Image courtesy of Ozgur Sahin, Columbia University
Rotary engine (top) harnesses energy from evaporation to drive a miniature car. Electron microscope image (bottom) shows the evaporation-powered “artificial muscle” made from bacterial spores on a polymer component.
Could water evaporating provide power? Biological systems are known to convert energy generated from the evaporation of water confined within nanoscale compartments into muscle-like mechanical work in response to changes in environmental humidity. Recently, scientists designed shape-changing engineered composites of bacterial spores and a polymer that used an evaporation-driven process to power locomotion and generate electricity.
By harvesting the energy from water evaporation in the environment researchers demonstrated self-sustained power generation from engines placed at air-water interfaces. These evaporation-driven engines could power robotic systems, sensor, devices, and machinery.
Evaporation is a powerful natural force able to extract and deliver water over great distances, from oceans to mountaintops. On a smaller scale, biological systems harness the energy of evaporation, such as when wheat releases pollen and pine cones release seeds. These biological systems convert energy generated from the evaporation of water confined within nanoscale compartments into mechanical work in response to changes in environmental humidity. Extending this energy conversion ability to human-made systems has proven to be difficult. One reason is the decreased evaporation rates in larger engineered assemblies (compared to confined spaces) coupled with the slow rate of change in humidity. As a result, scaled-up energy conversion systems based on water evaporation and adsorption cycles have been a neglected area of research. Scientists at Columbia University have effectively harvested energy from evaporation in the environment to drive engines that start and run autonomously when placed near a water surface. Arrays of artificial “muscles” composed of bacterial spores on a polymer strip enhanced the water evaporation rates when scaled-up. Researchers optimized the thickness of the spore layer for rapid water transport into and out of the nanopores. The energy of evaporation was harnessed to power the muscle. A small portion of the power operated a feedback mechanism to address the slow and variable rate of change in environmental humidity, enabling rapid cycling between spore swelling to stretch the muscle and shrinking to contract the muscle. Upon stretching at a desired humidity, the self-regulating feedback mechanism allows the humidity to decrease. Precise control of this process enabled self-sustained power generation from piston-driven and rotary engines when placed at air-water interfaces—powering a light source and moving a miniature car forward. These results could enable evaporation-driven engines to power robotic systems and sensors.
U.S. Department of Energy, Office of Science, Basic Energy Sciences, David and Lucile Packard Fellows Program (preparation and characterization of HYDRA samples, development of the rotary engine).
X. Chen, D. Goodnight, Z. Gao, A.H. Cavusoglu, N. Sabharwal, M. DeLay, A. Driks, and O. Sahin, “Scaling up nanoscale water-driven energy conversion into evaporation-driven engines and generators.” Nature Communications 6, 7346 (2015). [DOI: 10.1038/ncomms8346]
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