Humidity, that pesky mainstay of summer, is an untapped source of renewable energy. And now, researchers have developed the first ever engines driven by evaporation. The devices, described in Nature Communications this week, were used to propel a mini car and power little LEDs.
Evaporation is happening all the time. It’s how you cool down when you sweat, and it’s how snow is delivered to the highest peak in the Himalayas. In fact, this fundamental force of nature is the dominant form of energy transfer in the planet’s climate. "It's everywhere, and it's more powerful than other forces like wind and waves,” Columbia’s Ozgur Sahin says in a statement.
Sahin and colleagues created what they’re calling “hygroscopy driven artificial muscles,” or HYDRAs, using bacterial spores that shrink and swell as the humidity changes. Within the spores, water is confined in tiny cavities, and humidity triggers pressure changes. Using HYDRAs, the team created two kinds of devices—a floating, piston-driven engine and a rotary engine—that can help generate electricity from evaporation.
First, the team glued a dashed line of spores on one side of an extremely thin plastic tape; they did the same to the other side of the tape, but they made sure to offset the two lines of spores. That allows the tape to change the way it curves: straight when it’s moist, wavy when it’s dry. If at least one end of the tape is anchored, it tugs and releases depending on the environment—acting like an artificial muscle.
When water (blue) on the surface below evaporates, the evaporation engine drives a piston-like back-and-forth motion. When connected to a generator, that motion produces electricity. Joe Turner Lin
Then they placed multiple tapes next to each other in a plastic case that has shutters and floats on top of water (pictured here). Evaporation makes the air humid, causing the artificial muscles to elongate and open the shutters. This dries the air out, the spores shrink, the tapes contract, and the shutters close. This cycle of motion is self-sustaining. Coupling this evaporation-driven piston to a generator produces enough electricity to make a small light flash.
Next up, the team attached the spores to one side of the tabs protruding off a plastic wheel (pictured above). Half of the wheel is exposed to dry air, while the other is encased in a humid environment. That means half the tabs are curved, half are straight. As water evaporates off the wet tabs, the wheel rotates continuously. This so-called Moisture Mill helped roll a tiny, 0.1-kilogram (0.2-pound) toy car forward. You can watch the evaporation-powered car zip by in the cool video below.
If scaled up, evaporation energy could produce electricity from power generators floating on top of reservoirs or from rotating wind turbine-like machines sitting above water.