Evaporative Cooler

Evaporative coolers are major summertime residential water users. In Phoenix, 43 to 46 percent of the city's single-family homes use evaporative cooling, alone or in conjunction with refrigerated air conditioning.
During summer months, about 15 percent of those homes' water use is for evaporative cooling; a significant contributor to summertime peak water demand.
Background
Cooler Study
Results
Policy Questions
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Background
Evaporative cooling is one of the most ancient and one of the most energy-efficient methods of cooling a home. It long has been regarded as environmentally "safe," since the process typically uses no ozone-depleting chemicals, and demands one-fourth as much energy as refrigeration during the peak cooling months of the year. In dry climates, evaporative cooling, even the relatively inefficient "swamp box" household cooler, can be used to inexpensively cool large homes.
Direct evaporative cooling adds moisture to the air. In wet climates, or during the late summer rainy season in the desert, this results in uncomfortable and, for many individuals, unacceptable indoor humidity. In Phoenix, those who have the option to choose between evaporative cooling and refrigeration usually switch systems during July and do not go back to evaporative cooling until mid-September. Indirect evaporative cooling, without added humidity, is less effective, costs more, and if not used properly, can cause damage to refrigeration systems.
The most common form of residential evaporative cooling uses a vertical pad of cellulose fiber, a system for delivering water to the top of the pad, and a fan to draw air through the porous pad as the water runs down the medium and is absorbed. As dry air moves over the wet pad, water evaporates, and the air gives up its heat. The air moving from the wet pad into the home is cooler than the outdoor air. Where water is high in mineral content, a bleed-off system often is used on the water circulation system, constantly dumping part of the water and allowing the refill valve to replace it with fresh water. This decreases the mineral build-up on the pads and on the inside of the cooler, but at a 25 to 50 percent increase in water use.
The drop in temperature depends on how much water the air can absorb (a function of the relative humidity), how evenly the pad media is wetted, and how long the air is exposed to the pad (a factor of turbulence, wetness, and speed of air movement), the evaporability of the water (both temperature and hardness of the water affect this), and the ability of the building to "vent" warmer exhaust air back to the atmosphere.
There is general agreement, even among domestic manufacturers, that U.S.-made evaporative coolers are not as energy- or water-efficient as they could be. These products are targeted heavily to middle-, lower-middle-, and low-income households, and appear to be designed against a one- to two-year capital payback, rather than optimum operational efficiency. Operating savings in both water and energy, as well as in maintenance costs, often fall victim to production of price-competitive units.
Inefficiencies in less expensive units include: under-powered, inexpensive aluminum-wound fan motors rather than more energy-efficient, larger copper-wound motors; recirculating pumps that run faster and hotter than necessary to compensate for a lack of volume capacity; fans that run up to 20 percent over design capacity to move larger volumes of air at increased velocity to make up in wind movement what is lost in evaporative efficiency.
Slower movement of air through a thicker pad medium, adequately wetted by a properly sized pump, all contained within a high-reflectivity cabinet, and with all components operating at voltages higher than the usual household current, would result in energy and water savings as well as a longer-lived cooler and a more cost-effective long-term investment.
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Cooler Study
In 1992, the city of Phoenix asked the University of Arizona's Office of Arid Lands Studies to design a research study with the goal of accurately quantifying the water-use and operating characteristics of evaporative coolers currently in use in Phoenix. The university and the city conducted the study, with funding from the Arizona Department of Water Resources. The study represents the first statistically sound and repeatable attempt to quantify evaporative cooler water use in a broad sample of Phoenix residences.
The research was conducted between the spring of 1993 and the fall of 1994. This was an initial step in establishing programs to both improve understanding of actual water use by these appliances, and to provide a starting point for improvements to the water consumption efficiency of evaporative coolers. The field study involved metering the water used by a structured sample of home coolers to better estimate citywide evaporative cooler water demand. The intent was to secure a statistically valid sample of water use from existing systems, producing a valid "slice of the real world" with a high level of reliability to the city at large.
The research plan was designed to monitor the performance of 46 evaporative coolers in Phoenix in the summer of 1993. Twenty of those coolers also were monitored during the summer of 1994, as a check against variations in summer temperatures and rainfall. Previous surveys estimated that coolers with bleed-off systems represented about half the coolers in use, and that homes with evaporative cooling as the only cooling source represented about 13 percent of all homes. To have sufficient data points for each category, it was determined that half the test houses would be ones with bleed-off systems and half the homes would have dual cooling systems.
Meters were placed on the incoming water lines that provide make-up water to the evaporative cooler reservoir. To measure the volume of water flowing from coolers as bleed-off, it was necessary to design a measuring/recording device. Since bleed-off generally occurs at a slow flow rate with no water pressure, direct measurement using a water meter is not possible. Bleed-off capture boxes were devised to hold the discharge water. A pump emptied the catchment box when it filled, pumping the water through a meter. A baseline bleed-off rate was determined for each cooler, and an elapsed hour meter was wired into the pump to record total cooler use time during the season. This also acted as a back up to the bleed-off measurement boxes. If a box failed, the clock could be used to calculate an approximate bleed-off flow for the summer. No optimizing adjustments were made to water inflow or to bleed-off rates on the test homes.
To induce participation, the city performed pre-season maintenance on all the coolers, and provided on-call trouble shooting of operating problems during the summer. Criteria on home size, cooler size, and general operability of the cooler had to be met before the home was approved for the study. Once meters were installed and pre-season maintenance was accomplished, every reasonable effort was made to keep each participant involved. Only one volunteer withdrew.
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Results
Average daily water use by evaporative coolers for the two summers of the study was 66 gallons per day. This varied significantly, however, depending on whether the cooler was equipped to bleed off water or whether it evaporated all the water that came into the pan. Coolers with no system to bleed off water used an average of 3.5 gallons per hour of run time, while coolers that reduced the salt buildup by constantly dumping and replacing part of the water while the pump ran used an average of 10.5 gallons per hour of run time.
The percentage of household water use that was used by the cooler also varied significantly, from 2.5 percent to 66.8 percent of total summer use. The average for houses with no air conditioning option was 25.8 percent, and for all homes the average use was 15.8 percent. On average, the coolers ran 2,100 hours during the summer.
Average consumption of water for cooling, as well as total hours of cooler run time were both heavily influenced by the number of homes that could switch to air conditioning.
Water in the desert southwest is high in dissolved salts; mostly calcium. It is harmless in the drinking water supply, and expensive to remove. It can cause problems for coolers if it becomes too concentrated. The result of high total dissolved solids (TDS) is a build-up of calcium salt on cooler pads, pumps, pans and sides. It is corrosive to metal and will shorten the life of a cooler.
In the study, the TDS of fresh water coming into the cooler, measured in milligrams per liter of water, varied from 250 to 1,600. (The home with a 1,600 reading had a water softening system and cooler water was heavy with sodium chloride -- table salt.) The average TDS of incoming water (without the house with a softener system) was 371. Coolers with a bleed off system to dump salt buildup had water in the pan ranging from 375 to 4,043. Coolers with no bleed-off system ranged from 1,931 to 9,557.
Most homeowners whose cooler operated with a bleed-off system used the bleed-off water to irrigate trees or lawns. Water with TDS levels below 5,000 Mg/L can be used for most home irrigation needs.
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Policy Questions
While a number of domestic manufacturers have made great strides in recent years to make their coolers more efficient, some have seen the resulting cost increases cut into their market share, and have watched as competitors produced "knock off" imitations of their latest technology by employing cheap look-alike components.
Certainly there is room for programs that will encourage the development and production of more efficient and better-built cooling systems. The potential city, state or federal policy directions are many: develop and enforce standards of design and product quality; offer rebates or underwrite loans to citizens so they can purchase more efficient units; purchase high efficiency coolers outright for those who cannot afford the up-front costs often associated with more efficient technology; commit government redevelopment dollars to upgrading cooler efficiency in the homes of the poor and the near poor; assist in development of retrofit add-ons or upgrades to existing coolers; develop more water-efficient and energy-efficient cooling technology and transfer that technology to the private sector for marketing.
For more information on evaporative coolers, request "Evaporative Cooler Water Use," Cooperative Extension Bulletin 291045 (revised 5/94) from the University of Arizona College of Agriculture, Tucson, Arizona 85721 or from your county Extension Service office. The bulletin is available as a "PDF" file on the University's Internet site at: http://ag.arizona.edu/pubs/consumer/az9145.pdf.
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Last Modified on 02/27/2001 11:24:20