A footwear-embedded device that harvests and stores energy from human footsteps could be used to recharge smartphones and other power-hungry devices in the future.
Developed by American researchers, the power-generating shoes could help everyone who frequently needs to use their phone away from the grid. The military could make use of the technology to power not only mobile phones but also radios, GPS units and night-vision goggles. It could also help in developing countries and other areas without proper access to electricity.
"Human walking carries a lot of energy," said Tom Krupenkin, Professor of mechanical engineering at the University of Wisconsin-Madison, who led the research. "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."
A typical smartphone requires less than two watts, which means the power generated by walking would be enough to power even larger devices such as tablets, laptops or flashlights. However, engineers have struggled so far to make the technology efficient.
"We've been developing new methods of directly converting mechanical motion into electrical energy that are appropriate for this type of application," Krupenkin said.
The team took advantage of a phenomenon known as reverse electrowetting, which uses a conductive liquid interacting with a nanofilm-coated surface to directly convert the mechanical energy into electricity. Krupenkin pioneered this approach in 2011, proving it could be used to generate useable power. The only limitation of reverse electrowetting is that it requires a power source with quite a high frequency – something that human motion is not.
"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 admitted.
The researchers therefore designed a device that creates miniature bubbles at a very high speed that grow and collapse, pushing conductive fluid back and forth, generating electrical charge.
"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 explained.
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.
"The bubbler really shines at producing high power densities," said Krupenkin. "For this type of mechanical energy harvesting, the bubbler has a promise to achieve by far the highest power density ever demonstrated."
The power-generating shoes could be connected to the smartphone via a cable or even be turned into a wi-fi hotspot, acting as a middle-man between the smartphone and the network. That would decrease the energy requirements of smartphones, extending the time between recharging by up to 10 times.
"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," said Krupenkin.
The team is currently looking for industry partners who could help them commercialise the technology.