Indirect evaporative cooling is very similar to direct cooling but for the fact that this system uses some sort of heat exchanger. The hot, dry air is drawn into the cooler just as in a direct system and is cooled by evaporating cool water. In an indirect system, however, the cooled moist air never comes in direct contact with the air conditioned living space. Instead a closed circuit system is used in which the cooled air comes into contact with a heat exchanger or heat transfer fluid, which essentially trades the warmth of the indoor air for the evaporatively-cooled air from the cooler. The heat exchanger prevents any humidity from entering the living space.
Solar power enters the equation by running the electrical components of the system, including any fans or blowers used to collect outside air and distribute cooled air around the home. Evaporative coolers, direct or indirect, recycle the water after it percolates down through the moistened pads. At the bottom, in the reservoir where the water collects and is recycled, is a float valve -- similar to that you'll find in the guts of your toilet -- that signals to the system to add more water, which is naturally lost in some quantity during the evaporation process. An evaporative cooler, depending on the cooling loads, will use anywhere from 3 to 15 gallons of water per day.
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Indirect Evaporative Cooling / Direct Evaporative Solar Cooling / Two-Stage Evaporative Coolers