What's the News: We've all fantasized about a cell phone battery that won't quit. Now scientists hoping to harness the power generated when you walk are developing a device that might eventually use your footfalls to power small electronics
. But will it overcome the hurdles of efficiency and cost? How the Heck:
When your heel hits the ground, as much as 20 watts of power (that's 20 joules of energy per second) are generated as heat. To capture some of that energy, researchers developed a system that involves pressing droplets of liquid metal against an electrode.
Here's how it works: The team's device resembles an electrostatic capacitor, which consists of two flat electrodes wired together with a small space between. When a voltage is applied to the electrodes, their opposing charges freeze them in place, but pushing on them changes the circuit's capacitance, increases the voltage, and causes a current to run through the circuit, which can then be harvested.
But getting the electrodes close enough to each other to generate meaningful power is difficult because of the roughness of metals (see ScienceNOW's great explanation of this process for more). To address that problem, the researchers essentially replaced one of the electrodes a liquid metal alloy that could conduct electricity, and inserted an extremely thin insulating material in between it and the other electrode, resulting in a mere 10-50-nanometer wide gap between. This set up could yield far more power when compressed.
If such a device were inserted into a shoe, 2 watts of power could be harvested, the researchers say---more than 1000 times what had been harvested with other systems. A two-hour walk could recharge a cell phone battery, the lead researcher told ScienceNOW.
A prototype of the device, depicted here, is in development.
What's the Context:
Scientists and engineers have been interested in energy harvesting for decades: for a long time, DARPA was hoping to generate power from devices in soldiers' boots, and such gleaning is the topic of this vintage Wired piece on electricity-harvesting shoes. But systems developed so far only get small amounts of wattage from each step.
If you remember the early '90s, you probably remember shoes equipped with small LEDs that flashed when you walked. Those were powered by piezoelectricity, the charge that accumulates in crystals and metal when they are squeezed or stressed. Many previous harvesting schemes have tried to make use of piezoelectricity, which was discovered by Pierre Curie and keeps the beat in modern watches, but no one has succeed is getting significant amounts of power that way.
Not So Fast:
If such a device is developed, it wouldn't necessarily save people very much money, in terms of electricity bills, so it would have to be very cheap to have a wide adoption. Of course, for some people, the lure of being able to salvage some of the energy lost when walking could be create demand all on its own.
But getting the power from your shoes to your phone is another problem again---a dangling cord that plugs into your heel is no fashion statement. The researchers think they could set up a Wi-Fi hotspot in the shoe, which could take the burden of long-range transmitting off the phone and thus save energy, but that does restrict the device's usefulness to other electronics.
The Future Holds: The scientists have founded a company, InStep NanoPower
, to commercialize the technology, and they're working on a prototype shoe insert that they hope to start testing in a couple years. Reference: T. Krupenkin and J. A. Taylor, Nature Communications (23 August 2011) DOI: 10.1038/ncomms1454