce costs. Energy storage can be done with concentrated desiccant rather than hot water, further reducing installation costs. At 2002 U.S. energy prices and government subsidies for solar technologies that are now available in some U.S. states, a solar-powered liquid-desiccant air conditioner that uses single-glazed flat plate collectors could pay back the installation costs for its solar components in as little as eight years. In the longer term, advances both in solar collectors and liquid-desiccant regenerators will lead to a hightemperature system that could payback in less than eight years without government subsidies. Key Words: solar cooling, solar air conditioner 1. INTRODUCTION For the second time in three winters prices for fossil fuels have been extremely volatile. In February 2003, the U.S. market price for natural gas exceeded $9.70 per million Btu. Although this price is less than the $10.53 per million Btu reached in December 2000, it is three times higher than the February 2002 price. And, as noted by the U.S. Energy Information Administration, [a] major consideration for energy markets through 2025 will be the availability of adequate natural gas supplies at competitive prices to meet growth in demand 1. High prices and availability are not the only energy issue now facing the world. Climate change brought on by the burning of fossil fuels is a reality. Only the magnitude of the change remains debated. A cooling system powered by solar thermal energy could be an important part of a renewable solution to the preceding problems. With the availability of solar insolation coinciding roughly with cooling loads, solar energy appears to be an ideal energy source for an air conditioner. However, the technical challenges of developing an efficient heat pump that runs on low-temperature thermal energy and can deliver cooling when the energy source is not available have been formidable. Most past work on solar cooling has used absorption chillers to convert solar thermal energy into cooling. Work on solar/absorption systems has progressed steadily since the 1970s. The earliest work used single-effect absorption chillers. More recent demonstrations have adapted doubleeffect chillers to run on either solar energy or fossil fuel. Unfortunately, solar-powered absorption chillers will always have problems competing in the broad HVAC market. About 95% of the commercial buildings in the U.S. use packaged air conditioners, the majority of these systems being roof-top units. Roof-top systems are relatively small (most under 50 tons) and they match naturally to the air distribution system found in most buildings. In the U.S., gas-fired absorption chillers already have trouble competing in this market. Replacing natural gas with a solar energy source will not improve their competitiveness. A new generation of liquid-desiccant air conditioners (LDACs) is now coming to the market that is designed to

2 compete against roof-top systems. This technology, which is thermally activated, offers new opportunities for commercializing a solar cooling system. 2. A LOW-FLOW LIQUID-DESICCANT AIR CONDITIONER Perhaps the single biggest obstacle to moving liquiddesiccant technology from its secure base in industrial applications into the HVAC market has been the high maintenance requirements for the technology. The most common configuration for a liquid-desiccant conditioner (i.e., the component that dries and cools the process air) is a bed of porous contact media that is flooded with desiccant. The process air moves through this bed and is dried and cooled as it comes in contact with the desiccant. The process air that flows through a flooded packed-bed conditioner will entrain small desiccant droplets. These droplets are removed by a mist eliminator, i.e., a filter for liquid droplets. In industrial applications, these mist eliminators will be maintained to insure that desiccant droplets do not carryover downstream. However, it is unlikely that industrial maintenance procedures will be widely accepted in HVAC applications. In work funded by the Gas Research Institute, Lowenstein and Gabruk 2 studied a novel technology that could move liquid-desiccant systems from their industrial base into the broader field of comfort conditioning. The most dramatic departure from the prior art is the use of very low flow rates of liquid desiccant on the contact surfaces of the conditioner: flow rates that are one-tenth to one-fiftieth the levels in industrial systems. Low desiccant flow rates can be effective only if the desiccant is continually cooled as it absorbs water vapor from the process air. Thus, a low-flow conditioner must be internally cooled. Lowenstein and Gabruk solved this problem conceptually, by configuring the conditioner as a parallel-plate indirect evaporative cooler. In work funded by the National Renewable Energy Laboratory, AILR developed and tested a water-cooled liquid-desiccant conditioner that uses low-flow technology 3. This conditioner is built as a water-cooled heat exchanger. The plates of the heat exchanger are plastic extrusions that are 48 inches (1.22 m) long, 12 inches (0.30 m) wide and 0.1 inches (2.5 mm) thick with 132 cooling passages running their length. Process air flows through the 0.1inch (2.5 mm) spaces between plates. Liquid desiccant is delivered to the top edges of each plate. Wicks on the outer surfaces of the plates create thin, uniform desiccant films. The distribution and collection of desiccant is incorporated into the design of the heat exchanger and occurs without sprays or drip pans that would create droplets. A 6,000-cfm (2.83 m 3 /s) conditioner has 200 plates. In addition to a conditioner, a liquid-desiccant cooling system will have a regenerator and an interchange heat exchanger. This configuration is shown in Figure 1. The regenerator uses heat to remove the water that the desiccant absorbed in the conditioner. The interchange heat exchanger improves the efficiency of the air conditioner by using the hot concentrated desiccant that leaves the regenerator to preheat the entering weak desiccant. Conditioner Regenerator Interchange Heat Exchanger Fig.1: Generic Liquid-Desiccant Air Conditioner The regenerator also uses low-flow liquid-desiccant technology. It is again a liquid-to-air heat exchanger, but one that has a hot fluid flowing within the plates. This thermal energy desorbs water from the desiccant films that flow down on the outer surfaces of the plates. A scavenging air flow between the plates carries away the water vapor. The preceding scavenging-air regenerator will have a Coefficient of Performance (COP) that is close to A much more efficient regenerator with a 1.25 COP can be made by adding an atmospheric-pressure desiccant boiler upstream of the scavenging-air regenerator. The steam that leaves the boiler with a 212 F (100 C) saturation temperature provides the thermal energy for the scavengingair regenerator. This advanced system is commonly referred to as a 1½-effect regenerator (similar to the double-effect generators used in absorption chillers). The boiler, which must operate at about 300 F (150 C), will be available in about two years. A 1,200-cfm (0.57 m 3 /s) LDAC that uses a scavenging-air regenerator began operating in AILR s laboratory in November As of March 2003, the conditioner in this system has operated effectively with no signs of carryover of desiccant droplets. There have been material compatibility problems in the higher temperature

3 regenerator. Changes in both the selection of materials and in quality control procedures are being implemented that will eliminate these problems. Two 6,000-cfm (2.83 m 3 /s) LDACs will be tested in the field during the 2003 summer. Both systems will use scavenging-air regenerators and will process outdoor air. Solar collectors will be added to one site in THE COMMERCIALIZATION STRATEGY FOR A SOLAR LDAC A LDAC that uses low flow technology creates new opportunities for solar cooling. Two characteristics of this technology are most important: processes 100% ventilation air will purge indoor pollutants while keeping indoor relative humidity low. Our commercialization strategy for a solar cooling system starts with the premise that gas-fired LDACs will soon become the preferred technology for processing ventilation air in humid climates. To demonstrate this we modeled the performance of the 6,000-cfm (2.83 m 3 /s) roof-top LDAC that is shown in Figure 2. In addition to the three desiccant components shown in Figure 2, this packaged system has a hot-water boiler as its heat source and a cooling tower for rejecting heat to ambient. water heater cooling tower conditioner A LDAC can be configured as a small to mid-sized roof-top air conditioner, and A LDAC can serve very high latent loads *. The cost of a solar cooling system can be significantly reduced if the engineering time and contractor labor needed to design and install a new system are reduced. Our approach to achieving this is to target a very popular packaged cooling system roof-top air conditioners between 10 and 30 tons (35 and 105 kw). By making the feel and function of the liquid-desiccant system close to that of a conventional system, more contractors will accept the technology, and their learning curve to becoming proficient installers will be shortened. Furthermore, by keeping the solar collectors and cooling system near each other on the roof, installation costs can be reduced. A second important part of our commercialization strategy is to first target applications that have high latent loads. LDACs will have a strong advantage in these applications. Unlike vapor-compression technology, which dries air by cooling it below the dewpoint, a LDAC can dry air without over-cooling. Large latent loads can be served without reheating, and temperature and humidity can be independently controlled within the building. An effective latent cooling system also addresses the need for better air quality within buildings. ASHRAE Standard 62 recommends much higher ventilation rates, specifically to improve indoor air quality. However, in the more humid eastern half of the U.S., the humidity that accompanies this higher ventilation can push indoor relative humidity to uncomfortable and unhealthy levels. A solar LDAC that * The term latent is interchangeable with humidity. In thermodynamics, the energy needed evaporate water to create humidity is the Latent Heat of Evaporation. interchange heat exchanger regenerator Fig. 2: 6,000-cfm Liquid Desiccant Air Conditioner for Building Make-up Air The target application for this study is a 46,000 square foot (4,275 m 2 ) building in Miami, FL. The main air handlers for the building circulate 40,000 cfm (18.9 m 3 /s). The building is continuously ventilated from 6 AM through 7 PM, seven days per week at a rate of 12,000 cfm (5.66 m 3 /s). (At 0.26 cfm per square foot [ m 3 /s per square meter], the ventilation rate in this example would be representative of a retail store.) Two 6,000-cfm (2.83 m 3 /s) packaged LDACs process the ventilation air to the building. This conditioned ventilation air is supplied directly to the building. Approximately 12,000 cfm (5.66 m 3 /s) of return air is exhausted, and the remaining 28,000 cfm (13.2 m 3 /s) is conditioned by 84 tons of conventional DX roof-top units. The energy use for the preceding system is compared to a conventional design that uses 120 tons (422 kw) of DX

4 roof-top units to process a 70/30 mix of return air and outdoor air. The DX capacity for this system has been sized so that most of the latent load on the building is met. When necessary, reheat is provided by an 80% efficient boiler to prevent overcooling the building. All non-ventilation loads for the building are calculated using a TRNSYS building simulation that was developed in ASHRAE research project 1120-TRP 4. Both latent and sensible loads are calculated for each hour of the year. The operation of the DX roof-top units is modeled using manufacturer s data for a 15-ton, 11.5 EER model that operates at its minimum design air flow to maximize its latent performance. The LDAC is modeled using computer simulations that were developed at AILR. The performance predicted by these models compares favorably with laboratory data collected both at AILR and at an independent laboratory. The operating costs for the conventional and liquiddesiccant systems are calculated for gas and electricity prices that are the 2002 averages for commercial customers in Florida as reported by the U.S. Energy Information Administration: $0.065 per kwh and $0.785 per therm ($7.44 per GJ). Including parasitic power for fans and pumps, the conventional DX system has slightly lower operating costs than the system that preconditions the ventilation air with the liquid-desiccant system and uses a simple scavenging-air regenerator: $45,553 versus $47,223 for one year of operation. However, when the scavenging-air regenerator is replaced with a more efficient 1½-effect regenerator, the liquid-desiccant system gains a 30% advantage: $32,816 versus $45,553. This operating cost advantage provides a strong incentive for the advanced gas-fired LDAC to displace the DX system in this application. 5. THE PERFORMANCE OF A SOLAR LDAC Our ultimate goal is a solar cooling system that can capture a significant fraction of the cooling market without relying on government or utility subsidies or unrealistically high energy prices. However, we recognize that this goal will not be reached in a single step. If the long-term benefits offered by an emerging technology are sufficiently great, state and federal agencies in the U.S. have been willing to provide financial incentives to encourage its adoption. In the following analysis, we use North Carolina s existing TRNSYS is a computer program developed at the University of Wisconsin that simulates the time-dependent operation of coupled energy systems. 35% rebate for solar cooling and heating technologies as the subsidy that the solar LDAC will receive in the near term. In the longer term, we allow for improvements in the solar and desiccant technology, but eliminate subsidies. 5.1 Near-Term Competition between a Gas-Fired and Solar Cooling System in Miami Having demonstrated that a gas-fired LDAC that cools and dries a building s make-up air can be an attractive alternative to an electric DX system, we next show that the same LDAC can further improve its competitiveness if a significant fraction of its thermal input is solar energy. A 46,000 square foot (4,275 m 2 ) building in Miami, FL on which the liquid-desiccant system cools and dries 12,000 cfm (5.66 m 3 /s) of ventilation air again serves as the target application for comparing the two systems. This application produces a large number of operating hours a building in Miami requires cooling for almost the entire year and the building is occupied seven days per week which leads to faster paybacks for the solar components. The near-term competition assumes that the LDAC uses the less efficient scavenging-air regenerator operating with 190 F (88 C) hot-water provided by either a single-glazed flat-plate collector or an 80% efficient boiler. There is no significant storage of hot water. Energy is stored as concentrated lithium chloride solution at 43%. The regenerator runs whenever the collector can deliver 190 F (88 C) hot water and there is available storage for desiccant. The desiccant storage tank is stratified with weak desiccant drawn from the top of the tank and concentrated desiccant returned to the bottom. The LDAC processes 12,000 cfm (5.66 m 3 /s) outdoor air whenever the building is occupied and outdoor wet-bulb temperatures are above 55 F (13 C). The regenerator is run on natural gas whenever the concentrated desiccant in storage is depleted. A simple payback is used to assess the competitiveness of the solar cooling system. The key cost assumptions in this assessment are: The installed cost of flat-plate collectors is $18 per square foot ($194 per square meter) Desiccant storage costs $2.50 per pound of anhydrous lithium chloride ($5.50 per kg) The owner receives a rebate equal to 35% of the installed costs of the solar components The price for natural gas is $7.85 per million Btu ($7.44 per GJ), which is the average price paid by commercial customers in Florida for the first ten months of 2002.

5 The $18-per-square-foot ($194 per m 2 ) cost for installed flat-plate collectors assumes a relatively simple installation, which is made possible by locating the collectors and LDAC close to each other on the roof of the building. There is no significant hot water storage and no piping runs within the building. There are no issues regarding either the integrity of the roof or structural support. The performance of the single-glazed flat-plate collectors is simulated using TRNSYS. The TRNSYS modules for the flat-plate collectors have been modified so that they simulate equipment manufactured AET. The flat plate collectors are oriented facing directly south at an angle from the horizontal that equals the latitude of the site. No attempt was made to either optimize the installation angle or seasonally adjust the angle. Desiccant is stored in an uninsulated plastic tank. The cost of the tank is relatively minor compared to the cost for the desiccant. Large plastic tanks cost on the order of $1 per gallon ($0.27 per liter). A gallon of weak lithium chloride solution (i.e., 37% weight concentration), weighs contains 3.75 pounds (1.7 kg) of anhydrous salt. Lithium chloride is now selling for about $2.00 per pound ($4.40 per kg) in large quantities. Thus, the cost for the salt will be about 7.5 times the cost for the tank. The $2.50 per pound ($5.50 per kg) cost for storage includes the cost of the salt, the storage tank and a small markup by the contractor. Figure 3 presents the total ownership costs over eight years for the solar LDAC as a function of the size of the collector array. The capital component to the ownership costs only includes the incremental cost for adding the solar subsystem to a gas-fired LDAC. The $240,000 cost for a system with zero array area is the eight-year fuel cost for the gas-fired version of the air conditioner. For array areas below 5,000 square feet (465 m 2 ), the total costs stay constant at about $240,000. As shown in Figure 4, arrays up to this size are essentially fully utilized (i.e., over 90% of the energy that they collect is used to regenerate desiccant). The flat portion of the curve in Figure 3 occurs because the value of energy collected over eight years equals the array s installed cost whenever an array is fully utilized. With a 5,000 square-foot (465 m 2 ) array, the solar collectors are providing about 25% of the total thermal input to the liquid-desiccant regenerator. At this array size, the hourly thermal output of the array almost never exceeds the hourly thermal requirements of the regenerator. For arrays larger than 5,000 square feet (465 m 2 ), the availability of solar energy will start to exceed the thermal requirements of the regenerator for some hours. Although desiccant can be regenerated, stored and used at a later hour, Cost for Planning Period ($) $400,000 $380,000 $360,000 $340,000 $320,000 $300,000 $280,000 $260,000 $240,000 $220,000 flat plate collectors no storage 8-year planning period $200, Collector Area (s.f.) Fig. 3: Eight-Year Costs for a Near-Term Solar Cooling System that uses Flat Plate Collectors the cost for storage will lead to higher costs for the eightyear planning period. If storage is not used, costs again increase since the array in excess of 5,000 square feet (465 m 2 ) is not fully utilized. In the near-term application that we are studying here, storage will lead to lower total costs only if the cost of gas is higher or the planning period is lengthened. If the planning period is increased from 8 years to 12 years, the system with the lowest total costs now uses 14,000 square feet Solar Utilization flat plate collectors no storage 8-year planning period Collector Area (s.f.) Fig. 4: Utilization of Solar Collectors for a Near-Term Solar Cooling System that uses Flat Plate Collectors

6 (1,300 m 2 ) of array. With this much larger array, desiccant can be regenerated, stored and used at a later time. The lowest total costs will occur when 10,000 lbs (4,545 kg) of desiccant storage is used. The 12-year total costs with storage will be $304,000 versus $359,000 for the gas-fired alternative. The utilization of the arrays drops slightly to 88%, but solar energy now provides 66% of the total thermal input for regenerating desiccant. 5.2 Long-Term Competition between a Gas-Fired and Solar Cooling System in Miami A new technology cannot indefinitely rely on subsidies to promote its sale. If a solar air conditioner is to become a meaningful alternative to electric and gas cooling systems, a mature product must compete without subsidies. Although not now available, mature LDACs will use the more efficient 1½-effect regenerator that was described in an earlier section. The COP for this regenerator will be almost twice that of the simpler scavenging-air unit. However, to gain this improvement, the regenerator must have a 300 F to 320 F (150 C to 160 C) source of heat. This high temperature will require either a tracking, concentrating collector or an evacuated-tube collector. For the study presented here, we ve selected a tracking parabolic-trough collector to supply heat to the regenerator. Consistent with the assumption that the solar cooling system is mature with fairly high annual sales, we ve assumed that the installed costs for the collectors are $17.50 per square foot ($188 per m 2 ). As with the flat-plate collectors, this installed costs assumes a relatively simple installation. The performance of the tracking parabolic-trough collectors is again simulated using TRNSYS. The TRNSYS modules for parabolic-trough collectors have been modified so that they simulate the performance of the equipment manufactured by Industrial Solar Technology. The parabolic-trough collectors were oriented with their axes aligned north/south (which will collect more solar energy in the summer months than an east/west orientation). The axes of the collectors were in a horizontal plane. Given the extreme volatility present in the natural gas market at the time this paper was prepared in February 2003, it is difficult to forecast a long-term price for natural gas. For 2001, the average price paid by commercial customers in Florida was $10.61 per million Btu. This price is used in the long-term analysis. For the long-term analysis there will also be a planning period for which the total costs for the solar system are almost constant as the array size is increased (as is the case in Figure 3 for the flat-plate collectors). This planning period is 7.5 years, for which all systems with less than 2,500 square feet (230 m 2 ) of array (including the non-solar system) have a total cost close to $156,000. With 2,500 square feet (230 m 2 ) of parabolic-trough collectors, about 30% of the thermal input for the regenerator is supplied by solar energy. With 2,500 square feet (230 m 2 ) of parabolic-trough collectors, essentially all of the collected solar energy can be immediately used to regenerate desiccant. Storage does not improve the utilization of the arrays, a situation that is identical to that which occurred in the near-term analysis with 5,000 square feet (460 m 2 ) of flat-plate collectors. And, similar to the near-term analysis, storage will lead to a lower total cost only if either the planning period is lengthened or gas prices are increased. For the long-term analysis, we again increase the planning period to 12 years to study the benefits offered by storage. Keeping gas at $10.61 per million Btus, the lowest total costs for the planning period occur when the array is increased to 6,000 square feet (560 m 2 ) and 5,000 pounds (2,270 kg) of lithium chloride are used for storage. This system will have total costs of $217,000 a $32,000 savings over the non-solar system. 6. CONCLUSIONS A new thermally activated cooling technology is now coming to the market that will significantly expand the possibilities for solar cooling. This technology low-flow liquid-desiccant air conditioners will first be used as a packaged gas-fired roof-top unit that provides better control of indoor humidity when latent loads are high. Many factors affect the competition between a solar cooling system and one that primarily uses either gas or electricity. These include (1) hardware costs, (2) energy prices, (3) the length of the cooling season as well as the hourly variation of the cooling loads, and (4) the buyer s criteria for purchasing decisions (e.g., payback, return on investment, etc.). The study presented here is not a thorough investigation of how these factors impact the competitiveness of a solar cooling. Instead, we ve presented a fairly simple analysis. i.e., only one application was studied under one set of economic assumptions. However, this one application does reflect conditions in a very important market for solar cooling: humid climates with long cooling seasons. In this market, assuming fairly low energy prices (i.e., energy costs for 2001 and 2002 in Florida), a solar liquid-desiccant cooling system will payback in the near term with government subsidies in eight years. In the longer term, without subsidies but with more

7 advance technology for the LDAC and the solar collectors, the payback will be between seven and eight years. These paybacks should lead to a growing solar cooling industry. Obviously, a significant drop in energy prices will discourage sales of a solar cooling system. However, for the past five years energy prices have been at historically low levels. A more likely scenario is that energy prices will rise from these levels as the world emerges from a recession that is still depressing economic activity at the start of And, the need to limit greenhouse gas emissions will, in effect, further increase the price of fossil fuels. The solar LDAC is an economically viable technology for cooling and dehumidifying some of the world s buildings. Whether it displaces 5% or 50% of the fossil energy use for air conditioning will depend on both the maturation of the technology, global environmental issues and future energy prices. 7. ACKNOWLEDGEMENTS The work presented here is based in part on research funded by the U.S. Department of Energy under SBIR contract DE- FG02-01ER This support does not constitute an endorsement by the U.S. DOE of the view expressed in this paper. The author also acknowledges the financial support of the National Renewable Energy Laboratory for the development of the low-flow conditioner and regenerator, and the Oak Ridge National Laboratory for the development of the 6,000-cfm roof-top liquid-desiccant air conditioner. Finally, the author thanks Mr. Ken May of Industrial Solar Technologies for help in modeling parabolic trough collectors. 8. REFERENCES (1) Annual Energy Outlook 2003 With Projections to 2025, DOE/EIA-0383(2003), January 2003 (2) Lowenstein and Gabruk, The Effect of Absorber Design on the Performance of a Liquid-Desiccant Air Conditioner, ASHRAE Transactions, pt. 1, vol. 98, AN , 1992 (3) Lowenstein, Advance Liquid Desiccant Technology, DOE Peer Review, Nashville, TN, 2002 (downloadable as PDF file from IES/Lowenstein.pdf (4) Development of Equivalent Full Load Heating and Cooling Hours for GCHPs Applied in Various Building Types and Locations, ASHRAE 1120-TRP, December 2000