1.7.7 Evaporative Cooling
Induce cooling with water: the transformation of liquid water molecules into gas through evaporation uses energy to change the water’s physical state. In this process, the energy is taken from the air and transferred to the water. As a result, the air is cooler.
For evaporative cooling to work properly, the cooled outside air must be allowed to escape. By designing strategic ventilation pathways with openings to the exterior, the flow of cooled air can be directed.
There are 3 types of evaporative cooling systems: direct, indirect, and two-stage. A direct evaporative cooling system (also called an open circuit system) is essentially a large fan with water-moistened pads in front. The fan draws warm outside air through the pads and blows the now-cooled and more humid air throughout the space. An indirect system (also called a closed circuit system) works similarly to a direct system, but passes the cooled and humidified outdoor air through a heat exchanger which then transfers the temperature to the air circulated throughout the conditioned space. The outdoor air never comes in to contact with the conditioned environment, and no humidity is transferred. A two-stage system (also called an indirect-direct system) combines both of these techniques: first air passes through a heat exchanger that is cooled by evaporation on the outside. In the second stage, the pre-cooled air passes through a water-moistened pad. Two-stage systems produce more cooling than either direct or indirect systems alone. Since the outdoor air is pre-cooled before it passes through the water-moistened pads, less humidity is transferred.
An evaporative cooler can use as little as 10 percent of the energy consumed by other air conditioning systems. Payback occurs within 6 months to 5 years.
Two-stage evaporative coolers can reduce energy consumption by 60 to 75 percent over conventional air conditioning systems, according to the American Society of Heating and Engineers (ASHRAE).
Direct evaporative cooling is only effective where outdoor humidity is very low and works best in hot and dry climates. Indirect evaporative cooling can be used in hot humid climates. Depending on climate, evaporative cooling may not be able to meet peak cooling loads. However, when combined with air conditioning, it can meet total loads during milder cooling periods and decrease compressor loads under peak conditions.