Harnessing abundant but inaccessible energy from unlikely sources may soon be as simple as taking a walk in the park. Researchers at the University of Wisconsin-Madison have developed a new method of harvesting kinetic energy from human locomotion. “Human walking carries a lot of energy,” says Tom Krupenkin, researcher and UW Professor of Mechanical Engineering. “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.” The energy harvesting shoe could prove a simple but effective lifeline in remote ares where power access is limited or nonexistent.
Harvesting human kinetic energy is more complicated than it may seem. The “relatively small displacements and large forces of footfalls” that occurs during ambulation have proven elusive to capture. “We’ve been developing new methods of directly converting mechanical motion into electrical energy that are appropriate for this type of application,” Krupenkin says. One such method is reverse electrowetting, in which a conductive liquid reacts with a nanofilm-coated surface to generate electrical energy. To adapt this process for harvesting energy from a shoe, the team created and incorporated a device called a bubbler, which consists of two plates separated by a conductive liquid. Through tiny holes, pressurized gas enters the bubbler, which creates bubbles that move the upper plate and facilitate the generation of an electrical charge.
The team is enthusiastic about the bubbler breakthrough. “The bubbler really shines at producing high power densities,” says Krupenkin. “For this type of mechanical energy harvesting, the bubbler has a promise to achieve by far the highest power density ever demonstrated.” The team has revealed this method’s ability to generate around 10 watts per square meter in early experiments. To expand on this work, Krupenkin and his colleague J. Ashley Taylor have founded the start-up InStep NanoPower, which seeks industry investment to commercialize their discovery.