Wireless power efficiency boosted

MIT researchers working on wireless power have discovered a way to make it more efficient: Charge up lots of electronic gear at the same time.


Massachusetts Institute of Technology researchers developing wireless power transmission have found that the system becomes more efficient as more devices are being powered. The new work builds on their original discovery, announced in 2007, of using electromagnetic resonance between coils to transmit power without wires. Besides increasing the efficiency of the system, the experiment used smaller receiving coils, making it more practical for deployment in homes, offices and eventually portable devices.

The electromagnetic field between the coils isn't disrupted by objects or people, which in turn are safe from being zapped by the energy transfer.

In their original work, foreshadowed by that of the electrical genius Nicolai Tesla more than a century ago (more on Tesla's work below), the researchers fed AC power into a resonating conductive coil, about 2 feet across, creating a magnetic field with a matching coil 7 feet away. Resonance is the natural frequency at which energy can be added to an oscillating system (as seen in an opera singer's voice adding energy to a wineglass, where it's converted to mechanical vibrations. If the vibrations continue to increase, the glass can shatter). The second coil captured energy from the first, converting it into enough power to light up an attached 60-watt bulb.

The approach makes use of a number of basic properties in electricity and magnetics. For example, magnetic induction is used in power transformers that have coils very close to each other; an electric current in one creates an oscillating magnetic field, from which a second coil can capture energy. But as the distance increases, these non-resonant coils become vastly less efficient.

In the new work, as summed up in an MIT story, three of the researchers (Andre Kurs, Robert Moffatt and MIT Professor Marin Soljacic) set up a slightly larger sending coil and two receiving coils that were half the size of the original: 12 inches.

Powering one device with one receiving coil results in less than 20% efficiency in power transfer. But with two devices and two receiving coils, the efficiency jumped to 30%. The reason seems to lie in the fact that the two receiving coils, besides resonating with the sending coil, also resonate with each other. That additional resonance strengthens the magnetic field and increases the power transfer efficiency.

According to Kurs, adding still more devices should keep increasing the system's efficiency, in theory to 100%.

This newest experiment transmitted "on the order of 100 watts" of power. The only limiting factor is the sending coil's amplifier, which can easily be increased to several hundred watts or a kilowatt, according to Kurs. That would be enough to have a sending coil mounted in a medium-sized room, transmitting enough power to run several household devices at once: lamps, TVs, computers.

The receiving coils will continue to shrink to a size that eventually would make them practical for even mobile devices, according to the researchers.

The project team has formed a company called WiTricity to continue technology development and bring it to market. According to the MIT story, some large companies, including Intel and Sony, have used the research to launch their own wireless power projects. A commercial product is "reasonably close," Kurs says. Most of the current work is focused on engineering, reducing the size and thickness of the receiving coils, and creating automated tuning and electrical controls systems.

The new experiments were reported earlier this year in a paper, "Simultaneous mid-range power transfer to multiple devices," in Applied Physics Letters (subscription required). Kurs, a doctoral student in MIT's Department of Physics, is the lead author of the recent paper, with Moffat and Soljacic.

Tesla apparently attempted to create a powerful, wireless electric transmitter more than a century ago, in his famously dramatic experiments in May 1899 in Colorado Springs (the 2006 movie, "The Prestige," used these events as a key element in its plot.) Many insist he succeeded. But a 2000 PBS documentary, "Tesla: Master of Lightning," directed and co-written by Robert Uth, concluded it was unclear what Tesla had actually achieved in Colorado Springs.

"A great deal of mystery still surrounds Tesla's work at Colorado Springs," states an account on the PBS website. "It is not clear from his notes or his comments exactly how he intended to transmit wireless power. But it is clear that he returned back to New York City fully convinced that he could accomplish it."

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