First came cordless phones, then wireless internet. The next step to making our lives truly mobile is getting rid of the tangles of power cables lurking in all our homes. Instead of channelling current down wires, wireless electricity makes use of magnetic fields. When electricity is flowing down a coil of wire, it creates a magnetic field. The reverseis true as well – a magnetic field can produce an electric current in a coil. This form of power transmission is called magnetic induction and is used widely in gadgets like electric toothbrushes.In a sense, wireless electricity has therefore been around for years, but a team from MIT lead by Dr Marin SoljaÄiÄ‡ had bigger ideas for it. Magnetic induction is efficient only over a very smalldistance as the strength of the field tails off rapidly. ‘You have to put the toothbrush in a particular spot,’ comments Dr Aristeidis Karalis, a member of the research team. ‘There’s almost no difference between that and attaching it to a wire. It’s contactless but I wouldn’t necessarily call it wireless: there’s a mobility concept attached to wireless and that’s what we’re trying to address.’
The MITteam’s solution was to add sharp resonance to the equation, dramatically improving the efficiency of the energy transfer. Resonance is the phenomenon at play when an opera singer causes a glass to shatter at distance. This is possible only if the note sung has the same frequency as the resonant frequency of the glass, causing it to absorb the acoustic energy and to break.In the case of wirelesselectricity, electricity from a normal power socket runs through a coil, producing a magnetic field. Elsewhere in the room, a second coil transforms the magnetic field back into electrical current to power your TV or charge your phone. The pair of coils are fine-tuned to have the same resonant frequency, ensuring that only the receiver coil is targeted by the magnetic field. This means that noenergy leaks elsewhere and that there is no danger for humans, pets or anything else in your home.
Whilst bidding farewell to cables is a definite perk, it’s only one of the benefits that wireless electricity could bring. ‘A second, more interesting class of applications is where you actually improve performance,’ comments Karalis. ‘Inside a factory, efficiency is often limited bythe fact that there are wires running along all the machines and robotic arms. If we can get rid of those wires, then we can definitely improve the efficiency of the whole production line.’
Liberated from the constraints of wires, inventions that had previously been impossible will soon emerge. Only time will tell what these might be, but Karalis has a few ideas, including wirelessly chargedartificial organs and nanobots. SoljaÄiÄ‡, Karalis and their colleagues have gone on to found a company, WiTricity, aiming to commercialise their invention.‘It’s incredible how much interest we have from all kinds of companies, from electronics to industrial companies,’ says Karalis. Car manufacturers for example were quick to understand how wireless electricity could be applied to hybrid cars. ‘Itwould be very nice if you could just drive into your parking spot and your car would start charging automatically without you having to think about it,’ explains Karalis.
Efficiency and the environment
Whilst the imminent arrival of wireless electricity has generated much excitement, some critics are questioning its green credentials. Inevitably, transmitting power wirelessly is fundamentallyless efficient, but things aren’t quite so simple. ‘Efficiency is not a number, it’s a function of the application. So depending on what the application is you can tolerate a higher or lower efficiency,’ explains Karalis. ‘We always make sure the efficiency is reasonable.’‘If you are trying to recharge a car and you are transmitting kilowatts, you’ve got to be above 90% efficient,’ he elaborates....