Power Sheet Delivers Wireless Energy

Jan. 3, 2007 — These days it seems absurd to see a telephone handset tethered via curly-cue cord to its base station. Yet most of us still plug in to power up PDAs, laptops, cell phones, MP3 players, televisions and stereos.

Now researchers have built a wireless power transmitter that could cut the cord, so to speak.

The device is a 1-millimeter-thick sheet containing electronic parts made from flexible, plastic semiconductor materials.

Thin and lightweight, the sheet can be imbedded into desktops, walls, floors or ceilings, to power nearby electronic devices. The result: ubiquitous charging without any tripping over cords.

"The system selectively feeds power as high as 30 watts to electronic objects placed upon it," said associate professor Takao Someya of University of Tokyo, who developed the technology with colleague Takayasu Sakurai.

"Thirty watts is enough to drive a small laptop computer," said Someya.

Unlike transmitting information through the air — think wireless Internet — transmitting energy is tricky. The challenge is that the energy disperses, making the transfer inefficient.

And there is also the issue of safety, said assistant professor Marin Soljacic of the Massachusetts Institute of Technology.

"You want to deliver watts from a source to a device and hopefully not burn humans or cats or whatever lives in the room," he said.

Soljacic's team is working on a system of power transmission analogous to the wireless Internet — energy would flow over a few feet and electronic devices would simply have to be in the same room as a transmitter to be charged.

The power sheet overcomes both of these hurdles because it can transmit energy over a short distance (between the pad and the device), and transmit the energy using a magnetic field, which isn't harmful to humans.

First, the power sheet must be plugged into a power generator, battery or electrical outlet. When an object is placed on the sheet, an array of so-called "receiver" coils embedded in the sheet sense the object's location.

At the same time, the alternating current from the power source flows through a different array of so-called "sender" coils, producing a magnetic field.

The magnetic field causes tiny switches to flux back and forth and deliver the energy wirelessly to the receiver coils.

From there, the energy is delivered to the electronic object to be charged as it rests on top of the receiver coils.

The ability to transmit the power selectively to the position of the object, said Someya, significantly reduces the amount of energy that would otherwise disperse.

Someya and his team are working to improve the tiny components within the power sheet and think it could reach the market in about five years.