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Sustainable Energy

Power-Scavenging Batteries

Inexpensive chips harvest mechanical energy to charge batteries for wireless sensors.

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.

Good vibrations: This prototype wireless-sensor battery (top) incorporates four energy-harvesting chips. A close-up of one chip (bottom) shows a vibrating cantilever that converts mechanical energy into electrical energy.

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.

Robert Andosca, the founder and president of MicroGen Systems, says what sets its technology apart from other vibrational energy scavengers is that it uses a proprietary, nontoxic material; the most common piezoelectric material, called PZT, contains lead. 

Andosca also says manufacturing costs should be relatively low. Other piezoelectric energy-harvesting devices are large and must be assembled by hand, costing hundreds of dollars to produce. Because MicroGen’s cantilever arrays are based on silicon microelectrical mechanical systems, or MEMS, they are small and can be made inexpensively on the same equipment used to make computer chips. The company expects to make its energy harvesters for about a dollar each, depending on the volume.

MicroGen, a spin-off from the University of Vermont, has been using a foundry at Cornell University; it has sent samples of the devices to major semiconductor manufacturers. The company hopes to rent manufacturing time on a commercial MEMS line and begin selling its product in about a year.

The first market Andosca hopes to enter is batteries for tire-pressure sensors. These sensors are required by United States law in new cars because optimizing tire pressure saves fuel. “It’s a pain in the neck to change those batteries,” he says. Today, they must be replaced every three years; Andosca says that 64 million of these batteries are put into cars each year. He hopes that the energy-scavenging-and-storing chips would last the lifetime of the car.

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