Evaporative cooling is one of the oldest solutions humankind has used to achieve comfort in hot climates. For thousands of years, different strategies have been developed that take advantage of the cooling effect that occurs when water evaporates into the surrounding air — this can be observed in nature where temperatures are generally cooler near bodies of water, rain cools the atmosphere, and sweat cools our bodies as it evaporates from the skin.
Since evaporative cooling improves with higher air temperatures and lower humidity because air admits more evaporated water, it comes as no surprise that the first traces of its use were found in civilizations located in hot and arid climates, such as Ancient Egypt, the Roman Empire, and the medieval Islamic civilizations. Examples of this “technology” can also be found in traditional architectural designs all over the world.
However, when current, conventional air conditioning devices were invented in the early 1900s, these traditional cooling strategies were set aside. Today, we look back to this natural phenomenon that can achieve efficiency ratios above ten — that is to say, they provide more than ten times the amount of cooling than the energy required to operate them — while conventional air conditioning devices barely reach efficiency ratios of three.
If the world is to remain on track to meet sustainability goals, part of this strategy needs to look at reducing energy demand as we make a transition to renewable energy alternatives. As global temperatures continue to rise, once overlooked technology, evaporative cooling could help minimize the impact of cooling systems.
How is evaporative cooling applied today?
Today, many technologies apply this phenomenon during hot seasons, with direct evaporative cooling systems being the most widely used as they evaporate water directly into the air, they do not only cool the air, but also increase humidity. Because humidification may or may not be desirable, other more advanced technologies, called indirect evaporative cooling, avoid it by allowing water evaporation in an auxiliary airstream, which then is used to cool the air that has to be conditioned.
To do this effectively, these systems need to enhance air-water contact: through water spraying, such as fog systems, or from wetted surfaces, called evaporative cooling pads. The former can be applied in outdoor spaces, while, the latter requires air to be forced through the wetted media. The application of either depends on the situation for their use.
An interesting alternative to conventional wetted media is the use of vegetable surfaces or active living walls. Air is cooled and humidified as it passes through plants arranged on vertical surfaces. These are an ecological air conditioning system that also “biofilter” the air.
Evaporative cooling can be more efficient than conventional air conditioning
Evaporative cooling systems are cheap and have very low energy requirements to operate — only requiring apump that supplies the water and a fan that drives the air. Consequently, they are the most common air conditioning solution in “high volume” spaces such as farms, greenhouses, industrial buildings, and outdoor spaces where conventional air conditioning systems would not be feasible. In hot and dry regions, where outdoor air temperature can exceed 40⁰C and relative humidity falls below 40%, evaporative cooling systems can cool air temperatures to less than 10⁰C of the outdoor air temperature with almost no energy consumption.
If evaporative cooling technologies are so energy efficient, why do not they replace conventional air conditioning?
Their main limitation is its dependence on air conditions; in humid climates, the amount of water that can evaporate within the air decreases, hence limiting its cooling effect. In very hot conditions, evaporative cooling performs well, but may be insufficient to achieve acceptable indoor temperatures, or may result in excessive indoor humidity.
But the alternative, conventional air conditioning systems, perform worse under harsh climate conditions and require excessive energy in humid climates, as it is partially consumed for dehumidification.
The idea here is not to necessarily replace one with the other, but perhaps a combination of the two seems to be a prospective solution. Evaporative cooling, for example, could improve the performance of conventional air conditioning systems if used to precool the outdoor air where the former dissipates heat, improving their efficiency ratios.
In a warming world, innovative solutions such as this are desperately needed.
Written by: Ana Tejero González and Antonio Franco Salas
Reference: Ana Tejero-González and Antonio Franco-Salas Direct evaporative cooling from wetted surface: challenges for a clean air conditioning solution, WIREs Energy and Environment (2021). DOI: 10.1002/wene.423