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Wireless Power

New physics theory could cut the cord on power chargers for gadgets.
March 12, 2007

One night in 2002, MIT physicist Marin Soljacˇic´ heard the chirps of his cell phone letting him know that its battery was losing the last of its juice. Annoyed, he began to wonder if there were any physics principles that would allow the phone’s battery to be charged in a more convenient way. Over the next three years, Soljacˇic´, graduate student Aristeidis Karalis, and physics professor John Joannopoulos worked on and off to devise a theoretical scheme for charging gadgets wirelessly.

Marin Solijacic (left), Aristeidis Karalis, and John Joannopoulos illustrate how magnetism can transmit energy wirelessly.

“We are very good at transmitting information wirelessly,” says Soljacˇic´. But it’s been much more difficult to transmit power in the same way, because the radiation spreads out, and most of it is lost in the environment. Soljacˇic´ and his team propose developing a power “base station” that would plug into a wall, much like a Wi-Fi base station; but it would emit great energy at close range. Theoretically, when devices such as mobile phones or laptops came within range of the power base station, they would pick up its energy.

In the 1890s, before power cables commonly transported electricity over great distances, physicist and electrical engineer Nikola Tesla proposed beaming it through the air. Today, a form of wireless energy transfer called inductive coupling is used to charge electric toothbrushes. Electricity flowing through the wires in a toothbrush’s base station produces a magnetic field; that field induces a current in the wires of a nearby toothbrush handle, charging the toothbrush’s battery. This technique has limited range, however.

In his wireless energy system, explains Soljacˇic´, the base station would fill a space with a low-­frequency electromagnetic field in the range of a few megahertz. A gadget would be equipped with a receiver, like the power-­harvesting circuits used in RFID tags to collect ambient energy. Soljacˇic´���s circuit would be designed to resonate at the same frequency as the radiation emitted by the power station. When the device came within a couple of meters of the station, circuitry would absorb the energy, charging the device’s battery. The system could even power household electronics like televisions and toasters.

Most of the radiation emitted from the base station would stay in the vi­cinity, in a surrounding sphere with a radius of only a couple of meters. Soljacˇic´ says his calculations, and the known health effects of the proposed radio waves, suggest that his wireless power scheme poses no threat to people or other living things.

While the work is still in the realm of theory, Soljacˇic´ has applied for patents, and the group is working on an experimental demonstration. Soljacˇic´ says that he envisions houses with power hubs on the ceiling of each room. Then, as long as a mobile phone is inside, it will be charging, not chirping, through the night.

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