| Feature Article Cooling systems affect resources, climate, and health By Arunima Chatterjee and Melanie regional evapora- Lenart tion rates where it is generated, ac- Residential cooling and heating account cording to research for about 56 percent of the total energy at the National consumed in the typical home in the Renewable Energy United States, according to a 2005 U.S. Laboratory led by Department of Energy report. In the Paul Torcellini. This Southwest, this energy is increasingly value considers the going toward air-conditioning rather total water evapo- than the traditional evaporative coolers, rated from the known as swamp coolers. The shift has reservoirs serving implications for energy use, water use, Hoover and Glen and climate. Canyon dams and the amount of elec- Energy vs. Water tricity generated. Strictly in terms of energy use, the on- They estimate that going shift from swamp coolers to air the coal-fired plants conditioners has its costs. Air condition- that supply most of ers generally use two to four times more Tucson’s electricity electricity than swamp coolers. For a consume about half typical 2,000-square-foot Tucson resi- a gallon of water dence, the electricity used by a swamp for each kilowatt- cooler can be as low as 250 kilowatt- hour of electricity Figure 1. Swamp coolers typically consist of a box with vented sides. hours in an average month, while an produced. A fan draws ambient air through vents and through pads that are kept air conditioner consumes about 850 moist by a water supply, pump, and distribution lines. The cooled, moist air is then delivered to the building via a vent in the roof or wall. kilowatt-hours. In Tucson, this trans- Applying the coal Credit: Mike Buffington, Southwest Hydrology lates to a monthly electrical cost of $25 plant standard compared with $85. to the cooling of a 2,000 square-foot monsoon, when outside air is already home, the ERL analysis found that moist, the effectiveness of swamp But a scarcity of water in the Southwest monthly water consumption for an coolers is limited. One benefit of air makes the comparison more complex, air-conditioning system is about 425 conditioning is its ability to cool to a posing a challenge in determining the gallons, while an evaporative cooler thermostatically controlled temperature conservation strategy that can yield requires about 4,620 gallons, including regardless of the humidity. At some level, optimum savings for both energy and direct and indirect usage for both. The though, the cooling of a home usually water. T. Lewis Thompson of the Envi- source of energy used to power air con- equates to a warming of the planet, with ronmental Research Laboratory (ERL) ditioning is critical to this analysis, how- air conditioners doing more damage at The University of Arizona found that ever. If the same calculations are made than swamp coolers when the electric- during summer conditions in Tucson using hydropower, an air conditioner ity source, such as coal or oil, produces (May–September), a swamp cooler uses 55,250 gallons of water per month greenhouse gases. working at 75 percent efficiency uses an compared with an evaporative cooler’s average of 150 gallons of water per day, 20,745 gallons. Overall, the water used Climate Considerations while air-conditioning units do not di- directly by swamp coolers represents 5 The collective choice of cooling equip- rectly use water. The generation of elec- percent or less of a household’s annual ment can affect the local climate as well tricity, however, requires water, a behind- water use, based on a study of several as the global one. While air condition- the-scenes use that is easily overlooked. southwestern cities by the non-profit ers merely eject heat from the interior group Southwest Energy Efficiency of a home or office into the outside air, Hydropower, which supplies about 12 Project (SWEEP). swamp coolers can actually contribute percent of Arizona’s electricity, con- to cooling the environment, indoor and sumes about 65 gallons of water per Evaporative cooling works best in the outdoor. An evaporative cooler pulls kilowatt-hour generated because of high dry months of summer. During the continued on page 4 http://www.ispe.arizona.edu/climas/forecasts/swarticles.html Southwest Climate Outlook, September 2007  | Feature Article

Cooling systems, continued air through moist pads, lowering the lion joules of energy per hour into the a researcher who worked on this topic incoming air by as much as by 30 de- outside air, or a billion joules for every while at ASU, noted that the ongoing grees (Figure 1). Because people cooling 16 hours of operation. shift from swamp cooling to air condi- their homes with swamp coolers must tioning may eventually reduce some of leave some windows open, some of this Arizona State University (ASU) re- that daytime cooling. cooled air permeates outdoors. searchers were surprised to find daytime temperatures in parts of metropolitan Once the sun goes down, the desert A typical swamp cooler converts about 1 Phoenix were no higher, and in some cools down more quickly than the city, billion joules of energy a day from heat cases actually lower, than those in the which is carpeted by heat-trapping into other types of energy, including surrounding desert despite the expected pavement and vegetation. Some Phoe- kinetic and latent energy. As heat, this urban heat island effect. They surmised nix-area urban temperatures averaged amount of energy could warm a six-foot that their results reflected the evapora- up to 20 degrees F warmer than those deep, twelve-by-twelve-foot pool by 20 tive cooling from pools, urban lakes, in the nearby desert, so in this case the degrees Fahrenheit. Meanwhile, a typi- landscaped vegetation, and perhaps cal air conditioner ejects about 63 mil- even swamp coolers. Joseph Zehnder, continued on page 5

Figure 2. Evaporative coolers work by converting some of the heat energy in air into latent heat and kinetic energy that is trapped in the process of evaporation of water. A modern swamp cooler with an 85 percent efficiency can cool 100 degrees F daytime air down to about 68 degrees F. In the process, it uses about 145 to 150 gallons of water a day, assuming it operates on low during the day and is turned off at night. This data is for Tucson in June 2006. Credit: T. Lewis Thompson of The University of Arizona Environmental Research Laboratory http://www.ispe.arizona.edu/climas/forecasts/swarticles.html Southwest Climate Outlook, September 2007  | Feature Article

Cooling systems, continued urban heat island effect operates mainly supplemented with window-mounted temperatures rise, individuals will con- at night. air-conditioners as needed. tinue to seek a cool indoor refuge from outdoor heat—a cycle that could force Health Alternative Options society to seek energy- and water-saving When it comes to health issues, central Many factors influence total water solutions to cooling. air-conditioning may not be the best consumption in an evaporative cool- choice. Evaporative cooling contributes ing system, including residential design, Arunima Chatterjee is a graduate research- to better indoor air quality. The moist location of the cooler, and the use of er with The University of Arizona Environ- mental Research Laboratory. pads through which the outside air vegetation and other features to cool flows act as effective air filters, trapping air before it enters the cooling system. Melanie Lenart is a postdoctoral research dust and pollen. Since the pads are con- The cooling efficiency of such a swamp associate with The University of Arizona tinually moistened, trapped particles are cooler can increase dramatically by sen- Institute for the Study of Planet Earth. flushed out with the water cycle. Evapo- sible cooling of the air before it goes T. Lewis Thompson of The University of Ari- rative cooling also adds moisture to the through the moist pads of the cooler. zona Environmental Research Laboratory air, which helps keep wooden furniture, Sensible cooling can be achieved by also contributed to the analysis. fabrics, and plants from drying out strategic landscaping, rock beds, and along with skin, eyes, and throats. water channels. A slightly shorter version of this article ran in Southwest Hydrology’s September– Swamp coolers actually work best On a typical summer day in Tucson, air October issue on water and energy, avail- able at http://www.swhydro.arizona.edu/. when given plenty of ventilation to entering an evaporative cooler with 75 the outside air. This system thus pro- percent efficiency at a temperature of vides an ongoing stream of fresh air, 100 degrees F can exit with an air tem- References as long as outside air is not contami- perature of 75 degrees F. Many newer nated from a nearby wildfire or exces- evaporative cooling systems have an 85 U.S. Department of Energy, 2005. A sive pollution levels. percent efficiency (Figure 2). consumer’s guide to energy efficiency and renewable energy. http://www.eere. Air-conditioners, in contrast, are more For residences at the design stage, cool energy.gov/consumer/your_home/space_ efficient when recirculating indoor towers are a way of using the principle heating_cooling/index.cfm/ air. Thus air-conditioning systems can of downdraft evaporative cooling. Cool mytopic=12300 magnify indoor pollution, especially in towers usually have a wet pad in the top households that permit smoking. Even of the tower. The cool air is heavier than Quackenboss, J.J., M. Krzyzanowski, in non-smoking households, central warm air and sinks by means of gravity, and M.D. Lebowitz, 1991. Exposure air-conditioning units can pull in con- creating its own airflow and eliminating assessment approaches to evaluate re- taminants from walls and concentrate the need for blowers or fans. The only spiratory health effects of particulate volatile organic compounds from car- power required is for a 12-volt pump matter and nitrogen dioxide. Journal pets, cosmetics, and cleaning products, to circulate water over the cooler pads. of Exposure Analyses and Environmental among other materials. Generally, cool towers without fans are Epidemiology 1: 83-107. 20 to 30 feet tall and between 6 and 10 Researcher Michael Lebowitz and his feet wide. These systems require 100 Southwest Energy Efficiency Project, colleagues at the UA College of Medi- to 150 watts and cool 1,000 to 2,500 2004. New evaporative cooling systems: cine conducted a series of studies on square feet. An emerging solution for homes in hot dry how indoor air quality varies with fac- climates with modest cooling loads. tors including type of cooling equip- The need to consider energy as well as http://www.swenergy.org/pubs/. ment. They concluded that sensitive water demand for cooling options seems populations, such as asthmatics and likely to increase during this century. Torcellini, P., N. Long, and R. Judkoff, people with cardio-vascular problems, The Intergovernmental Panel on Cli- 2003. Consumptive water use for U.S. should only use central air-condition- mate Change projects that average sum- power production, National Renewable ing if the units have a high-efficiency mer temperatures in the Southwest will Energy Laboratory Publication NREL/ filter, such as a High Efficiency Par- rise by at least several degrees in decades CP-550-35190. http://www.nrel.gov/ ticulate Air (HEPA) filter. Other- to come, and even more if society fails docs/fy04osti/35190.pdf wise, they should use swamp cooling to stabilize greenhouse gas emissions. As http://www.ispe.arizona.edu/climas/forecasts/swarticles.html Southwest Climate Outlook, September 2007