Evaporation engines could produce more power than coal — but there's a catch

MIT Technology Review via Xi Chen

Technology that can tap into the renewable power of natural water evaporation could produce a huge portion of the nation's energy needs—at least theoretically (see "Scientists Capture the Energy of Evaporation to Drive Tiny Engines").

Prototype "evaporation-driven engines" generate power from the motion of bacterial spores that expand and contract as they absorb and release air moisture.

If it could be done efficiently and affordably, the devices could provide more than 325 gigawatts of electricity-generating capacity, outpacing coal, according to a study published Tuesday in Nature Communications.

That, however, would require covering the surface of every lake and reservoir larger than 0.1 square kilometers in the lower 48 states, excluding the Great Lakes, with arrays of the devices. Obviously, that would directly conflict with existing economic and recreational uses, and raise a host of serious aesthetic and environmental concerns. Notably, interfering with evaporation on a large enough scale, across a big enough lake, could even alter local weather.

But study coauthor Ozgur Sahin says that the paper is more of a thought experiment designed to underscore the potential of the technology and the importance of advancing it beyond lab scale, rather than any sort of literal development proposal.

Sahin, an associate professor of biological sciences and physics at Columbia University, believes it could make a significant contribution to clean energy and climate goals, even if it's never rolled out at anywhere near the potential extent highlighted in the study.

He says that early use cases could include remote reservoirs already generating hydroelectric power, where it's not as likely to interfere with other uses. It could offer the added benefit of reducing water loss through evaporation, increasing the amount available for energy generation, irrigation, and other needs.
The team of scientists also created a tiny, evaporation-powered car, dubbed Eva.

Sahin and colleagues at Columbia have been working on this technology for years. In a 2015 paper, the team described an evaporation engine that relied on Bacillus subtilis spores adhered to stacks of film attached to shutter mechanisms.

When the device is placed above water, the spores absorb moisture from natural evaporation and expand, opening the shutter and allowing moisture to escape.

The spores then dry out and contract, closing the shutter once again, and allowing additional air moisture to flow in and restart the process. When connected to a generator, the continual oscillating motion generates a tiny amount of power.

As MIT Technology Review previously reported: "An eight-centimeter by eight-centimeter water surface can produce about two microwatts of electricity (a microwatt is one-millionth of a watt), on average, and can burst up to 60 microwatts."

The team has continued to work on improving the efficiency and scalability of the technology, exploring additional materials and means of spore adhesion. Because the technology is largely based on biological materials, the eventual cost could be lower than solar photovoltaic cells and other technologies that require specially manufactured materials, Sahin believes.

Crucially, Bacillus subtilis spores continue to perform the necessary mechanical motion even when they're dead or dormant.

In addition, the technology largely avoids the intermittency limitations of wind and solar power because, while evaporation rates change, they don't stop. Moreover, since the devices decrease the evaporation rate, they also raise the temperate of surface water.

Modeling in the new study showed that by deliberating altering the rate of this process, they could create a kind of thermal water battery that balances out generation and demand. When throttled up, the heat in the water would increase evaporation, boosting power generation.

“We could match power demand on an hourly basis about 98 percent of the time in warm and dry places,” Sahin says. “Which means you don’t need an external battery to adjust for intermittency.”

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