An energy-harvesting technology developed by University of Wisconsin-Madison researchers captures the energy of human motion to power mobile electronic devices. The footwear-embedded energy harvester could be especially useful for the military, as soldiers currently carry heavy batteries to power their radios, GPS units, and night-vision goggles in the field.

The advance also provides a potential source of power to people in remote areas and developing countries that lack adequate electrical power grids.

UW-Madison Mechanical Engineering Associate Professor Tom Krupenkin used the footwear energy harvester to directly power an LED flashlight.

“Human walking carries a lot of energy,” Krupenkin says. “Theoretical estimates show that it can produce up to 10 watts per shoe, and that energy is just wasted as heat."

Krupenkin says tapping into just a small amount of that energy is enough to power a wide range of mobile devices, including smartphones, tablets, laptop computers, and flashlights. A typical smartphone, for example, requires less than two watts.

The researchers use a “reverse electrowetting” technology approach; a conductive liquid interacts with a nanofilm-coated surface, directly converting mechanical energy into electrical energy.

A "bubbler" device consists of two flat plates separated by a small gap filled with the conductive liquid. The bottom plate is covered with tiny holes through which pressurized gas forms bubbles. The bubbles grow until they are large enough to touch the top plate, causing the bubble to collapse.

The speedy, repetitive growth and collapse of bubbles pushes the conductive fluid back and forth, generating electrical charge.

The researchers say their bubbler method can potentially generate high power densities — lots of watts relative to surface area in the generator — which enables smaller and lighter energy-harvesting devices that can be coupled to a broad range of energy sources.

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Also: Learn about NASA's Piezoelectric Energy Harvesters.