| Cutting the cord: MIT researchers
have shown that it's possible to wirelessly power a 60-watt lightbulb from
two meters away. Above, a coil (background) creates a magnetic field that
is able to pass through an obstruction. The foreground coil resonates at
the frequency of the magnetic field, picking up its energy to power the
Researchers at MIT have shown that it's possible to wirelessly power a 60-watt lightbulb sitting about two meters away from a power source. Using a remarkably simple setup--basically consisting of two metal coils--they have demonstrated, for the first time, that it is feasible to efficiently send that much power over such a distance. The experiment paves the way for wirelessly charging batteries in laptops, mobile phones, and music players, as well as cutting the electric cords on household appliances, says Marin Soljacic, professor of physics at MIT, who led the team with physics professor John Joannopoulos.
The research, published in the June 7 edition of Science Express (the online publication of Science magazine), is the experimental demonstration of a theory outlined last November by the MIT team. (See "Charging Batteries without Wires.") "We had strong confidence in the theory," says Soljacic. "And experiment indeed confirmed that this worked as predicted."
The setup is straightforward, explains Andre Kurs, an MIT graduate student and the lead author of the paper. Two copper helices, with diameters of 60 centimeters, are separated from each other by a distance of about two meters. One is connected to a power source--effectively plugged into a wall--and the other is connected to a lightbulb waiting to be turned on. When the power from the wall is turned on, electricity from the first metal coil creates a magnetic field around that coil. The coil attached to the lightbulb picks up the magnetic field, which in turn creates a current within the second coil, turning on the bulb.
This type of energy transfer is similar to a well-known phenomenon called magnetic inductive coupling, used in power transformers. However, the MIT scheme is somewhat different because it's based on something called resonant coupling. Transformer coils can only transfer power when they are centimeters apart--any farther, and the magnetic fields don't affect each other in the same way. In order for the MIT researchers to achieve the range of two meters, explains Soljacic, they used coils that resonate at a frequency of 10 megahertz. When the electrical current flows through the first coil, it produces a 10-megahertz magnetic field; since the second coil resonates at this same frequency, it's able to pick up on the field, even from relatively far away. If the second coil resonated at a different frequency, the energy from the first coil would have been ignored.
The researchers' approach, says Soljacic, also makes the energy transfer efficient. If they were to emit power from an antenna in the same way that information is wirelessly transmitted, most of the power would be wasted as it radiates away in all directions. Indeed, with the method used to transfer information, it would be difficult to send enough energy to be useful for powering gadgets. In contrast, the researchers use what's known as nonradiative energy that is bound up near the coils. In this first demonstration, they showed that the scheme can transfer power with an efficiency of 45%.
Wireless power transfer is an idea that's more than 100 years old. In the 1890s, physicist and electrical engineer Nikola Tesla proposed beaming electricity through the air. However, soon thereafter, power cables became the commonly accepted means of transporting electricity across distances. But with the widespread adoption of small, portable devices with batteries in need of constant recharging, people's attention is again turning to wireless power. In fact, the startup Powercast, based in Ligonier, PA, has, using a different approach from that of the MIT team, developed a wireless power system that can transmit low wattages across a distance of about a meter. To start, the company is targeting devices with low power consumption, such as sensors, but it's hoping to ramp up to more power-hungry gadgets in the future.
One concern that people might have, says Sir John Pendry, professor of physics at Imperial College in London, is health effects. "There will be safety issues, real or imagined," he says. "After all, the power has to pass through space in some form or other, and pass through any bodies lying in its path. The [MIT] team has minimized this problem by making sure that the power is mainly in the form of a magnetic field, a form of energy to which the body is almost entirely insensitive."
Based on calculations, Soljacic believes that the scheme is safe, even for people with implanted medical devices, such as pacemakers. Although the researchers have not made a detailed study to test how the system interferes with pacemakers, Soljacic says that they don't expect it to interact strongly with objects that don't resonate at the same frequencies used to transfer power.
At this point, the team has applied for a number of patents and is planning to commercialize the technology, although the researchers expect that it could take a few years before devices with such wireless power systems will make it to consumers. In the meantime, the team is exploring different materials and alternate coil geometries to try to extend the range and ramp up the power.
2) Antenna resonates at a frequency of about 10MHz, producing electromagnetic waves
3) 'Tails' of energy from antenna 'tunnel' up to 2m (6.5ft)
4) Electricity picked up by laptop's antenna, which must also be resonating at 10MHz. Energy used to re-charge device
5) Energy not transferred to laptop re-absorbed by source antenna. People/other objects not affected as not resonating at 10MHz
Cutting the last cord could resonate with our increasingly gadget-dependent lives
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.
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.
Making no waves
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.
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.
3) Le WiTricity, ou comment faire passer du courant sans fil
Promis juré c'est pas dangereux!