Footsteps Could Power Mobile Devices

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The bubbler method combines reverse electrowetting with the fast process of bubble growth and collapse. As this process of bubble growth and collapse occurs in the bubbler device, it creates an internal high-frequency source independent from the mechanical energy source. "So the high frequency that you need for efficient energy conversion isn't coming from your mechanical energy source but instead it's an internal property of this bubbler approach," Krupenkin says.

The researchers' bubbler device - which contains no moving mechanical parts - consists of two flat plates separated by a small gap that's filled with a conductive liquid. The bottom plate has tiny holes through it as well as a dielectric-coated electrode on its surface. Pressurized gas blows up through the holes in the bottom plate, forming bubbles on each hole. These bubbles grow until they're large enough to touch the top plate, which causes the bubble to collapse, and this extremely fast process of bubbles growing and collapsing rapidly repeats itself. As the gas bubbles push the conductive fluid back and forth across the bottom surface, the flow of that fluid generates electrical charge motion and gives rise to electrical current due to reverse electrowetting.

The researchers say their bubbler method can potentially generate extremely high power densities, which enables smaller and lighter energy harvesting devices that can be coupled to a broad range of energy sources.

The proof-of-concept bubbler device generated around 10 watts per square meter in preliminary experiments, and theoretical estimates show that up to 10 kilowatts per square meter might be possible, according to Krupenkin.

"The bubbler really shines at producing high power densities," he says. "For this type of mechanical energy harvesting, the bubbler has a promise to achieve by far the highest power density ever demonstrated."

Krupenkin and Taylor are seeking to partner with industry and commercialize a footwear-embedded energy harvester through their startup company, InStep NanoPower.

The harvester could directly power various mobile devices through a charging cable or it could be integrated with a broad range of electronic devices embedded in a shoe, such as Wi-Fi hot spot that acts as a "middleman" between mobile devices and a wireless network. The latter requires no cables, dramatically cuts the power requirements of wireless mobile devices, and can make a cellphone battery last 10 times longer between charges.

"For a smartphone, just the energy cost of radio-frequency transmission back and forth between the phone and the tower is a tremendous contributor to the total drain of the battery," Krupenkin says.

Additional authors on the Scientific Reports paper include UW-Madison mechanical engineering graduate students Tsung-Hsing Hsu and Supone Manakasettharn.



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