Wireless power could have cellphone users beaming
19:00 07 June 2007
NewScientist.com news service
Robert Adler

Your cellphone or laptop computer may soon recharge itself the same way it transfers information - wirelessly.

Researchers at the Massachusetts Institute of Technology (MIT), in the US, report that they can now send substantial amounts of power - enough to light a 60-watt bulb - across a room by magnetic induction between two devices tuned to resonate with each other.

They hope to use this phenomenon of "strong coupling" to recharge or even run mobile devices wirelessly.

Induction - the ability of a changing magnetic field to produce an electric current - was discovered by Michael Faraday in 1831. It is what makes electric generators, transformers, and motors work. Until now, induction has only been practical at close range, for example between the charger and handset of an electric toothbrush. At longer distances, the power losses are too great to make it worthwhile

Power-storing coil
Inspired by a mobile phone with a rundown battery, Marin Soljaĉić (pronounced soul-ya-cheech), a theoretical physicist at MIT, wondered if he could improve the efficiency of induction over longer distances.

From his experience with lasers, he knew that objects that resonate at the same frequency readily exchange energy. He set out to see if he could use electromagnetic resonance to transmit electrical power.

Soljaĉić and his research group have now built a coil with just the right properties. Powered by mains current, the coil naturally oscillates at 10 MHz. Unlike an antenna - which radiates the energy it receives - their device stores energy internally, in the form of oscillating currents and charges.

The coil generates a strong electromagnetic field, but most of the electric component of that field is trapped inside the coil, while an oscillating magnetic field surrounds it. The oscillating magnetic field efficiently transmits power across the lab to a receiver tuned to the same frequency.

Minimising the external electric field is crucial for safety. "We wanted to use the magnetic field for coupling, and have the electric field confined," says theoretician André Kurs, a member of the MIT group, "because a magnetic field does not interact with most objects, including biological tissues."

Real world applications
"I think it’s brilliant," says Douglas Stone, a theoretical physicist at Yale University, not affiliated with the MIT group. "This is something anybody could have thought about for a century."

Stone agrees with the MIT researchers that while there is much work to be done before your gadgets recharge themselves wirelessly, this technology will move from the lab to the real world. "There’s no fundamental problem," says Stone. "It’s going to work."

Journal reference: Science Express (7 June 2007, p 1)
http://www.newscientisttech.com/article/dn12014-wireless-power-could-have-cellphone-users-beaming.html