Scientists at Columbia University 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. Their details are published in the journal Nature Communications.
The inventions pave the way for a new generation of renewable devices that extract energy from natural evaporation and transform it into something useful.
It works using bacterial spore that expand when they become humid to drive an engine. The machines build on Sahin’s discovery last year that spores of common soil bacteria swell when they absorb water in humid environments and shrink when they release the water in drier air. The change in spore size can be used to push and pull objects.
“Water wants to evaporate. It has a desire to evaporate. If you make a surface wet, it will dry up, that’s the natural course”.
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’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. The scientists call them hygroscopically driven artificial muscles, or hydra.
They 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. The hydra are put inside a plastic case that has little shutters overhead. When placed on water, evaporating moisture makes the hydra elongate and open the shutters above them. This allows the moisture in the case to escape, causing the hydra to contract and the shutters to close again. The cycle then repeats.
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.
In tests, the scientist found the engine generated enough electricity to make an LED bulb flash on and off.
With its current power output, the floating evaporation engine could supply small floating lights or sensors at the ocean floor that monitor the environment, speculating 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 Columbia team’s another new evaporation-driven engine – the Moisture Mill – contains a plastic wheel with protruding tabs of tape covered on one side with spores. Half of the wheel sits in dry air, causing the tabs to curve, and the other half sits in humid environment, where the tabs straighten. As a result, the wheel rotates continuously, effectively acting as a rotary engine.
The researchers now built a small toy car, powering it with the Moisture Mill and were successful in getting the car to roll on its own, powered only by evaporation.
In the future, the researcher said, it may be possible to design engines that use the mechanical energy stored in spores to propel a full-sized vehicle. Such an engine, if achieved, would require neither fuel to burn nor an electrical battery.
A larger version of the Moisture Mill could also produce electricity, suggesting a wheel that sits above a large body of water and evaporates saltwater, causing the wheel to rotate and generate electricity. This development would steadily produce as much electricity as a wind turbine.
