|Fig. 1: This figure shows how energy from the sun which creates evaporation of the water which will then be captured by an engine above the water. (Source: M. Humphrey. After Cavusoglu et al. )|
Evaporation, with an average global energy flux of about 80 W m-2, is a powerful process in nature that affects ecosystems, water resources, weather and climate.  While the global fresh water supply has been somewhat of an issue concerning our future, if one is to look at the bright side of the situation, recent technological developments in the area of water responsive materials are showing signs of converting energy from evaporation into usable power. As we continue to shift our reliance from fossil fuels to more environmentally friendly forms of energy such as solar and wind, a breakthrough in another renewable energy source, such as harnessing power from evaporation is very important.
Through a cycle of absorbing and rejecting water via evaporation these water-responsive materials can be incorporated into evaporation driven engines that harness energy when located over a fresh body of evaporating water.  This can be seen in Fig. 1 which illustrates the process of harvesting the energy. This "Evaporation Engine" works by controlling humidity with a shutter that opens and closes, this causes bacterial spores to expand and contract. The contractions are then transferred to a generator that makes electricity. Energy harvested from this "Evaporation Engine". would be used to supply additional power along with our naturally occurring energy resources such as solar and wind. It is predicted that in warm weather conditions, 20°C, 250 W m-2 can be generated.  However, the current measured power per square meter is 8.4 W. It is important to understand that until this process can produce more power per square meter then it will be more cost effective to invest in other forms of renewable energy, such as solar cells.
One benefit of evaporation is that it can be generated only when needed, in contrast, to solar and wind power which are only available when the sun is shining or the wind is blowing. Also, wind and solar power require expensive batteries that are comprised of toxic materials in order to store the power they generate. Evaporation technology can also save water, in fact, it is estimated that in some of the drier regions of the United States as much as 5.9 mm of H20 can be saved per day with the usage of the Evaporation Engine.  Essentially, drier regions that already maintain reservoirs to hold water in case of drought could benefit twice, retaining more of that supply while producing energy at the same time. Some dilemmas with the Evaporation Engine, are that they would be quite the eyesore, as well as almost totally eliminating the use of lakes and reservoirs for recreation and they are extremely expensive. Additionally, this process requires the sun to provide the heat in the water which then creates more energy that can be captured through evaporation, but if the water is covered then the water will cool and there will not be energy to capture.
Like all forms of power there are positive and negative repercussions from harvesting energy from evaporation. However, the possibility of combining renewable energy forms such as; solar, wind and evaporation is very possible in the future.
© Michael Humphrey. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 A.-H. Cavusoglu et al., "Potential for Natural Evaporation as a Reliable Renewable Energy Resource," Nat. Commun. 8, 617 (2017).