A new way to harvest footfall energy could someday let shoes generate enough power to keep cell phones and laptops topped up.
University of Wisconsin-Madison researchers have come up with a microfluidics technique that scavenges considerably more energy from human footfalls and converts it into electric power. Previous attempts to make energy-harvesting shoes have yielded less than a watt of power, but the new approach could lead to a shoe-mounted generator that produces up to 10 watts, says Tom Krupenkin, a mechanical engineering professor who led the work.
“A lot of energy is simply wasted as heat while we walk,” says Krupenkin. “If one can convert this into electrical energy, numbers come out to be up to 10 watts per foot.” Cell phones and smart phones need about one to two watts, while small laptops need 10 to 12 watts. Power-generating shoes could be an important breakthrough for soldiers, who currently carry heavy batteries to power their radios, GPS units, and night-vision goggles.
Walking exerts a lot of force on the heel and toe, and cushioned soles can compress by about a centimeter with every step. Energy harvesters convert this force and displacement into electrical energy. The most promising approaches to tap into the human gait have involved piezoelectrics and electroactive polymers, materials that convert mechanical stress into electric power. But neither material works well with the relatively high displacements, but low frequency, of footfalls, Krupenkin says.
The new concept, presented in a Nature Communications paper, involves microscopic droplets of a conductive fluid flowing between electrodes coated with dielectric films. The droplets—the researchers used mercury or a gallium-based alloy called galistan—can be sandwiched between flat plates coated with the film or can be enclosed in a coated microchannel. When the area of overlap between the droplets and electrodes changes, an electric current is produced.
“It’s a unique approach to energy harvesting,” says Andrew Haughian, a partner at Vancouver, Canada-based venture capital firm Pangaea Ventures, which is evaluating the technology for potential investment. “The biggest opportunity I see would be in [developing countries], where the power grid is not reliable.”
It might be years before you can buy a power-generating shoe, though. So far, the researchers have only made an array of 150 droplets that gives a few milliwatts of power. However, they calculate that a device with 1,000 droplets in a four-meter-long, one-millimeter-wide channel, which would cover an area of 40 square centimeters and fit in a shoe sole, could generate a few watts.
“The process is interesting, and the work itself is very good,” says Paul Wright, a mechanical engineering professor at the University of California at Berkeley. However, he says, “to be useful to society, they would need to scale up the approach and show that it still works.”
Krupenkin and his colleagues have established a startup, InStep NanoPower, to develop and possibly commercialize the technology. The company has a first-generation benchtop-sized prototype device. They expect the third generation harvester could be embedded in footwear. “This type of product will have to be a collaborative project between Instep and a shoe manufacturer,” Krupenkin says. “We can’t expect anything on the market earlier than two years.”
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