Footsteps Could Power Mobile Devices

Estimated reading time: 4 minutes

When you're on the go and your smartphone battery is low, in the not-so-distant future, you could charge it simply by plugging it into … your shoe.

That's right. An innovative energy harvesting and storage technology developed by University of Wisconsin-Madison mechanical engineers could reduce our reliance on the batteries in our mobile devices, ensuring we have power for our devices no matter where we are.

In a paper published November 16, 2015, in the journal Scientific Reports, Tom Krupenkin, an associate professor of mechanical engineering at UW-Madison, and J. Ashley Taylor, a senior scientist in mechanical engineering, described a new energy-harvesting technology that's particularly well-suited for capturing the energy of human motion to power mobile electronic devices.

The technology could enable a footwear-embedded energy harvester that captures energy produced by humans during walking and stores it for later use.

Power-generating shoes 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 could provide a source of power to people in remote areas and developing countries that lack adequate electrical power grids.

"Human walking carries a lot of energy in it," Krupenkin says. "Theoretical estimates show that it can produce up to 10 watts per shoe, and that energy is just wasted as heat. A total of 20 watts from walking is not a small thing, especially compared to the power requirements of the majority of modern mobile devices."

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. For example, a typical smartphone requires less than two watts.

However, traditional approaches to energy harvesting and conversion don't work well for the relatively small displacements and large forces of footfalls, according to the researchers. "So we've been developing new methods of directly converting mechanical motion into electrical energy that are appropriate for this type of application," Krupenkin says.

The researchers' new energy-harvesting technology takes advantage of "reverse electrowetting," a phenomenon that Krupenkin and Taylor pioneered in 2011. With this microfluidic approach, as a conductive liquid interacts with a proper nanofilm-coated surface, the mechanical energy is directly converted into electrical energy.

The reverse electrowetting method can generate high power densities but it requires an energy source with a reasonably high frequency, such as a mechanical source that's vibrating or rotating quickly. "Yet our environment is full of low-frequency mechanical energy sources such as human and machine motion, and our goal is to be able to draw energy from these types of low-frequency energy sources," Krupenkin says. "So reverse electrowetting by itself didn't solve one of the problems we had in this space."

To overcome this problem of needing a high-frequency mechanical energy source, the researchers developed what they call the "bubbler" method, which they described their Scientific Reports paper.

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