Evaporating water to produce electricity

Can the energy of evaporating water be harnessed to propel self-sufficient devices or produce electricity? A new experiment raises hopes.

Columbia University scientists report the development of two novel devices that derive power directly from evaporation -- a floating, piston-driven engine that generates electricity causing a light to flash and a rotary engine that drives a miniature car.

When evaporation energy is scaled up, the researchers predict, it could one day produce electricity from giant floating power generators that sit on bays or reservoirs, or from huge rotating machines akin to wind turbines that sit above water.

"Evaporation is a fundamental force of nature. It is everywhere, and it is more powerful than other forces like wind and waves," said lead author Ozgur Sahin.

Sahin found that when bacterial spores shrink and swell with changing humidity, they can push and pull other objects forcefully.

They pack more energy, pound for pound, than other materials used in engineering for moving objects. Building on these findings, Sahin and colleagues sought to build actual devices that could be powered by such energy.

To build a floating, piston-driven engine, the researchers first glued spores to both sides of a thin, double-sided plastic tape akin to that in cassette tapes, creating a dashed line of spores.

They did the same on the opposite side of the tape, but offset the line so dashes on one side overlapped with gaps on the other.

When dry air shrinks the spores, the spore-covered dashes curve. This transforms the tape from straight to wavy, shortening the tape. If one or both ends of the tape are anchored, the tape tugs on whatever it is attached to.

Conversely, when the air is moist, the tape extends, releasing the force. The result is a new type of artificial muscle that is controlled by changing humidity.

The team then placed dozens of these tapes side by side, creating a stronger artificial muscle that they then placed inside a floating plastic case topped with shutters.

Inside the case, evaporating water made the air humid. The humidity caused the muscle to elongate, opening the shutters and allowing the air to dry out.

When the humidity escaped, the spores shrunk and the tapes contracted, pulling the shutters closed and allowing humidity to build again. A self-sustaining cycle of motion was born.

"When we placed water beneath the device, it suddenly came to life, moving on its own."

The spore-covered artificial muscles function as an evaporation-driven piston. Coupling that piston to a generator produced enough electricity to cause a small light to flash.

"We turned evaporation from a pool of water into light," Sahin added.

The researchers speculate that an improved version with stickier plastic tape and more spores could potentially generate even more power per unit area than a wind farm.

The findings were reported online in Nature Communications.