No power outlet? No problem! If a pair of University of Wisconsin–Madison engineers has its way, juicing up a dying smartphone could soon be as easy as plugging it into your shoe. In a paper published in the November 16, 2015 issue of Scientific Reports, Tom Krupenkin, a professor of mechanical engineering, and J. Ashley Taylor, a senior scientist, detail a new technology that’s “particularly well-suited” for harvesting energy from human motion to power mobile electronics. “Human walking carries a lot of energy,” Krupenkin said in a statement. “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.”
Considering that an average smartphone operates on less than two watts, even a small proportion of that energy could keep myriad portables, including laptops, humming along nicely.
Power-generating shoes could be particularly useful in the military, since soldiers typically carry heavy batteries to power devices such as radios, GPS units, and night-vision goggles in the field. A footwear-embedded energy harvester, Krupenkin and Taylor say, could also provide a source of power for people in remote regions or developing countries with inadequate electrical grids.
While the concept itself isn’t new, the researchers’ approach takes advantage of “reverse electrowetting,” a mechanical-to-electrical energy conversion behavior that Krupenkin and Taylor observed in 2011.
The phenomenon, which occurs when a conductive liquid interacts with a nanofilm-coated surface, can generate usable power, but only if the mechanical source is vibrating or rotating “reasonably” quickly.
To enhance the effect, Krupenkin and Taylor created a device that generates an electrical charge by spawning and then collapsing tiny bubbles through a conductive fluid.
Though their proof-of-concept “bubbler” generated around 10 watts per square meter in preliminary experiments, theoretical estimates show that up to 10 kilowatts per square meter might be possible, according to Krupenkin.
Krupenkin and Taylor are currently looking for industry partners to help them commercialize their footwear-embedded energy harvester through their startup company, InStep NanoPower.
There are a couple of ways their harvester could power mobile devices: directly via a cable or by embedding electronics such as a Wi-Fi hotspot.
The latter tack could even extend the life of a cellphone battery by as much as 10 times 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 said.