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The First Evaporation-Powered Engine

Author Nicholas Derenzo Illustration Jameson Simpson


The first steam engine was patented more than three centuries ago, paving the way for the Industrial Revolution. Now, researchers at Columbia University are once again harnessing the power of water—this time with a twist. The new innovation? Scrapping the need for a heat source and instead using a simple puddle of room-temperature water to generate electricity. To achieve this seemingly impossible goal, bioengineers have turned to an unlikely (and decidedly low-tech) ally: the common grass bacteria, Bacillus subtilis. This microbe’s spore boasts the unique property of dramatically changing size when exposed to even the most minuscule shift in humidity. Led by bioengineer Ozgur Sahin, the team has put these spores to work in what they’re calling HYDRAs, or hygroscopy-driven artificial muscles, which function by contracting and expanding—a process that has already been used to power a primitive piston engine and run a miniature model car. Best of all, the device costs less than $5 to construct. Here’s how scientists hope to exercise their bacterial might and muscle their way into the renewable energy game.


1. To create HYDRAs, bacterial spores are glued to both sides of thin strips of plastic tape, resulting in something that looks like kinky ramen noodles. When the air surrounding the HYDRA gets even slightly more humid, the strips absorb the water molecules and nearly quadruple in size, much like a Slinky being stretched out.   

2. HYDRAs are attached to shutters and placed inside an enclosure, over a puddle of water. As the water evaporates, the HYDRA expands and forces open the shutters, allowing moisture to escape. As humidity decreases, the HYDRAs dry out, contract and close the shutters. The HYDRAs are connected to a tiny electromagnetic generator that yields about 50 microwatts of energy per cycle (a microwatt is a millionth of a watt). That’s not a lot of energy, but it has already been shown to turn on an LED light in early trials.

3. The inventors have also created a “moisture mill,” which uses HYDRAs in a rotary engine. Strips are affixed to a paddleboat-like wheel, half of which is tucked into a semicircular cavity, and then placed on a two-axle car. Water is introduced, and humidity builds up in the protected half, which spins the wheels and scoots the car along. It may be years before we actually fill up our tanks on tap water, but the concept is proving that evaporation-powered engines might be a little more science and a little less fiction.                                       

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