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Wireless power: Electricity in the air

More than a century ago, electrical genius Nikola Tesla performed pioneering research and development on many of the things we take for granted today, from X-rays and alternating current electricity to efficient motors and generators and the precursors of radio. But when he turned his vivid imagination to sending electricity over the air with radio waves to power all sorts of devices and appliances without cords, it was an expensive failure.

Fast-forward to the present, where a company called Powercast is doing just what Tesla dreamed of: transmitting power via radio waves. "In a real sense, we're picking up where Tesla left off," says Harry Ostaffe, vice president for marketing and development at the Pittsburgh-based vendor. "We are sending power over the air for devices where it is expensive or inconvenient to change batteries."

Powercast transmitter and receivers
The Powercaster transmitter and two Powerharvester receiver chips (not to scale).

Called power harvesting, the technique uses the company's book-size Powercaster transmitter to send either 1 or 3 watts of electricity into the air at the 915MHz radio frequency. At the receiving end, the power is pulled from the air by one of the company's Powerharvester chips, which convert RF energy to DC power.

At the moment, Powercast has two chips: One works best at close range and puts out up to 4.2 volts of continuous electricity for directly powering a very low-power device or charging a battery. The other can be used at longer distances from the transmitter to create an intermittent pulse of up to 5.25 volts for directly powering a low-power device.

These chips can grab small amounts of usable power -- from microwatts to low milliwatts -- out of thin air. That's not enough to run an MP3 player or phone, but it is sufficient for a device that uses very little power, like a Kindle e-book reader, according to Ostaffe.

Rather than developing systems for consumer electronics products, however, Powercast focuses on powering the various sensors that monitor our world, from temperature and pressure sensors in oil refineries to smoke alarms in homes and offices. The typical office today might have a door-position sensor that's part of its security system, a smoke detector, and a motion sensor to turn off the lights if nobody's in the room -- all of which could be powered wirelessly with Powercast's technology.

The company's goal is to eventually develop technology that can extract usable amounts of power from ambient sources such as Wi-Fi signals. But for now its system needs a transmitter to work, which means it's subject to range limitations, Ostaffe says.

Powercast - Harry Ostaffe
Powercast's Harry Ostaffe: "We're picking up where Tesla left off."

"Our biggest enemy is the inverse square law," he jokes. This fundamental law of physics describes how energy radiating outward from a point source -- such as light or in this case radio waves -- is dissipated over distance. The energy available for the receiver falls off very quickly the farther you get from the transmitter.

"Right now, our usable range is about 40 to 45 feet," he says. That's long enough to cover part of a house or a few offices in a building. The typical office facility would probably need 10 or 15 transmitters per 40,000 square feet of floor space, set up around the building's periphery with antennas aimed toward the center.

NIST's Seiler agrees that RF-to-DC power has potential for certain kinds of devices. "It's promising but is limited by range and the amount of power an RF wave can hold. But it could power many smaller devices," he says.

Powercast isn't the only company developing RF-to-DC power technology. Nihon Dengyo Kosaku of Japan, for instance, has been working on a similar system that relies on a special rectifying antenna. Powercast, however, claims to have a head start on its competitors, saying it has chips ready to be integrated into products.

By 2012, Powercast hopes to have a household sensor product available to power smoke detectors that will never need to have their batteries changed. The company has also been working on shrinking its transmitters for home use. Ostaffe envisions a miniaturized transmitter "the size of a child's night light that would be powerful enough to power the smoke alarms in a modest-size home."

Nevertheless, distributing power over the air remains a tantalizing step or two away from mainstream availability. Although Powercast has the designs and chips ready, it needs a manufacturer to make and sell the actual products for consumers or businesses.

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