Is evaporation the next great renewable?

Anyone who has observed a puddle of water gradually dry and disappear has seen natural water evaporation in action. Now a group of scientists from Columbia University in New York have developed a strategy for harnessing the power of evaporation into a source of renewable energy, according to a paper published online this week in Nature Communications.

Biologist and physicist Ozgur Sahin and his team have developed a model that indicates the total power available from evaporation-driven engines to be as much as 325 gigawatts – more than 69% of US electrical energy generation rates. The strategy could provide power at real densities of up to 10 watts per square metre, three times that of modern wind power.

About half of the solar energy absorbed at the Earth’s surface drives evaporation, fuelling the water cycle that affects renewable energy resources such as wind and hydropower.

The researchers restricted their studies to existing lakes and reservoirs larger than 0.1 square kilometres in the United States, excluding the Great Lakes.

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“Recent advances in water-responsive materials and devices demonstrate the ability to convert energy from evaporation into work,” they write. “These materials perform work 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 placed above a body of evaporating water. With improvements in energy conversion efficiency, such devices could become an avenue to harvest energy via natural evaporation from water reservoirs.”

Further, they estimate evaporative water losses from bodies of water equipped with such devices could be nearly halved.

A key challenge for current renewable energy resources is intermittency: wind turbines and solar photovoltaic panels produce power only when the wind and sun are available. Since the supply of power must match demand on a real-time basis to maintain a stable electrical grid, energy storage is a critical component for stable renewable energy systems. 

The Columbia researchers found that power output from evaporation-driven engines could be controlled by using water’s heat capacity to store and release energy. “Strikingly, we find that storing energy thermally in the water below an evaporation driven engine could substantially reduce intermittency by varying power supply to match power demand,” they say.

The study cautions, however, that using evaporation-driven materials and devices on lakes or reservoirs could affect freshwater resources, altering water withdrawal rates, water quality and recreational use. 

“However, the potential area available for open-water energy harvesting is substantial,” they write. “Lakes and reservoirs cover at least 95,000 square kilometres (excluding the Great Lakes) of the contiguous US – and are found across a geographically diverse range of locations. Some of these regions suffer from periods of water stress and scarcity, which might favour implementation of these energy harvesting systems due to the reduction of evaporative losses.”

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