Researchers harvest electricity from evaporating water

Scientists have found a way to harness the power of evaporating water to produce electricity that is powerful enough to light up a small LED and power a miniature car.
Columbia University scientists have developed 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.
Ozgur Sahin, an associate professor of biological sciences and physics at Columbia University, had found in previous research that when bacterial spores shrink and swell with changing humidity, they can push and pull other objects forcefully.

In the new study, to build a floating, piston-driven engine, the researchers first glued bacterial 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's 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.

Sahin and Xi Chen, a postdoctoral fellow in his lab, then placed dozens of such tapes side by side 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," Chen said.
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 said.
With its current power output, the floating evaporation engine could supply small floating lights or sensors at the ocean floor that monitor the environment, Chen said.
The Columbia team's other new evaporation-driven engine - the Moisture Mill - contains a plastic wheel with protruding tabs of tape covered on one side with spores.

The researchers also built a small toy car powered with the 'Moisture Mill' and were successful in getting the car to roll on its own, powered only by evaporation.
In the future, Sahin said, it may be possible to design engines that use the mechanical energy stored in spores to propel a full-sized vehicle.