A new form of renewable energy could be added to solar, wind, and hydro power. Better still, evaporation as an energy source should be more continuous than most other renewables. So far, the technology has only been demonstrated on a tiny scale, but a new study shows that, if it can be scaled up at a practical cost, it could provide two-thirds of the electricity used in the United States.
Changing liquids to gasses involves the absorption of energy. Water has an unusually high specific latent heat, meaning a high amount of energy required to cause a particular amount to change state. Consequently, it normally represents an energy sink, not a source.
Two years ago, however, Columbia biophysicist Dr Ozgur Sahin demonstrated what he calls the Evaporation Engine. The engine uses bacterial spores that swell when they absorb water. If the spores are attached on each side of a piece of tape with the lines offset, changes in humidity flex the tape, pulling on a piston or rotary engine.
Normally, it would take too long for the environment to change sufficiently for anything useful to come of this, but Sahin placed his tapes inside a container part filled with water. Some were attached to a shutter. When sunlight evaporated some of the water, the air became humid, stretching the tape and opening the shutter. Outside air caused the humidity to fall, which in turn contracted the tapes, closing the shutter. Even after some of the energy produced was used to control the shutters, enough was left to drive miniature cars or power a small light.
There's probably a market for toys powered this way, but practical application is a different matter. At the time, Sahin and his team thought it might be useful for off-grid electricity production. Now, in Nature Communications Sahin has thought bigger. Much bigger. If the engines were placed on lakes and reservoirs across America, he calculates they could produce 15 Watts per square meter in the right circumstances, and 325 gigawatts nationally, even without tapping the Great Lakes. This equals 69 percent of the electricity America now consumes.
Covering vast areas of lakes with evaporation machines would be expensive, but there would be benefits as well. Somewhat ironically, the machines reduce the rate of evaporation, preserving fresh water in dry areas – exactly the places where the evaporation is fastest, and possibly help paying for the system.
Perhaps most importantly, although evaporation is powered by sunlight and wind, it doesn't stop even on still nights, although it will slow down. Consequently, a grid powered by evaporation would need less battery storage than one depending on solar or wind.