While scientists at the turn of the 20th century were experimenting with the
wireless transmission of information, like radio, Croatian-born inventor Nikola
Tesla had a grander vision (Fig. 1). He imagined
the wireless transmission of power—to supply "light, heat, or motive
power anywhere—on sea, or land, or high in the air," he told The New
York Times in 1904.
With the help of the October 2006 film The Prestige, where a fictionalized
version of the physicist helps an aspiring illusionist develop a magic trick,
it's rumored that Tesla was able to light a field of 200 incandescent bulbs
from a power source 25 miles away during early experiments at his Colorado Springs
laboratory.
No one's sure that happened. Yet Tesla did
hold several patents relating to the wireless
transmission of energy, part of his wireless
"World System," which would send both information and electricity around the globe.
Over time, Tesla's ideas were met with some skepticism as he became reclusive
and made bizarre claims like being able to communicate with Mars. Even though
the press came to rely on him solely for scientific prophecy, much of Tesla's
vision for wireless power transfer is coming into fruition today, through startup
companies and research institutions.
WIRELESS POWER
The most widespread use of wireless power transfer, albeit on a very small scale,
is in passive RFID tags. The tag has no power source, so it relies on the electrical
current of an incoming radio frequency signal to power up its CMOS chip and
transmit a response.
Bigger wireless-power initiatives debuted at the 2007 Consumer Electronics Show with companies like Powercast and Fulton Innovation's eCoupled technology. A Powercast transmitter
can send juice up to 10 feet over RF signals. A receiver that's
either built in or attached with an adapter converts the waves into
electricity to charge small devices like phones or cameras.
Rather than radio frequency, eCoupled coils transfer energy through inductive
coupling, which relies on a shared electromagnetic field (see "Charge
Your Gadgets Without Plugging Them In."). Visteon has already used eCoupled
induction coils to create wireless cell-phone and MP3 player chargers, which
are expected to be released later this year.
While these applications are practical, researchers at the Massachusetts Institute
of Technology have come much closer to Tesla's grander vision. At the same time
that The Prestige was released last fall, the researchers—led
by professor Marin Stoljacic, a great admirer of Tesla's—outlined a way
to juice bigger devices without wires. By June, the team lit a 60-W light bulb
from a power source seven feet away (Fig. 2).
MIT's "WiTricity" (for "wireless electricity") also relies on magnetic induction to transfer power, but with an added component—resonance. All objects resonate. When two objects resonate at the same frequency, they exchange energy efficiently.
It's similar to how a wine glass bursts if it resonates in tune
with an opera singer's voice.
The team used two copper coils. The sending coil gave off a non-radiative magnetic
field oscillating at megahertz frequencies that resonated in unison with the
receiving coil. The coils exchanged energy solely with each other, and any power
not picked up by the receiving coil remained bound to the area of the sending
coil. Power didn't radiate into the environment, and objects placed between
the coils didn't hamper the energy transfer.
The researchers acknowledged that the transfer seems similar to standard magnetic induction used in transformers today,
but they also said that such methods can only send power over
short distances. Resonant coupling enables magnetic induction to work over greater distances.
And to those who argue Tesla already demonstrated resonant coupling in wireless
power transfer, the team wrote in its paper that Tesla's incarnations, like
Tesla coils, radiated energy inefficiently in all directions. Stoljacic and
his team intend to demonstrate greater efficiency than the 40% that their experiment
achieved.