Five Questions You Should Have About Evaporation as a Renewable Energy Source

What’s the big deal with evaporation-driven engines?

Water-strapped cities with growing populations and energy needs could benefit the most. Greater Phoenix, for instance, is served by this reservoir and irrigation system fed by the Colorado River. (Central Arizona Project)
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When Ozgur Sahin and his colleagues at Columbia University started talking about evaporation generators as a source of renewable energy, our eyes bugged. Could the United States really, as they said in Nature Communications, get 69 percent (approximately 325 gigawatts) of its energy needs from water that evaporates from our reservoirs, lakes and rivers?

The short answer is no. Sahin’s numbers were based on extrapolation of a small-scale study of a machine he invented that generates power via evaporation. This small, flat, “evaporation engine” sits on the surface of a body of water and uses variations in humidity to open and close vents, which can run a generator. To get the number, Sahin multiplied the power he was getting from this device by the total area of lakes, rivers and reservoirs in the U.S. But of course, we’re not going to cover every lake and river. We—and the natural ecosystem—need those for other things.

But that doesn’t mean we can’t benefit from the technology, and use it at a smaller scale, as a source of renewable energy. How might that look? What are we waiting for? Here are five questions you might have about evaporation power, answered.

You can get energy from evaporation? How does that work?

The drinking bird toy your high school physics teacher had on her desk is proof that you can. A body of water absorbs heat from the sun—around half of all the sun’s energy gets used this way—and gradually gives up vapor to the air. The simplest iteration of the evaporation engine is covered with strips of tape, which are themselves covered with bacterial spores. As water vapor collects beneath the strips of tape, the bacteria absorb it and elongate. This causes the tape to flex, simultaneously opening up a vent to the air and pushing a lever, which can be converted from mechanical energy to electrical. The vent releases vapor, the spores dry out, and over just a few seconds, the tape condenses, the vent closes, and the cycle begins again.

The paper Sahin published this year referenced not just his own energy capturing technology, but any type of evaporation harvester. In the case of Sahin’s engine, which he and his colleagues published in Nature Communications in 2015, it works via the expansion and contraction of bacterial spores. Unlike a turbine, which relies on heat to drive the engine, “muscles” made of the spores expand and contract based on humidity—when humidity goes up, the spores expand, elongating the strips of tape-like material they’re attached to, and opening up a sort of vent. Now vented, humidity decreases, the spores contract, the vent closes, and the cycle resets. As this is happening, the motion of the strips push a small wheel, and the rotation drives a generator.

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The evaporation engine sits on the surface of the water (blue) here. When water on the surface below evaporates, it drives a piston-like back and forth motion, which produces electricity if connected to a generator. (Xi Chen)

Could this replace solar or other renewable energy sources?

Just like solar, wind, hydro, and nearly everything else, evaporation energy comes from the sun. Solar power is unique in that it’s obtained directly, says Axel Kleidon, an earth systems scientist at the Max Planck Institute who was a reviewer of the latest Nature Communications paper. All the others feature some sort of intermediary process that decreases efficiency. At the rate solar prices are dropping, it’s unlikely that evaporation power will be cost effective relative to solar panels.

Kleidon studies the energy conversions of natural processes at a large scale. For example, he says, wind power relies on sunlight that has been converted to heat, and then wind, by the atmosphere, each time accruing a loss unseen in solar energy. Additionally, the more wind turbines you put up, the less energy remains in the atmosphere for each turbine to pull out of it. The same would be true for evaporation energy.

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The southern and western United States have the greatest capacity to produce evaporation-generated power from lakes and reservoirs. (Ahmet-Hamdi Cavusoglu)

If it’s not going to greatly reduce the need for other energy sources, then what can we gain from it?

There’s no one answer to human energy needs. Even if we don’t produce 70 percent of our energy this way, it can still contribute. A small percentage of the total wattage they calculated would still impact the renewable energy industry. Wind power, right now, accounts for tens of gigawatts, and solar even less, so even a small percentage of the total available evaporation energy would make a big dent.

But there are benefits beyond the power, too. As you harvest energy, evaporation rates slow down. Especially in the American West, where the environment is dry and water sources are limited, covering reservoirs can help reduce overall evaporation, leaving more water for irrigation and human consumption.

Furthermore, this type of energy could address one of renewables’ current challenges, that of energy storage. Evaporation occurs not just during the day, but also at night, when accumulated warmth from the day’s sun drives vapor into the cooler night air. Solar, and to a lesser extent, wind power die down at night, which is when we need the energy the most. Evaporation energy could complement other on-demand solutions to this problem, like lithium ion batteries, blue batteries or geothermal power.

What side effects might this have for lakes, rivers and ecosystems?

This is not something that was addressed in Sahin’s research. His group ran the numbers, and he says the context is for others to parse as the technology gets further developed. Environmental assessments will need to be done on a location-by-location basis. In some cases, that will mean studying the wildlife that lives on and around a body of water. In others, the recreational, industrial or transportation use of the water must be addressed.

Even the evaporation itself could impact the humidity of the surrounding area. On a large scale, points out Sahin, atmospheric moisture is dominated by the oceans. But small pockets of drier air, where evaporation is being slowed by this technology, could have minor effects on plants or agriculture there. And it could have a significant effect on the temperature of the water it covers. But it all depends on what percent of each water body is covered.

What barriers are still in the way of implementing this technology?

Make it more efficient. Scale it up. Do ecological assessments. We’re at the early stages of a major process. Although it’s reasonable to think the technology will scale well, just by repeating blocks of the proposed devices, it has only been studied at a small scale—the 2015 research featured a single rotary engine. There may be further opportunities to increase efficiency, like optimizing materials and reducing the cost of production, or combining the systems into larger engines. And environmental studies will have to assess the effect on ecosystems where it may be deployed.

About Nathan Hurst

Nathan Hurst blends a love of storytelling with a passion for science and the outdoors, covering technology, the environment, and much more. His work has appeared in a variety of publications, including Wired, Outside, Make: and Smithsonian.

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