The technology could enable a footwear-embedded energy harvester that captures energy produced by humans during walking and stores it for later use.
Power-generating shoes could be especially useful for the military, as soldiers currently carry heavy batteries to power their radios, GPS units and night-vision goggles in the field. The advance could provide a source of power to people in remote areas and developing countries that lack adequate electrical power grids.
“Human walking carries a lot of energy,” Krupenkin says. “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.”
Krupenkin says tapping into just a small amount of that energy is enough to power a wide range of mobile devices, including smartphones, tablets, laptop computers and flashlights. For example, a typical smartphone requires less than two watts.
The researchers’ bubbler device — which contains no moving mechanical parts — consists of two flat plates separated by a small gap filled with a conductive liquid. The bottom plate is covered with tiny holes through which pressurized gas forms bubbles. The bubbles grow until they’re large enough to touch the top plate, which causes the bubble to collapse.
The speedy, repetitive growth and collapse of bubbles pushes the 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 says.
The researchers say their bubbler method can potentially generate high power densities — lots of watts relative to surface area in the generator — which enables smaller and lighter energy-harvesting devices that can be coupled to a broad range of energy sources.
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, according to Krupenkin.
InStep NanoPower, LLC and Vibram S.P.A. have the first practical footwear embedded energy harvester. The harvester is based on a revolutionary new method of converting mechanical energy into electrical energy recently developed by InStep NanoPower researchers.
The mechanical energy produced by humans during walking, which is normally simply lost as heat, is captured and converted into electrical power ready to be utilized by wireless personal electronics.
We have proposed and successfully demonstrated a novel approach to direct conversion of mechanical energy into electrical energy using microfluidics. The method combines previously demonstrated reverse electrowetting on dielectric (REWOD) phenomenon with the fast self-oscillating process of bubble growth and collapse. Fast bubble dynamics, used in conjunction with REWOD, provides a possibility to increase the generated power density by over an order of magnitude, as compared to the REWOD alone. This energy conversion approach is particularly well suited for energy harvesting applications and can enable effective coupling to a broad array of mechanical systems including such ubiquitous but difficult to utilize low-frequency energy sources as human and machine motion. The method can be scaled from a single micro cell with 10−6 W output to power cell arrays with a total power output in excess of 10 W. This makes the fabrication of small light-weight energy harvesting devices capable of producing a wide range of power outputs feasible.