From: Kristin Kusek, Harvard University
Published February 27, 2014 12:15 PM

Potential new source of renewable energy found in humidity

A new type of electrical generator uses bacterial spores to harness the untapped power of evaporating water, according to research conducted at the Wyss Institute of Biologically Inspired Engineering at Harvard University. Its developers foresee electrical generators driven by changes in humidity from sun-warmed ponds and harbors.

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The prototype generators work by harnessing the movement of a sheet of rubber coated on one side with spores. The sheet bends when it dries out, much as a pinecone opens as it dries or a freshly fallen leaf curls, and then straightens when humidity rises. Such bending back and forth means that spore-coated sheets or tiny planks can act as actuators that drive movement, and that movement can be harvested to generate electricity.

"If this technology is developed fully, it has a very promising endgame," said Ozgur Sahin, Ph.D., who led the study, first at Harvard's Rowland Institute, later at the Wyss Institute, and most recently at Columbia University, where he's now an associate professor of biological sciences and physics. Sahin collaborated with Wyss Institute faculty L. Mahadevan, Ph.D., and Loyola University faculty Adam Driks, Ph.D. The researchers reported their work in late January in Nature Nanotechnology.

Water evaporation is the largest power source in nature, Sahin said. "Sunlight hits the ocean, heats it up, and energy has to leave the ocean through evaporation," he explained. "If you think about all the ice on top of Mt. Everest — who took this huge amount of material up there? There's energy in evaporation, but it's so subtle we don't see it."

But until now no one has tapped that energy to generate electricity.

As Sahin pursued the idea of a new humidity-driven generator, he realized that Mahadevan had been investigating similar problems from a physical perspective. Specifically, he had characterized how moisture deforms materials, including biological materials such as pinecones, leaves and flowers, as well as man-made materials such as a sheet of tissue paper lying in a dish of water.

Sahin collaborated with Mahadevan and Driks on one of those studies. A soil bacterium called Bacillus subtilis, wrinkles as it dries out like a grape becoming a raisin, forming a tough, dormant spore. The results, which they reported in 2012 in the Journal of the Royal Society Interface, explained why.

Unlike raisins, which cannot re-form into grapes, spores can take on water and almost immediately restore themselves to their original shape. Sahin realized that since they shrink reversibly, they had to be storing energy. In fact, spores would be particularly good at storing energy because they are rigid, yet still expand and contract a great deal, the researchers predicted.

Read more at Harvard's Wyss Institute.

Rain on window and Hygrometer images via Shutterstock. Morphed by Robin Blackstone.

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