Evaporative Cooling an Air Conditioning Alternative
Evaporative coolers, one of the many innovations listed by the Partnership for Advancing Technology in Housing Technology (PATH), was featured recently on ToolBase.org as an alternative to conventional air conditioning.
“Evaporative coolers, commonly called ‘swamp coolers,’ use the natural cooling effect of water evaporation to provide a low first-cost, energy-saving and environmentally-friendly alternative to conventional air conditioners in arid climates,” according to ToolBase E-News, a publication of the NAHB Research Center.
Two types of coolers are available: direct and indirect.
In a direct evaporative cooler, a blower forces air through a permeable, water-soaked pad. As the air passes through the pad, it is filtered, cooled and humidified. Indirect coolers have a secondary heat exchanger that prevents humidity from being added to the air stream entering the house.
Evaporative coolers can be used as a sole cooling system, as an alternative cooling system to a conventional refrigerant air conditioner or combined with a refrigerant system. However, conventional air conditioners should not be operated simultaneously with direct evaporative coolers, because the two systems work in opposition, with the former dehumidifying and evaporative coolers providing humidity.
Windows or ceiling vents need to be open when an evaporative cooling system is operating. The large volume of fresh air added to the home replaces a significant amount of the air that is exiting.
Many systems incorporate a bleed-off valve that purges water about every six hours. This leads to an additional five gallons of water used per hour, but may be necessary to avoid mineral build-up. Bleed-off valves are generally recommended.
Indirect, or two-stage evaporative coolers do not add humidity to the air, but they cost more than direct coolers and operate less efficiently.
Two-stage evaporative coolers combine indirect with direct evaporative cooling by passing air inside a heat exchanger that is cooled by evaporation on the outside. In the second stage, the pre-cooled air passes through a water-soaked pad and picks up humidity as it cools.
Because the air supply to the second stage evaporator is pre-cooled, less humidity is added to the air, whose affinity for moisture is directly related to temperature. The result, according to the manufacturer, is cool air with a relative humidity between 50% and 70%, depending on the regional climate. A traditional system would produce about 80% relative humidity.
Among the attributes of evaporative coolers cited by PATH:
- Direct coolers cost about $700 to $1,000 installed, compared to several thousand dollars for conventional air conditioning and ductwork. In addition, their operating costs are about one-third those of conventional air conditioning — including the cost of water, depending on electric and water costs. Indirect evaporative coolers are much costlier.
- Evaporative coolers use about one-fourth the electricity of conventional air conditioners.
- Evaporative coolers can improve a home’s indoor air quality by drawing a large supply of fresh outdoor air through the home. However, they consume between 3.5 and 10.5 gallons of water per hour of operation.