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Goodbye wires...
CAMBRIDGE, Mass. --- Imagine a future in which wireless
power transfer is feasible: cell phones, household robots,
mp3 players, laptop computers, and other portable electronics
capable of charging themselves without ever being plugged
in, freeing us from that final, ubiquitous power wire. Some
of these devices might not even need their bulky batteries
to operate. A team from MIT's Department of Physics,
Department of Electrical Engineering and Computer Science,
and Institute for Soldier Nanotechnologies (ISN) has experimentally
demonstrated an important step toward accomplishing this
vision of the future. The team members are Andre Kurs, Aristeidis
Karalis, Robert Moffatt, Prof. Peter Fisher, and Prof. John
Joannopoulos (Francis Wright Davis Chair and Director of
ISN), led by Prof. Marin Soljačić. Realizing their
recent theoretical prediction, they were able to light a
60W light-bulb from a power source seven feet (more than
2 meters) away; there was no physical connection between
the source and the appliance. The MIT team refers to their
concept as "WiTricity" (as
in Wireless Electricity). The work will be reported in the
June 7 issue of Science Express, the advance online
publication of the journal Science.
The story starts one late night a few years ago, with Soljačić
(pronounced Soul-ya-cheech), standing in his pajamas, staring
at his cell-phone on the kitchen counter. "It was probably
the sixth time that month that I was awakened by my cell-phone
beeping to let me know that I had forgotten to charge it.
It occurred to me that it would be so great if the thing
took care of its own charging." To make this possible,
one would have to have a way to transmit power wirelessly,
so Soljačić started thinking which physical phenomena could
help make this wish a reality.
Various methods of transmitting power wirelessly have been
known for centuries. Perhaps the best known example is electromagnetic
radiation, like radio waves. While such radiation is excellent
for wireless transmission of information, it is not feasible
to use it for power transmission. Since radiation spreads
in all directions, a vast majority of power would end up
being wasted into free space. One can envision using directed
electromagnetic radiation, such as lasers, but this is not
very practical and can be even dangerous. It requires an
uninterrupted line of sight between the source and the device,
as well as a sophisticated tracking mechanism when the device
is mobile.
In contrast, WiTricity is based on using coupled resonant
objects. Two resonant objects of the same resonant frequency
tend to exchange energy efficiently, while interacting weakly
with extraneous off-resonant objects. A child on a swing
is a good example of this. A swing is a type of mechanical
resonance, so only when the child pumps her legs at the natural
frequency of the swing is she able to impart substantial
energy. Another example involves acoustic resonances: imagine
a room with 100 identical wine glasses, each filled with
wine up to a different level, so they all have different
resonant frequencies. If an opera singer sings a sufficiently
loud single note inside the room, a glass of the corresponding
frequency might accumulate sufficient energy to even explode,
while not influencing the other glasses. In any system of
coupled resonators there often exists a so-called "strongly
coupled" regime of operation. If one ensures to operate
in that regime in a given system, the energy transfer can
be very efficient.
While these considerations are universal, applying to all
kinds of resonances (e.g., acoustic, mechanical, electromagnetic,
etc.), the MIT team focused on one particular type: magnetically
coupled resonators. The team explored a system of two electro-magnetic
resonators coupled mostly through their magnetic fields;
they were able to identify the strongly coupled regime in
this system, even when the distance between them was a several
times larger than the sizes of the resonant objects. This
way, efficient power transfer was enabled. Magnetic coupling
is particularly suitable for everyday applications because
most common materials interact only very weakly with magnetic
fields, so interactions with extraneous environmental objects
are suppressed even further. The fact that magnetic
fields interact so weakly with biological organisms is also
important for safety considerations," Kurs points out.
The investigated design consists of two copper coils, each
a self-resonant system. One of the coils, attached to the
power source, is the sending unit. Instead of irradiating
the environment with electromagnetic waves, it fills the
space around it with a non-radiative magnetic field oscillating
at MHz frequencies. The non-radiative field mediates the
power exchange with the other coil (the receiving unit) specially
designed to resonate with the field. The resonant nature
of the process ensures the strong interaction between the
sending unit and the receiving unit, while the interaction
with the rest of the environment is weak. Moffatt explains: “"The
crucial advantage of using the non-radiative field lies in
the fact that most of the power not picked up by the receiving
coil remains bound to the vicinity of the sending unit, instead
of being radiated into the environment and lost.” With
such a design, power transfer has a limited range, and the
range would be shorter for smaller-size receivers. Still,
for laptop-sized coils, power-levels more than sufficient
to run a laptop can be transferred over room-sized distances
nearly omni-directionally and efficiently, irrespective of
the geometry of the surrounding space, even when environmental
objects completely obstruct the line-of-sight between the
two coils. Fisher points out: “As long as the laptop
is in a room equipped with a source of such wireless power,
it would charge automatically, without having to be plugged
in. In fact, it would not even need a battery to operate
inside of such a room.” In the long run, this could
reduce our society’s dependence on batteries, which
are currently heavy and expensive.
At first glance, such a power transfer is reminiscent of
relatively commonplace magnetic induction, such as is used
in power transformers, which contain coils that transmit
power to each other over very short distances. An electric
current running in a sending coil induces another current
in a receiving coil. The two coils are very close, but they
do not touch. However, this behavior changes dramatically
when the distance between the coils is increased. As Karalis
points out, "Here is where the magic of the resonant
coupling comes about. The usual non-resonant magnetic induction
would be almost 1,000,000 times less efficient in this particular
system."
WiTricity is rooted in such well-known laws of physics that
it makes one wonder why no one thought of it before. "In
the past, there was no great demand for such a system, so
people did not have a strong motivation to look into it,"
points out Joannopoulos, adding, "Over the past several
years, portable electronic devices, such as laptops, cell-phones,
iPods, and even household robots have become widespread,
all of which require batteries that need to be recharged
often."
As for what the future holds, Soljačić adds, "Once,
when my son was about three years old, we visited his grandparents'
house. They had a 20 year old phone and my son picked up
the handset, asking, 'Dad, why is this phone attached
with a cord to the wall?' That is the mindset of a
child growing up in a wireless world. My best response was, ‘It
is strange and awkward, isn’t it? Hopefully, we will
be getting rid of some more wires, and also batteries, soon.'"
This work was funded by the Army Research Office (Institute
for Soldier Nanotechnologies), National Science Foundation
(Center for Materials Science and Engineering), and the Department
of Energy.
Contact: Marin Soljačić, soljacic@mit.edu
Andre Kurs, akurs@mit.edu
Written by Franklin Hadley
Director of Outreach
Institute for Soldier Nanotechnologies
Massachusetts Institute of Technology
Tel: +1-617-324-6413
E-mail: fhadley@mit.edu
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