Taking the lead

Posted on 13 Sep 2007 at 16:27

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American medical company Abiomed (www.abiomed.com) has used a similar technique to power artificial hearts. By using an external primary coil and a secondary coil implanted in the body just under the skin, the artificial heart's battery is recharged without wires penetrating the skin. As spokesperson Liza Heapes explained, "Eliminating the need for skin-piercing tubes has dramatically reduced complications caused by infection. Indeed, none of the 14 patients in [our] clinical trial died of device-related infections."

Inductive coupling is extremely efficient when the two coils are very close to one another or share an iron core, but typically it is ineffective over anything more than a few millimetres' distance. However, this doesn't mean induction has no useful applications for wirelessly powering consumer electronics. A handful of companies, including Splashpower (www.splashpower.com), Edison Electric (www.aiye.com.cn) and Fulton Innovation (www.ecoupled.com) are working on 'induction mats', which are essentially the primary coil of a transformer. Gadgets fitted with a suitable secondary coil can simply be placed on the mat to recharge.

These induction mats will power only devices that are placed directly on or very near them, but that's still a breakthrough compared with juggling lots of conventional chargers. However, the concept comes with its own set of problems - specifically, that different devices require different voltages and charge times, and also that the efficiency of an inductive coupling system depends not only on proximity but also on the precise positioning and orientation of the primary and secondary coils. Fulton Innovation explained how it overcame these issues.

Resonant Coupling

Inductive coupling is clearly a very useful way of transferring electricity, but it would be even more useful if it were able to transmit power over a longer range - say, from a transmitter in the ceiling to a vacuum cleaner on the floor. In November 2006, a team at the Massachusetts Institute of Technology (MIT) announced a theoretical way in which such a power transfer might work.

In June this year, the team revealed details of an experiment in which they lit a light bulb by transmitting power wirelessly over a distance of up to two metres. The setup used induction, and even at the maximum distance the experiment's efficiency was as high as 40 per cent.

The key to the success of MIT's system, dubbed WiTricity, is resonance, a type of oscillation found in mechanical systems such as a drum or a building, and also electrical circuits. Almost any object has a natural frequency at which it will tend to oscillate when provided with energy. If the energy is actually provided at this frequency, a particularly energetic oscillation occurs. An example of this is during an earthquake, which typically shakes the ground a few times every second. Buildings that are around eight to 10 stories high tend to have a similar natural frequency, causing them to resonate powerfully and sometimes collapse.

Tesla discovered the importance of resonance in the efficient transmission of power without wires. In 1900 he filed a patent for a "means for increasing the intensity of electrical oscillations", in which he referred to the importance of resonance in "the attainment of the best result". The MIT team says that by using two coils resonating at the same frequency, the energy transfer at the full range of their experiment is almost a million times more efficient than it would be without resonance.