MIT Researches Wireless Energy

MIT Professor of Physics Marin Soljaccicc and his colleagues Aristeidis Karalis and John Joannopoulos have worked out a theoretical scheme for a wireless-energy transfer that could power devices within a few meters of a small power "base station" plugged into the mains. Beaming power through the air is used for some types of radio-frequency identification (RFID) tags. Called inductive coupling, 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 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. A power base station would be plugged into an electrical outlet and emit low-frequency electromagnetic radiation at 4-10Mhz. 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. To a nonresonant device, the radiation is undetectable. The energy that's accessed by the device is nonradiative. 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 near-field and far-field. The intensity of low-frequency radiation drops quickly as a person moves farther away from the base station. The far-field radiation that propagates out in all directions isn't very strong at low frequencies, hence is essentially useless. 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. Ideally, says Soljaccicc, the system would be 50% 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 constantly charge from any location in a home.