Wireless Battery Technology
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.)
http://www.verizonwireless.com/insiders-guide/tech-smarts/battery-life-wireless-charging-technology-inductive-charging-phones/
http://en.wikipedia.org/wiki/Inductive_charging
http://www.technologyreview.com/news/406878/charging-batteries-without-wires/
