If you’ve ever emerged from a pool on a hot day and gotten a chill, you’ve directly experienced the principle behind an evaporative cooling tower. When water is converted from the liquid to the vapor state (as it is when it evaporates from your skin) the portion that evaporates contains more heat than the portion that remains.1
A tower makes this process go faster. Hot water is sprayed at the top of the tower onto a material with a large surface area (called “fill”). During this process the hot water is cooled, both by evaporation and by contact with cooler, outside air.2 As the water from the top trickles down, it is cooled substantially relative to the hot water that was originally sprayed in at the top of the tower.
The huge hourglass shaped stacks at nuclear power plants operate this way and the “smoke” seen rising from them is actually steam.
Why evaporative cooling is used in air conditioning
In a normal air conditioner, heat is transported outside of a building with a refrigerant into a radiator-like set of cooling coils. These coils heat up the air they come into contact with and this change in temperature is then the basis for the entire air conditioning cycle.3 A fan might be used to “drive” more outside air across the cooling coils to enhance the process, but air is not the best way to move heat energy around.
Water is much better, and a system that cools the air conditioner coils (the ones containing refrigerant) can handle a larger cooling load more efficiently.4 It is the efficiency obtained that makes an evaporative cooling tower attractive.
Commercial or residential?
The cost savings improve when units are large and there is a high demand for cooling. This meant that in the past, evaporative cooling towers were found only in commercial applications (large buildings, apartment housing, retail spaces, offices). It was too much trouble to install an additional piece of equipment (the tower) in residential HVAC applications.
The situation is changing however. As the price of energy increases, the importance of saving energy also rises. Under these conditions, residential evaporative cooling becomes economically feasible.5 (Check out ACboy’s evaporative cooler main page for more info. –Dave)
A government report from 2006 (Department of Energy) found they could be used successfully and at a substantial savings by home owners, although the payback period (time to recover additional costs over a “regular” system) varied considerably based on climate and whether units were optimally controlled or manually (set by home owner).6
The most impressive statistic from the study was achieving an incredible 97.6 annually adjust SEER rating for the best combination. (Compared to a SEER rating of 13 for a standalone, traditional unit without evaporative cooling.)
Local climates matter a great deal, since the water must evaporate to get the best effect. This means that in very humid conditions (atmosphere already at water holding capacity) the technique will not work well, or at all. In drier climates (low humidity) the highest benefits will be obtained.7