motion powered energy harvester fits in shoe

a device inserted into the soles of shoes may generate 2 watts of usable electricity from the wearer’s movements while walking, using a technology being developed by a research team at the university of wisconsin-madison

led by tom krupenkin and j. ashley taylor, a team of microelectronics researchers at america’s university of wisconsin-madison have developed a new energy-harvesting technology that utilizes the energy generated by human motion, in a way that is hundreds of times more efficient than motion-powered piezo-electric systems. with their company instep nanopower, the team hopes to produce a device that fits in the sole of a shoe and uses the reverse electrowetting process to generate about two watts of usable electricity from the simple act of walking.

device prototype

‘humans, generally speaking, are very powerful energy-producing machines,‘ explains krupenkin, professor of mechanical engineering. ‘while sprinting, a person can produce as much as a kilowatt of power,’ and up to 10 watts of power is lost as heat each time a foot hits the ground. the difficulty is in engineering technology that efficiently converts mechanical to electrical energy on this scale.

the research team utilizes an innovative energy-harvesting technology called ‘reverse electrowetting’. in electrowetting, a conductive liquid droplet, placed on an electrode, becomes physically deformed by an applied electric charge. the process is based on the same principles of electrostatic capacitors, but involves the replacement of a physical electrode with a electrically conductive liquid. in the reversed process used by the krupenkin, the changing physical form of the droplets on dieletric-coated plates generates charge, which can be harvested as electrical power.

the energy generated increases in proportion to the number of droplets used. while the initial tests involved 150 droplets, producing a few milliwatts of power, krupenkin expects that a device with 1000 droplets could generate up to 10 watts while fitting in an area of 40 square centimeters (6.2 square inch).

left: water droplet on the nanostructured surface developed by the research team (note: the device uses mercury or a gallium-alloy to function, not water) right: diagram of the functional basis of the technology, liquid microdroplets caught between electrode plates

the electricity generated can be used to directly power a range of mobile devices via physical connection to the microfluidic device; or alternatively the harvester can be integrated with a wifi hot spot to reduce the power consumption of wireless devices, increasing the duration of a battery charge by up to 10 times.

instep nanopower is working on a prototype device that fits into the sole of a shoe, capable of generating about two watts of power and thus recharge a standard cell phone battery over the course of a two hour walk. a commercially ready device is expected within two years.

via nature