When we think of renewable energy sources that could help replace the burning of climate-altering carbon-based fuels, solar panels and wind turbines come to mind. But Columbia University scientists have identified another, thus-far untapped energy source that might have just as much promise — the massive amount of water that continually evaporates from the nation's lakes and reservoirs.
That natural phenomenon, they say, could be tapped into by devices containing sheets covered with bacterial spores, which contract and expand in response to changes of moisture — almost like the flexing of a muscle. That mechanical "muscle" action, in turn, could be used to generate gigantic amounts of electricity, they say.
In a newly published article in the journal Nature Communications, the researchers estimate that inland bodies of water in the United States have at least the theoretical potential to generate as much as 325 gigawatts of electricity, an amount equals to nearly 70 percent of our nationwide electrical consumption in 2015.
"Our artificial muscle absorbs water evaporating from the surface of the lake," says chemical engineer and study lead author Ahmet-Hamdi Cavusoglu, via email. "As it absorbs water, the muscle swells and expands. When the muscle absorbs all the possible water, the shutters above the muscle open so that water can evaporate into the air from the muscle. As the water leaves the muscle, the muscle shrinks, pulling at a turbine to generate energy, similar to a rower on a rowing machine pulling at the turbine."
In practice, that amount of power might be difficult to achieve, since as the article notes, it would require completely covering the entire surface of just about every domestic lake and reservoir. But as even a much smaller deployment of evaporation engines — say, covering 10 percent of those water surfaces — still could add a substantial amount of electrical capacity to the nation's supply.
"If we were to cover only 10 percent of the top ten lakes and reservoirs we studied with the most efficient evaporation engines possible," says Cavusoglu, now is an associate director of the Academic Venture Exchange and director of partnerships at the Innovation Accelerator Foundation, "we could harness an average power of 7.1 gigawatts from an area of about 560 square miles [1450 square kilometers], which is greater than the utility solar power sector in the U.S. in 2013."
And that's just if they used lakes and reservoirs. Evaporation energy conceivably could be harvested from other sources as well, ranging from rivers and coastlines to farm fields that have been moistened by irrigation.
"Water is pretty much ubiquitous in the environment," says Ozgur Sahin, an associate professor of biological sciences and physics at Columbia and one of the study's co-authors. (Sahin and colleagues pioneered the use of spores and evaporation to create tiny engines, as detailed in this 2015 Nature Communications article.)
Using evaporation as a renewable energy source could have other advantages as well, the researchers note. Evaporation is driven by solar energy, which is absorbed by a body of water and stored, almost like a gigantic battery, and then released gradually. That means an evaporation engine could produce electricity with fewer fluctuations than solar panels or wind turbines, whose production drops whenever the wind dies down or the sun goes behind a cloud.
Additionally, evaporation engines could alter the amount of water vapor given off by a lake or reservoir or another source, providing aid to water-strapped parts of the nation. The engines could also control the electrical output.
"An interesting property we realized is that by slowing down evaporation, we can raise the temperature of water, effectively storing energy as heat that we could later release on demand," says Cavusoglu. "Depending on the local weather conditions, we could release this stored energy rapidly as additional evaporation to match fluctuations in electrical energy demand. This would be similar to how hydroelectric dams vary their power output by changing the flow rate of water through the dam."
So far, evaporation engines have been tested only in the laboratory, and there's a lot more research and development work ahead before the technology could be scaled up to the size where it might become a major source of electricity.
"First, better and bigger materials need to be made that can effectively harness energy from evaporation," says Cavusoglu. "Currently, the Sahin lab is researching how to make better spore-based materials and building larger engine prototypes. We hope to see larger prototypes being demonstrated in the next few years."