Wi Fi Power distribution
- From: Jack Linthicum <jacklinthicum@xxxxxxxxxxxxx>
- Date: Tue, 31 Jul 2007 13:48:36 -0700
On Jul 16, 10:27 pm, "Jonathan" <wr...@xxxxxxxxxxxxx> wrote:
Science News Online
Week of July 21, 2007; Vol. 172, No. 3
The Power of Induction
Cutting the last cord could resonate with our increasingly gadget-
dependent lives
Davide Castelvecchi
Marin Soljacic was understandably nervous. The young physicist was
about to give his first public presentation of an idea that sounded
almost too good to be true. There was no telling how his audience, at
a Berkeley, Calif., symposium, would receive his daring proposal.
Design two antennas to be as inefficient as possible at transmitting
radio waves, Soljacic began.
a8654_136.jpg
UNPLUGGED. Alternating current fed into a wire loop (blue) generates a
field that induces currents in the coil (red, at left), creating a
magnetic field that reaches a second coil (red) several meters away
(at right), creating a local field that induces a current in the
second loop (blue), lighting a bulb.
Science
Separate the antennas by a few meters and, with some fine-tuning, you
can safely and efficiently transfer electricity from one to the other-
without wires. Put this system inside your home, and you would have a
wireless network for electrical power. You could recharge your laptop
or turn on a light without plugging anything in.
The crucial bit would be the fine-tuning: The two antennas would have
to be tweaked so that one would create a pulsating magnetic field with
a specific frequency and geometry, which the other would then
transform into an electric current.
When Soljacic first presented the principle, it was unproved. All he
could show were his calculations. "I expected that some people would
think I was a crackpot," says Soljacic, a physicist at the
Massachusetts Institute of Technology (MIT). "This was pretty far
out."
Perhaps it also didn't help that the participants at the symposium-a
celebration of the 90th birthday of Charles Townes, who pioneered the
laser in the 1950s-included 18 Nobel prize winners and dozens of other
luminaries. Much to Soljacic's relief, he sold the scientists on his
presentation.
A year and a half later, a bulb lit up in an MIT lab-unplugged.
Soljacic and his collaborators had demonstrated a new way of coaxing
magnetic fields into transferring power over a distance of several
meters without dispersing as electromagnetic waves. The demonstration
ushered in a technology that might eventually become as pervasive as
the gadgets it could power. Laptops, cell phones, iPods, and digital
cameras might someday recharge without power cords. With the
proliferation of wireless electronics, perhaps it was just a matter of
time before power transmission would go wireless, too.
The device that Soljacic and his collaborators put together had a
disarming simplicity. On one side of the room, hanging from the
ceiling, was a ring-shaped electrical circuit, about half a meter
across, plugged into the wall. Hanging adjacent to the circuit, but
with no physical connection to it, was a slightly larger copper coil
looking like an oversize mattress spring. A few meters away hung a
similar system with an ordinary lightbulb attached to the circuit.
When the physicists sent power through the first circuit, the bulb lit
up.
As expected, some energy was lost on its way to the lightbulb.
However, a surprising amount reached its destination, the team reports
in the July 6 Science. "The efficiency was 40 percent at the biggest
distance we probed [more than 2 meters]," Soljacic says. At shorter
distances, the efficiency was much higher.
The coils of this demonstration device would be too big to fit inside
a laptop, let alone a cell phone. But this was only the first and
simplest of several prototypes that the physicists have in mind. More
tests are to come. The MIT team and other physicists say that in
principle they see no obstacle to making such devices more compact and
more efficient.
Making no waves
The idea of transmitting energy wirelessly isn't new. For almost two
centuries, scientists have known that rapidly changing magnetic
fields, such as those produced by an alternating current flowing
through a wire, can induce an electric current in another wire. That's
how the coils inside power transformers transmit energy from one coil
to another without touching. But this form of induction usually works
efficiently only when the two coils are very close to each other.
In the early 1900s, long before the power grid made electricity widely
available, electricity pioneer Nikola Tesla devised a grand scheme to
transfer large amounts of power over long distances from a tower 20
stories tall, to be built on Long Island in New York. To this day,
historians puzzle over how Tesla's system was supposed to work, or
whether it could have worked at all, says Bernard Carlson, a historian
of science at the University of Virginia in Charlottesville who is
writing a biography of the great engineer. "We can't even begin to
understand what he was doing with this power stuff," Carlson says.
The project died when Tesla's financial backers pulled the plug,
possibly because Tesla seemed unclear as to how to bill customers
receiving wireless power. Ironically, Tesla also invented the
alternating current (AC) system of power production, transmission, and
distribution that would become the standard for the modern grid.
But electromagnetic radiation can indeed carry energy through air or
empty space and over large distances. One familiar example is the
energy we receive from the sun, mostly as visible light. Another is
radio waves, first detected by Heinrich Hertz in 1888. An
electromagnetic wave is a synchronized dance of an electric field and
a magnetic field. Because an oscillating magnetic field generates an
oscillating electric field, and vice versa, the two fields sustain
each other as the wave propagates.
Radio waves and light waves, however, tend to shoot out in all
directions. This makes for very inefficient power transmission,
because the farther the waves travel, the larger the volume of space
throughout which their energy spreads. Technologies such as lasers and
parabolic antennas can confine the energy of electromagnetic waves in
tight beams, that can transfer power. But beams have disadvantages.
One problem is that anything that happens to cross a beam's path may
get fried.
Soljacic's wireless power system harnesses oscillating electric and
magnetic fields in a novel way. Although it doesn't radiate energy as
a radio antenna does, it transmits power across greater distances than
a conventional transformer can.
A typical antenna-the simplest type being essentially a rod-has a size
comparable to the wavelength of the radiation it emits. The electric
and magnetic fields it creates are in phase. They rise and fall in
sync with each other, a property that's crucial to the self-sustaining
feedback that allows a wave to propagate.
The circuit in Soljacic's device carries an alternating current with a
frequency of about 10 megahertz (MHz). It generates a magnetic field
that induces a current in the adjacent coil, which then amplifies the
magnetic field.
Electromagnetic waves of 10 MHz have a wavelength of about 30 m.
Because the coils are much smaller than that, they don't generate
conventional waves, explains Aristeidis Karalis, an MIT graduate
student who helped with Soljacic's theoretical model and computer
simulations. Instead, "the electric field is at its maximum when the
magnetic field is zero, and vice versa," which is the opposite of
being in phase, Karalis says. This arrangement means that the fields'
energy stays mostly in the vicinity of the coil, and only a small
percentage of the total power disperses as waves.
The MIT team introduced two additional ingredients into its design,
the first to make it safe and the second to make it efficient.
For safety, they took the advice of John Pendry, an Imperial College
London physicist who visited the MIT lab in 2005. Pendry recommended
designing the system to minimize exposure to electric fields, since
rapidly changing electric fields can heat up the surroundings,
including any people close by. "With the electric field you'd get hot,
like in a microwave oven," Pendry says, whereas the body "hardly
responds to magnetic fields."
In the team's designs, the magnetic fields change slowly enough to not
create strong electric fields. The magnetic fields themselves are
comparable in strength to Earth's magnetism, Karalis says, and only
one-thousandth as strong as the field inside a magnetic resonance
(MRI) machine. On the other hand, both MRIs and Earth have constant,
not rapidly oscillating, fields. But the MIT scientists say that their
fields stay within safety guidelines issued by the Institute of
Electrical and Electronic Engineers.
Resonating power
The second ingredient is Soljacic's use of resonance-the innovation
that makes efficient energy transfer possible. Just as guitar strings
and wine glasses vibrate at specific frequencies, electric circuits
have their own natural AC oscillation modes. The diameter of the MIT
coils and the spacing between their turns are suitably adjusted, so
the coils act as electrical circuits with a natural AC frequency of 10
MHz, putting them in sync with the magnetic oscillations and with each
other. One coil can then transfer energy to the other by the same
principle that enables a violin played at just the right pitch to
break a wine glass.
a8654_2352.jpg
ABRACADABRA. The first demonstration of energy transfer based on
magnetic resonance. The receiving circuit (right) picks up 40 percent
of the power consumed by the emitting circuit (left) and lights the
bulb.
Science
When Pendry revisited the MIT lab this March, he got a firsthand view
of the bulb lighting up. "What they've done is take some very basic
physics concepts [and] brought these ingredients together. It's the
synthesis which is the novel thing," says Pendry.
Shanhui Fan, a physicist at Stanford University, says that the use of
magnetic resonance as a means of transferring energy is a completely
new concept, and "very clever." Although it's a simple principle,
nobody seems to have thought of it before, he says. "Many great things
look simple from hindsight."
Soljacic and his colleagues have applied for two patents, and they
have branded their idea with the name WiTricity to suggest an
electrical-power version of Wi-Fi wireless-Internet technology.
But if the physics is simple, why didn't anyone think of it sooner?
Soljacic suggests that before the spread of cell phones and laptops,
there was little need for a wirefree power source. In fact, Soljacic
admits that what got him thinking hard about wireless power was the
frustration of being awakened at night by a beeping cell phone that
needed to be recharged.
In a smaller way, wireless power has already crept into our lives and
our wallets. The access cards of many office buildings and public-
transportation systems now carry embedded radio frequency
identification (RFID) tags. RFID tags have no batteries. They are
semiconductor chips that draw a tiny amount of energy-typically
microwatts-from radio waves generated by the device that reads them,
and in response beam back an identification code.
In a similar vein, Powercast, a start-up company in Ligonier, Pa.,
recently began marketing a new kind of chip that can harvest several
milliwatts from radio waves. The company's chips have a patented
design that converts up to 70 percent of the radiofrequency energy
picked up by a small antenna into direct current (DC) power, says
Powercast's Keith Kressin.
Powercast's small, dedicated radio sources can be hidden in fixtures
such as desk lamps. One chip can provide enough power to keep a cell
phone charged while it sits in standby mode a few inches from the
emitter, Kressin says. Eventually, the technology could be used in
environmental sensors and in medical implants.
In comparison, the MIT team's system could potentially furnish a room
with hundreds of watts of wireless power, which could drive a wide
range of devices. The system's ultimate limitation derives from the
physics of the magnetic fields. A few meters from the source, the
fields' strength quickly drops. "Eventually, you have to face the fact
that the fields decay very fast," Soljacic says.
Efficiency is limited primarily by the power dissipated as heat in the
copper coils. The physicists plan to experiment with different
materials and designs to reduce electrical resistance.
If Soljacic's "far-out" idea bears fruit and engineers manage to
squeeze WiTricity into electronics products, then in a few years
homes, workplaces, and coffee shops could be pulsating with magnetic
energy, greatly reducing the tangles of cords that clutter floors and
eliminating the need to plug gadgets in. A simple, relatively low-tech
idea could make everyone's life a little more hasslefree.
As Pendry puts it, "The power cord is the last cord that needs to be
cut. Everything else has been severed."
If you have a comment on this article that you would like considered
for publication in Science News, send it to editors@xxxxxxxxxxxxxxxx
Please include your name and location.
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