Evaporative Cooling for Residential and Light-Commercial

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1 EMERGING TECHNOLOGIES This article was published in ASHRAE Journal, October Copyright 2011 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Posted at This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit Evaporative Cooling for Residential and Light-Commercial Cooling on a Small Scale By Alissa Cooperman; John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE With evaporative cooling of the condenser, the outdoor air wetbulb temperature, rather than the dry-bulb temperature, becomes the heat sink temperature. This results in a lower condensing temperature, saving energy. Evaporative cooling of condensers is common in large chillers and in industrial-scale refrigeration equipment, where some level of maintenance cost is easily offset by the energy cost savings. However, in smaller systems, evaporative cooling of condensers generally has been avoided (in favor of air cooling), primarily due to maintenance issues. Depending on the climate, significant energy savings could be obtained through the use of evaporative condenser cooling in residential and light commercial airconditioning systems. Basics Three basic ways (and probably others and variants on these) to implement evaporative condenser cooling are: Directly evaporatively precool the cooling air before it goes through a conventional air-cooled finned condenser coil; Flood water over the condenser coil while air is blown through it; and Use a cooling tower to evaporatively cool cooling water, which in turn cools a water-cooled condenser. The first method can be compared to air cooling with equal cooling airflow rates through identical air cooled finned condenser coils, at Air-Conditioning, Heating and Refrigeration Institute (AHRI)-prescribed conditions (outdoor dry and wet bulb temperatures of 95 F [35 C] and 75 F [23 C], respectively, with the cooling air evaporatively precooled in one case and not evaporatively precooled in the other). As shown in Figure 1, in the air cooling case (no evaporative precooling), cooling air enters the condenser at 95 F (35 C) and warms to 110 F (43 C) (a 15 F [8 C] rise) as it removes heat from the condensing refrigerant (this temperature rise corresponds to a cooling airflow rate of approximately 900 cfm [425 L/s] per ton of cooling capacity). The condensing temperature must be above the leaving air temperature. In this case it is assumed to be 120 F (49 C). With the air evaporatively precooled, the dry-bulb temperature approaches the wet-bulb temperature. For this example, it is assumed that the dry-bulb temperature is reduced to 80 F (27 C) (reduced by three-fourths of the difference between the dry- and wet-bulb temperatures). To a first order, everything else is the same, so the leaving air temperature is 95 F (35 C), which is 15 F (8 C) higher than the entering air temperature. The condensing temperature is 105 F (41 C), which is 10 F (6 C) higher than the leaving air temperature and 15 F (8 o C) lower than the condensing temperature with air cooling. This difference in condensing temperature typically results in about a 30% increase in the compressor energy efficiency ratio (EER). With Method 2, as the air goes through the condenser and warms up, more water can evaporate, effectively increasing the specific heat of the air several-fold (the combined sensible heat capacity and incremental latent heat capacity as the saturation humidity ratio increases as the air temperature increases). If the air does not have to get as warm to carry away the heat, the condensing temperature can be even lower. Figure 2 compares this method of evaporative condenser cooling with air cooling using the same assumptions that are the basis for Figure 1 (air drybulb temperature within 5 F [3 C] of the wet-bulb temperature, 10 F [6 C] temperature difference between the leaving air and condensing temperatures, 900 cfm [425 L/s] per ton cooling airflow rate). With evaporative cooling throughout the condenser air side, the air temperature rise is only 4 F (2 C) and the resulting condensing temperature is 94 F (34 C), 26 F (14 C) less than the air-cooled case, with an increase in compressor EER of more than 50%. Method 3 (cooling tower + water cooled condenser) is the most common configuration for large systems. Following standard design assumptions for cooling towers and water cooled condensers, the cooling water will leave the cooling 84 ASHRAE Journal ashrae.org October 2011

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3 EMERGING TECHNOLOGIES 120 Condensing Temperature Air-Cooled 120 Condensing Temperature Air-Cooled Temperature ( F) Condensing Temperature Evaporatively Precooled Cooling Air DB Temperature Air-Cooled Cooling Air DB Temperature Evaporatively Precooled Temperature ( F) Cooling Air DB Temperature Air-Cooled Condensing Temperature Evaporatively Cooled Cooling Air DB Temperature Evaporatively Cooled Percent of Total Heat Rejection Percent of Total Heat Rejection Figure 1 (left): Comparison of air-cooled and evaporatively precooled condenser performance. Figure 2 (right): Comparison of air-cooled and evaporatively cooled (Method 2) condenser performance. tower 7 F (4 C) higher than the wet-bulb temperature, the cooling water will leave the condenser 10 F (6 C) higher than the entering temperature and the condensing temperature will be 5 F (3 C) higher than the leaving water temperature, therefore: condensing temperature = 75 F + 7 F + 10 F + 5 F = 97 F This is 23 F (13 C) lower than the condensing temperature with air cooling, similar to Method 2. Technology Status Evaporative precoolers and evaporatively cooled condenser coils are the most commonly used technologies in the evaporatively cooled residential condenser market. An evaporative precooler operates by drawing outdoor air over a wetted evaporative medium. This stage brings the outdoor air close to its wet-bulb temperature, reducing the inlet air temperature to the condenser, thereby lowering the condensing temperature (Method 1, as described previously). It can be comprised of a wraparound evaporative medium and a sump pump that keeps the medium wetted. A fan draws the outdoor air through the medium. Alternatively, the condenser coils can be sprayed with water as outdoor air is drawn over them or immersed in cooled water. These methods increase heat transfer between the refrigerant and the heat sink due to increased evaporative heat transfer coefficients and the latent heat as the water changes from liquid to vapor. One such technology uses an evaporative medium to cool the sump water, in which the condenser coils are submerged, to within 5 F to 10 F (3 C to 6 C) of the wet-bulb temperature (essentially a small cooling tower). The evaporatively cooled water cools the submerged condenser coils with the water and high side refrigerant flowing counter to each other (Method 3, as described previously). 1,2 Evaporative condenser (EC) technology is sensitive to water quality. Hard water leads to deposits on the outside of the condenser coils, and clogging of the spray nozzles. One company s solution to this problem involves condenser coils suspended such that the vibrations of normal operation shake any collected scale loose, wide spray nozzles to prevent blockage by scale, and sacrificial anode rods to prevent coil corrosion. 3 Another company is exploring a hybrid air and water cooled condenser. Scale is less likely to build up on cooler condenser coils; one company noted scale buildup on the outermost, hot- 86 ASHRAE Journal October 2011

4 test condenser coils versus their inner cooler coils. In its new hybrid EC design, the refrigerant temperature is first cooled through air cooling, and then by water cooling. This company is aiming to make its new hybrid evaporative condenser resistant to scale buildup. 4 Though EC technology consumes water, the overall water consumption of the unit is a small percentage of a household s yearly water consumption. For example, an average family of three in the U.S. consumes about 100,000 gallons ( L) of water annually. An EC consumes about 2,000 gallons (7600 L) of water annually, or about 2% of average annual water consumption. 3 Despite component sensitivities, EC units perform better than air-cooled units over a wider temperature range. As the temperature increases beyond 100 F (38 C), air-cooled units suffer from capacity reductions of up to 24%; it is customary to oversize these units such that capacity reductions due to outdoor temperature rise do not hinder cooling ability. With the negligible cooling capacity losses experienced by EC units as the temperature rises, the required unit size is a half to whole ton smaller than a vapor compression unit serving the same space. 3 A trend in the residential EC market is coupling the technology with geothermal heat pumps and swimming pools, as heat reservoirs. When coupled with a swimming pool, the EC air conditioner provides essentially free pool heating. 4 Energy Savings In ideal climate conditions, evaporatively cooled condensers provide increased efficiency over conventional vapor compression cycles. The ideal climate is one in which relative humidity is low and outdoor temperature is very high. This makes evaporatively cooled condensers ideal for California and desert climates. EC technology works in all climates. Manufacturers of EC technology have reported that their products have been sold and used throughout the United States, providing savings on cooling. A study of evaporative condenser technology in California by Pacific Gas and Electric found that EC units are more efficient than conventional air-conditioning units having seasonal energy efficiency ratios (SEER) of both 10 and 12 and demand less energy (10 SEER was the minimum required efficiency at the time that the study was done). Between 85 F and 110 F (29 C and 43 C), the SEER 10 unit studied lost approximately 16% of its cooling capacity, whereas the evaporatively precooled condenser lost 7% and the evaporatively cooled condenser lost only 4%. October 2011 ASHRAE Journal 87

5 EMERGING TECHNOLOGIES Across the same temperature range, electric power demand was lowest for the evaporatively cooled condenser, 1.64 kw to 1.94 kw. The SEER 10 unit required 3.08 kw to 3.59 kw, or almost double the electric power required to operate the evaporatively cooled condenser unit. The units studied were lab and field tested; the field results confirmed the performance seen in the lab. 1 Of the technologies compared, SEER 10 and SEER 12 conventional air-conditioning units, an evaporatively precooled condenser, and an evaporatively cooled condenser, the latter provided the greatest monetary savings. Climate zone-dependent annual savings from $30 to more than $100 relative to SEER 10 technology were realized using DOE modeling software. Additionally, the savings increased with the severity of the climate region. The hotter and dryer it was, the better the unit outperformed its air-cooled counterpart. For the climatic regions of California, widespread use of EC technology has the potential to save energy and money. 1 Overall, EC units demand less energy and maintain their design capacity at higher temperatures than vapor compression units. However, both types of units demand more electricity as the outdoor air temperature rises. The primary difference is that the demand profile for an EC air conditioner is flatter than that of a conventional model. Projected demand savings for an evaporatively cooled condenser are at least 35% higher than those of an evaporatively precooled condenser: 0.5 kw to 1.1 kw compared to 1.1 kw to 1.7 kw, respectively. 1 Market Factors Clear benefits, energy and monetary savings, can be found from installing an evaporative condenser. The savings depend on climactic conditions: EC performs best in hot and dry places. However, its efficacy can be realized throughout U.S. climatic regions. One manufacturer has installed units throughout Texas, Nevada, and California, as well as in New Orleans and New York City. 5 Another manufacturer of evaporative condensers has witnessed the highest monetary savings from the use of his product in Key West, Fla. 6 A manufacturer of evaporative precooling for condensers has installed its product across the U.S. 7 The efficiency advantages of the technology are reduced in more humid climates, but still provide energy savings. 5 In addition, the unit s water consumption, though relatively minimal, is of concern for hot and dry climates where water is scarce. EC units come with maintenance concerns: scale 88 ASHRAE Journal October 2011

6 build-up, nozzle blockage, bacteria or other growth in the water, and lifetime of the evaporative media. Solutions to these problems include flexible condenser tubes, suspended condenser tubes that vibrate during normal operation, wide water spray nozzles, condenser tube coatings, sacrificial anodes, and sump water purging. Concerns about the water used by evaporative condensers can be offset by considering that the reduction in energy consumed is equivalent to less energy produced and thus less water consumed by the power plant. One study found that one-third of the water used by EC technology would be offset by the reduced water use at the power plant. Additionally, systems with smart purging based on either time intervals or water mineral content would help to conserve water. 8 At the time of the PG&E study, a SEER 10 residential AC unit cost approximately $2,500 whereas SEER 12 units were approximately $150/ton to $250/ton ($43/kW to $71/kW) more, units with evaporative precooling were about $180/ ton to $200/ton ($51/kW to $57/kW) more, and evaporative condensers were about $300/ton to $350/ton ($85/kW to $100/kW) more expensive. The average AC unit with an evaporatively precooled condenser was about $3,500 whereas the average AC unit with an evaporative condenser was about $ Of the EC technologies on the market, an evaporatively cooled condenser is the most interchangeable with current systems. Unlike direct or indirect evaporative cooling of the indoor air, neither airflow nor ventilation requirements change. An air conditioner with an evaporatively cooled condenser can be connected to current ducting Shelton, S Phone conversation. ADVEN, LLC. 6. Bacchus, R Phone conversation. RR&P LLC. 7. Nicolini, M Phone conversation. Premier Industries. 8. Reichmuth, H., et al Assessment of Market-Ready Evaporative Technologies for HVAC Applications. New Buildings Institute. Alissa Cooperman is a technologist and John Dieckmann is a director in the Mechanical Systems Group of TIAX, Cambridge, Mass. James Brodrick, Ph.D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C. References 1. Hoeschele, M., et al Evaporative Condensers: The Next Generation in Residential Air Conditioning. Davis Energy Group and Pacific Gas and Electric Company. tinyurl.com/hoeschele. 2. Davis Energy Group Evaluation of Residential Evaporative Condensers in PG&E Service Territory. Davis Keesee, M., and D. Bisbee The AquaChill. Energy Efficiency, Customer Research & Development, Sacramento Municipal Utility District. AquaChill. 4. Word, M Phone conversation. Thermal~Flow. October 2011 ASHRAE Journal 89