Etats - Unis: a wirelessly powered lightbulb
Comment faire passer du courant sans fil
1) et 2) ADIT,, juin et juillet 2007
3) réseau Sortir du nucléaire, juillet 2007
en cours d'installation...

    1) Researchers at MIT have created a revolutionary device that could remotely charge batteries and power household appliances.
     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 bulb.
     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.

Voir aussi:

Wireless power
1) Power from mains to antenna, which is made of copper
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
2) The Power of Induction
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.

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.

     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.

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.

     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."

3) Le WiTricity, ou comment faire passer du courant sans fil
 Promis juré c'est pas dangereux!

Nouvelle technologie
     Nous en parlions dans cette actualité, la recherche sur le transfert d'énergie sans fil avance, même si tout n'est pas encore parfait. Des chercheurs du MIT viennent de mettre en application leur théorie de transfert d'énergie baptisée «nonradiative resonant energy transfer», et plus affectivement «Witricity ».
     La démonstration met en scène une ampoule de 60 W recevant l'énergie électrique suffisante pour briller à deux mètres de sa source d'énergie, sans fil aucun. L'idée est d'émettre des ondes électromagnétques à basse fréquence, 10 MHz, pour exploiter l'effet de résonnance induit au sein du point de réception. Ce dernier entre alors en résonnance et créé de l'énergie à son tour.
     Tout n'est pas encore satisfaisant cependant, car la transmission d'énergie à 2 mètres de distance ne s'effectue pour l'instant qu'avec un très pauvre rendement de 40%. Mais les chercheurs sont optimistes pour la suite, surtout que cette technologie est annoncée comme inoffensive pour le corps humain.

     En effet, le fait de rester dans les basses fréquences est sans danger pour les êtres vivants, car la longueur d'onde est alors de l'ordre de 30 m, explique le professeur Marin Soljacic, responsable du groupe d'étude qui planche sur le sujet: «D'ordinaire, si vous utilisez un transmetteur comme un téléphone portable à 2 GHz, une longueur d'onde bien plus petite, l'engin irradie un mélange de champ magnétique et de champ électrique
     Et c'est ici que réside l'astuce: à une distance inférieure à la longueur d'onde de l'émetteur, le champ émis est presque uniquement magnétique. «Le corps humain ne répond vraiment qu'aux champs électriques, c'est pourquoi vous pouvez cuire un poulet au micro-onde. Mais dans l'état actuel de nos connaissances, le corps a une réaction presque totalement nulle face aux champs magnétiques, en termes de quantité d'énergie absorbée
     «Le but est de diminuer la taille du matériel utilisé, ainsi que d'augmenter les distances et les rendements» explique Soljacic, qui admet que le système actuellement en démonstration reste très rudimentaire.