Charging Batteries without Wires

New MIT research reveals a way to send wireless energy to mobile phones and laptops.
By Kate Greene

Small, battery-powered gadgets make powerful computing portable. Unfortunately, there's still a continual need to recharge the batteries of phones, laptops, cameras, and MP3 players by hooking them up to a tangle of wires. Now researchers at MIT have proposed a way to cut the cords by wirelessly supplying power to devices.

"We are very good at transmitting information wirelessly," says Marin Soljačić, professor of physics at MIT. But, he says, historically, it's been much more difficult to transmit energy to power devices in the same way. Soljačić, who was a 2006 TR35 winner (see "2006 Young Innovator"), and MIT colleagues Aristeidis Karalis and John Joannopoulos have worked out a theoretical scheme for a wireless-energy transfer that could charge or power devices within a couple of meters of a small power "base station" plugged into an electrical outlet. They presented the approach on Tuesday at the American Institute of Physics's Industrial Physics Forum, in San Francisco.

The idea of beaming power through the air has been around for nearly two centuries, and it is used to some extent today to power some types of radio-frequency identification (RFID) tags. The phenomenon behind this sort of wireless-energy transfer is called inductive coupling, and it occurs when an electric current passes through wires in, for instance, an RFID reader. When the current flows, it produces a magnetic field around the wires; the magnetic field in turn induces a current in a nearby wire in, for example, an RFID tag. This technique has limited range, however, and because of this, it wouldn't be well suited for powering a roomful of gadgets.

To create a mid-range wireless-energy solution, the researchers propose an entirely new scheme. In it, a power base station would be plugged into an electrical outlet and emit low-frequency electromagnetic radiation in the range of 4 to 10 megahertz, explains Soljačić. A receiver within a gadget--such as a power-harvesting circuit--can be designed to resonate at the same frequency emitted by the power station. When it comes within a couple of meters of the station, it absorbs the energy. But to a nonresonant device, the radiation is undetectable.

Importantly, the energy that's accessed by the device is nonradiative--that is, it doesn't propagate over great distances. This is due to the low frequency of the radio waves, says John Pendry, professor of physics at Imperial College, in London. Electromagnetic radiation comes in two flavors: near-field and far-field. The intensity of low-frequency radiation drops quickly as a person moves farther away from the base station. In other words, the far-field radiation that propagates out in all directions isn't very strong at low frequencies, hence is essentially useless. (Wi-Fi signals, in comparison, are able to remain strong for tens of meters because they operate at a higher frequency of 2.4 gigahertz.)


However, the near-field radiation, which stays close to the base station, contains quite a bit of energy. "If you don't do anything with it, it just sits there," says Pendry. "It doesn't leak away." This bound-up energy, which extends for a couple of meters, is extracted when a resonant receiver on a gadget comes within range.

At this point, the work is still theoretical, but the researchers have filed patents and are working to build a prototype system that might be ready within a year. Even without a prototype, though, the physics behind the concept is sound, says Freeman Dyson, professor of physics at the Institute for Advanced Study, in Princeton, NJ. "It's a nice idea and I have no reason to believe that it won't work."

Pendry suspects that people might be squeamish about the idea of wireless energy radiating throughout the air. "Whenever there's powerful energy sources, people worry about safety," he says. Depending on the application, he says, either the electric or the magnetic portion of the near-field radiation could be handy. Using the electric field would pose a health risk, and would be better employed in applications in which people aren't nearby, he says. Conversely, using the magnetic field would be much safer and could be implemented just as easily. "I can't think of any reason to worry [about health concerns]," he says, "but people will."

Soljačić also suspects that the wireless power systems would be safe, based on his calculations and on the known health effects of low-frequency radio waves.

Ideally, says Soljačić, the system would be about 50 percent as efficient as plugging into an outlet, which would mean that charging a device might take longer. But the vision for this sort of wireless-energy setup, he says, is to place power hubs on the ceiling of each room in the house so that a phone or laptop can be constantly charging from any location in a home.