Special Shoes Could Harvest Usable Energy From Footsteps

Wouldn’t it be nice if you could charge your smartphone battery by simply tapping into some of the energy produced by your body while walking or running? A new technology, currently being developed by scientists at the University of Wisconsin-Madison, promises just that. Aimed at reducing our dependency on conventional energy-guzzling batteries, it ensures that we have proper access to power everywhere we go, thanks to special shoes that can harvest energy from the user’s footsteps.

Published last November in the Scientific Reports journal, the study focuses on the development of an innovative technology that can capture energy from human movements to power a variety of mobile electronic devices. According to the team, a special energy harvester, embedded inside footwear, could be used to collect the energy generated by humans during walking or running, storing it for later use. Speaking about the project, Tom Krupenkin, a UW-Madison professor of mechanical engineering, said:

Human walking carries a lot of energy. 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 technology, the researchers believe, could be used to charge a range of electronic devices, including smartphones, laptops, tablets, flashlights and so on. A smartphone, for instance, requires under 2 watts to be fully charged. As the scientists point out, currently-available technologies lack efficiency, especially when it comes to harvesting energy from large forces of footfalls and small displacements. Central to the current technology is a phenomenon called “reverse electrowetting”, pioneered by Krupenkin and his team back in 2011. The project’s leader added:

So we’ve been developing new methods of directly converting mechanical motion into electrical energy that are appropriate for this type of application.

In reverse electrowetting, a highly-conductive liquid interacts with a special nanofilm-covered surface, converting the available mechanical energy into usable electricity. Despite its efficacy as a power-generating system, the method usually requires a relatively high frequency energy source, like a fast-rotating mechanical source or one that vibrates quickly. However, as Krupenkin points out:

…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. So reverse electrowetting by itself didn’t solve one of the problems we had.

To that end, the researchers developed another highly-specialized technique, described in the paper as the “bubbler method”. It basically combines the process of reverse electrowetting with alternating bubble growth and collapse. The device, according to the researchers, contains two closely-placed flat plates, separated by a gap filled with a specific conductive fluid. The contraption’s bottom plate is perforated with tiny holes that allow pressurized gas to pass through, forming small bubbles. When the bubbles become larger in size, they collapse upon contact with the top plate. The movement of the bubbles in turn pushes the liquid back and forth within the gap, resulting in the production of electrical charge. The team explained:

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.

During laboratory testing, the bubbler contraption generated nearly 10 watts of power per square meter of its area. The scientists, however, believe that the technology could be fine-tuned to produce up to 10 kilowatts of electrical power per square meter. If embedded inside shoes, the advanced energy-harvester could prove useful for people living in remote areas and locations that have insufficient access to electrical power grids. Furthermore, these power-generating footwear could come in handy for the military, as soldiers are often required to carry heavy batteries for their GPS trackers, radios and even night-vision goggles. Krupenkin said:

The bubbler really shines at producing high power densities. For this type of mechanical energy harvesting, the bubbler has a promise to achieve by far the highest power density ever demonstrated.