  
Goodbye Wires! The technology behind the PowerMat and other Wireless
Chargers
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.
MIT team experimentally demonstrates wireless power transfer,
potentially useful for powering laptops, cell phones without cords
Franklin Hadley, Institute for Soldier Nanotechnologies
June 7, 2007
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
Soljacic.
Realizing their recent theoretical prediction, they were able to light a 60W
light bulb from a power source seven feet (more than two meters) away; there was no physical connection between
the source and the appliance. The MIT team refers to its 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.
Late-night beeps
The story starts one late night a few years ago, with Soljacic (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 Soljacic started thinking about which
physical phenomena could help make this wish a reality.
Radiation methods
Various methods of transmitting power wirelessly have been known for
centuries. Perhaps the best known example is electromagnetic radiation, such as 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 even be 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.
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The key: Magnetically coupled resonance
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 electromagnetic 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 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, a graduate student in physics, 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), which is 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, an MIT undergraduate in physics, 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, a graduate student in electrical
engineering and computer science, points out, "Here is where the magic of the resonant coupling comes about. The
usual non-resonant magnetic induction would be almost 1 million times less efficient in this particular
system."
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Old physics, new demand
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, Soljacic 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.
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