In what has been called a “very, very impressive breakthrough,’” bioengineers at Columbia University have built tiny working engines – complete with living parts – to harvest energy from the evaporation of room temperature water. Tests have proven that an evaporation-driven floating engine can power an LED light and that a plastic wheel “Moisture Mill” engine can drive a miniature (0.1 kg) car forward.
The “living parts” of the evaporation engines are slightly modified, naturally-occurring spores of the bacteria Bacillus subtilis that expand and contract in response to humidity. The engineers developed “hygroscopy-driven artificial muscles” (HYDRAs) made from thin strips of plastic tape coated in spore layers. The HYDRA change curvature and overall length in response to changes in humidity, and the engine captures the energy of that microscopic movement to start and run autonomously when placed at air–water interfaces.
“The engine is placed over a puddle of room-temperature water, creating a small enclosure. As the water on the surface naturally evaporates, the inside of the engine becomes slightly more humid. This triggers strips of HYDRAs to expand as they soak up some of the new-found humidity. Collectively, these HYDRAs pull on a cord which is attached to a small electromagnetic generator, transforming the cord's movement into energy. The HYDRAs also pull open a set of four shutters on top of the engine, releasing the humid air. With the shutters open, humidity inside the engine drops. This causes the HYDRAs to shed their water-vapor and contract, which pulls the shutters back closed. And the process repeats, just like an engine's cycle.”
The researchers hope that this proof of concept translates into “off-the-grid” applications. The amount of energy created by an evaporation engine is tiny (roughly 50 microwatts) but the fact that it provides enough energy to power an LED light by just passively sitting over room temperature water might lend it to applications in the developing world. In addition, the materials used to make the engine are cheap, and the other “raw material,” evaporating water, is ubiquitous in nature.
While it is still very early days for the technology, suggestions have been made that it might one day compete with solar or wind energy. Researchers envision scaled-up versions of the engines that eventually 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.” It has also been suggested that with its current power output, a floating evaporation engine could supply small floating lights or sensors on the ocean floor that monitor the environment, or that it might be used to power small robots or devices. Regardless of immediate uses, researchers plan to continue investigating evaporation technology by improving the efficiency of component materials to capture more of the energy potential and make better use of the spores.