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Evaporative Coolers

An evaporative cooler (also swamp cooler, desert cooler and wet air cooler) is a device that cools air through the evaporation of water.

Today’s evaporative coolers come in one of three general designs: direct, indirect, and indirect/direct. They can use about one-quarter as much energy as central air conditioners and cost half the price. Evaporative coolers require regular maintenance to keep the water reservoir clean.

Direct, single-stage evaporative coolers

Direct, single-stage evaporative coolers work by drawing outside air through water-saturated pads. The flowing air evaporates some of the water, giving up heat in the process, which can reduce the air temperature by 15°F to 40°F. The cooled, humidified air is then blown into the home. The air is not recirculated inside the house but is blown through the house and an exit must be provided for it; thus, a window or vent must be opened to allow air to leave the house. Lower air flow helps the cooler to work more efficiently. An evaporative cooler should have at least two speeds and a vent-only option. The vent-only operation allows the unit to be used as a house fan during mild weather.

Indirect coolers

Indirect coolers use an air-to-air heat exchanger so they don’t add humidity to the cool air. The main fan supplies outside air that is cooled by passing through the heat exchanger into the home. A second fan draws exhaust air from the home and/or outside air through the wet pads, which are in contact with the heat exchanger and cool it without raising the humidity of the air that flows into the home.

Indirect/direct coolers

Indirect/direct coolers cool in two stages. In the first stage, the air passes through an indirect cooler, which lowers the temperature without adding humidity. The air then enters the second, direct cooling stage where it flows through the wet pads to be further cooled and humidified before flowing into the home. One model, developed by Building America partner Davis Energy, reportedly provides up to 5 tons of cooling while using less than 1600 watts. The model is eligible for utility rebates up to $1,100 in several south-western states. The installed cost for an efficient indirect/direct evaporative cooler is similar to or less than that of a new central air conditioning system. Evaporative coolers do use water and water costs should be included when considering installing an evaporative cooler.

Another innovative evaporative technology is the water-cooled evaporative condenser, which is a scaled-down residential version of the 250-ton chillers used on commercial buildings. Inside the housing, a mist of water is continually sprayed on the condenser coils to remove heat from the refrigerant and at the same time reduce the work of the compressor. Unlike traditional air conditioners, which use 10% more power for each 10° F increase in temperature above 95° F, an evaporative-cooled condenser draws about the same power over a wide range of outdoor temperatures. It uses about half the energy costs of conventional “air-cooled” condensing units. One model has an EER of 17 at 95°F.

There are now evaporative coolers on the market that use photovoltaic (PV) panels to create the electricity used to run the blower and the water pump. For hot, desert areas, the combination of evaporative cooling and solar power is a perfect match: the afternoon, when the most solar energy is available, is also the hottest part of the day, when cooling is most needed. And, because evaporative coolers use a fraction of the energy of air conditioners, PV cells can provide enough electricity to run the system effectively.