Power-Scavenging Batteries  

Technology Review has a look at small scale devices harnessing ambient energy from their environment - Power-Scavenging Batteries.

MicroGen Systems, a startup based in Ithaca, New York, is developing energy-harvesting chips designed to power wireless sensors like those used to monitor tire pressure and environmental conditions. The chips convert the energy from environmental vibrations into electricity that's then used to charge a small battery. The chips could eliminate the need to replace batteries in these devices, which today requires a trip to a mechanic or, for networks of sensors that are widely distributed, a lot of legwork.

The core of MicroGen's chips is a one-centimeter-squared array of tiny silicon cantilevers that oscillate when the chip is jostled. At the base of the cantilevers is a bit of piezoelectric material: when it's strained by vibrations, it produces an electrical potential that can be used to generate electrical current. The cantilever array is mounted on top of a postage-stamp-sized, thin-film battery that stores the energy it generates. The current passes from the piezoelectric array through an electrical device that converts the current to a form compatible with the battery. When the chip is shaken by, say, the vibrations of a rotating tire, it can produce about 200 microwatts of power.

"If you can get it down to a small size, 200 microwatts is potentially quite useful," says David Culler, chair of computer science at the University of California, Berkeley, and a pioneer in developing wireless sensor networks for environmental monitoring and other applications. However, he notes, engineers are developing "zillions of harvesters" that produce energy from light, heat, radio-frequency waves, or vibration, and convert it into electrical energy that can be used right away or stored on a battery. Culler believes solar power is the technology to beat for most wireless-sensor applications.