Energy harvest. Compressing or sliding plates sandwiching electrically conductive droplets efficiently coverts mechanical motion to electricity.

T. Krupenkin and J. A. Taylor, Nature Communications (23 August 2011); (shoes) InStep NanoPower

Power Walk, Gain a Watt

Instead of walking to the corner store to buy batteries for your flashlight, soon you could just walk—anywhere. A pair of Wisconsin researchers has come up with a novel energy-harvesting device that converts mechanical motion into electrical energy at a high enough power to recharge cell phones, cameras, and myriad other portable electronic devices. Such energy harvesters won't do away with batteries. But if they become practical, they could dramatically lengthen the time between when we need to plug our devices into the wall.

Walking may seem a languid activity, but it still generates plenty of energy. Previous studies have shown that as much as 20 watts of power per foot is lost as heat with each stride. Researchers have tried for decades to harvest some of this lost energy and convert it into electrical power. But the various types of energy harvesters developed so far generate mere microwatts to hundreds of milliwatts of power, well below the amount needed to recharge a typical portable electronics battery.

One type of energy harvester, and an inspiration for the new work, is known as an electrostatic capacitor. It consists of a pair of thin, flat, metal electrodes with a tiny gap sandwiched in between. The electrodes are wired together to complete an electronic circuit. When a constant voltage is applied between the two electrodes, one electrode holds positive charges, the other negative. Those charges interact to keep two electrodes essentially locked in place. But if mechanical force is applied to move one electrode, the change lowers the surface area where the two electrodes interact. A result is that a property known as the capacitance decreases, which in turn raises the voltage between the two electrodes. That excess voltage drives a current through the external circuit, which is collected.

One key to this technology is that the smaller the gap between the two electrodes, the more the capacitance and voltage increases, so the more power can be harvested before the electrodes move back to their starting position and the cycle can be run again. But aligning two solid-state electrodes extremely close together is very difficult because of the rough surfaces of the materials. Typically they wind up a micrometer or so apart, too large to harvest much power.

So University of Wisconsin, Madison, engineers Tom Krupenkin and J. Ashley Taylor decided to try doing away with one of the solid electrodes and replacing it with an electrically conductive liquid. They started with a conductive solid substrate, which they topped with droplets of an electrically conductive liquid. On top of it they placed a metal electrode coated with a 10- to 50-nanometer-thick film of an insulating material. That meant that the gap between the metal electrode and the conductive liquid electrode was a mere 10 to 50 nanometers. The bottom conductive substrate and the top electrode were then connected into a circuit. So when the solid electrode was pushed down, compressing the liquid droplets, or pushed laterally over the top of them, the device produced a very large capacitance and voltage. If scaled up to the size that would fit in a typical shoe, this would enable the Wisconsin researchers to harvest 2 watts of power, they report today in Nature Communications. At that rate, a person could completely recharge a standard cell phone battery by going for a 2-hour stroll, Krupenkin says.

"Using a liquid droplet is a clever way to do it," says Zhong Lin Wang, a pioneer of energy-harvesting devices at the Georgia Institute of Technology in Atlanta. The top power output reported by the Wisconsin researchers is 1000 times as high as other technologies have achieved, Wang notes. Krupenkin adds that another advantage of the technology is that it can be easily scaled up to harvest energy from a larger device with hundreds or thousands of liquid drops.

Krupenkin and Taylor have formed a company called InStep NanoPower in Madison to commercialize the new technology. They are currently working to produce a prototype device that can be fitted into the sole of a shoe, which they expect will be ready in 2 years.