LET'S face it: power cables are unsightly dust-traps. PCs, TVs and music players are becoming slicker every year, but the nest of vipers in the corner of every room remains an ugly impediment to true minimalism.
Then there is the inconvenience of charging phones, MP3 players and PDAs. A minor hassle, admittedly, but it is easy to forget to top up the batteries and before you know it you have left the house with a dead gadget. Wouldn't life be simpler if power was invisibly beamed to your devices whenever you walked into a building with an electricity supply? Wireless communication is ubiquitous, after all, so why can't we permanently unshackle our electronics from power cables too?
Poor transmission efficiencies and safety concerns have plagued attempts at wireless power transfer, but a handful of start-ups - and some big names, like Sony and Intel - are having another go at making it work. The last few years have seen promising demonstrations of cellphones, laptops and TVs being powered wirelessly. Are we on our way to waving goodbye to wires once and for all?
The idea of wireless power transfer is almost as old as electricity generation itself. At the beginning of the 20th century, Nikola Tesla proposed using huge coils to transmit electricity through the troposphere to power homes. He even started building Wardenclyffe Tower on Long Island, New York, an enormous telecommunications tower that would also test his idea for wireless power transmission. The story goes that his backers pulled the funding when they realised there would be no feasible way to ensure people paid for the electricity they were using, and the wired power grid sprang up instead.
Wireless transmission emerged again in the 1960s, with a demonstration of a miniature helicopter powered using microwaves beamed from the ground. Some have even suggested that one day we might power spaceships by beaming power to them with lasers (New Scientist, 17 February 1996, p 28). As well as this, much theoretical work has gone into exploring the possibility of beaming power down to Earth from satellites that harvest solar energy (New Scientist, 24 November 2007, p 42).
Long-distance ground-to-ground wireless power transmission would require expensive infrastructure, however, and with concerns over the safety of transmitting it via high-power microwaves, the idea has been met with trepidation.
While we won't be seeing a wireless power grid any time soon, the idea of beaming power on a smaller scale is rapidly gaining momentum. That is largely because, with wireless communication, like Wi-Fi and Bluetooth, and ever-shrinking circuits, power cables are now the only limit to becoming truly portable. "The move was inevitable once wireless communication became popular," says David Graham, a co-founder of Powerbeam in San Jose, California.
With this new impetus, engineers and start-up companies have jumped at the challenge, and while beamed power is still in its infancy, three viable options seem to be emerging. The use of radio waves to transmit electricity is perhaps the most obvious solution, since you can in principle use the same kinds of transmitters and receivers used in Wi-Fi communication. Powercast, based in Pittsburgh, Pennsylvania, has recently used this technology to transmit microwatts and milliwatts of power over at least 15 metres to industrial sensors. They believe a similar approach could one day be used to recharge small devices like remote controls, alarm clocks and even cellphones.
A second possibility, for more power-hungry devices, is to fire a finely focused infrared laser beam at a photovoltaic cell, which converts the beam back to electrical energy. It's an approach PowerBeam has adopted, but so far its efficiency is only between 15 and 30 per cent. While that could serve more power-hungry appliances, it would in practice be too wasteful.
The technology has been used to power wireless lamps, speakers and electronic photo frames that require less than 10 watts to function. Over time, as both the lasers and photovoltaic cells improve, the company hopes efficiencies of up to 50 per cent will be possible. "There's no reason we couldn't power a laptop eventually," says Graham. Unlike some other possible techniques, a sharply focused beam loses minimal energy over large distances, preserving its efficiency: "A hundred metres is no big deal."
Inconvenient beams
Others are sceptical that this technique would be practical for truly portable devices, which are constantly moving around and between rooms. "An infrared beam would not be convenient to charge a mobile phone - it's too directional," says Menno Treffers, chairman of the Wireless Power Consortium in the Netherlands. Powerbeam's solution is to fit a small fluorescent bulb to the receiving device so that a camera on the transmitter can track the light and steer the laser beam accordingly. Another problem is that a separate beam is needed for each device you want to power, which would be tricky to engineer, says Aristeidis Karalis at the Massachusetts Institute of Technology, who is developing an alternative wireless power transmission system.
The third possibility for wireless power is magnetic induction - the most attractive option for beefy domestic applications. A fluctuating magnetic field emanating from one coil can induce an electric current in another coil close by, which is how many devices, like electric toothbrushes and even some cellphones, recharge drained batteries. The snag, however, has been that while efficiency is good at close contact, it can drop to zero at even a few millimetres from the transmitter.
Enter Karalis and his colleagues. It has long been known that such mechanical energy transfer is improved enormously if two objects resonate at the same frequency - it's how an opera singer can smash a glass if she hits the right pitch. Karalis wondered whether the same idea could improve the efficiency of magnetic induction at greater distances.
The team's set-up consisted of an inducting coil connected to a capacitor. The energy within this circuit oscillates rapidly between an electric field in the capacitor and a magnetic field in the coil. The frequency of this oscillation is controlled by the capacitor's ability to store charge and the coil's ability to produce a magnetic field. If the frequency in the energy-transmitter's circuit is different from that of the receiver's circuit, they are non-resonant. The result is that the energy sent by the transmitter will not be in phase with the energy that is already held at the receiver, which could result in the two cancelling each other out, limiting a coherent build up of energy inside the receiver. But if the transmitter and receiver are resonant, the team reasoned, the oscillating fields of their two coils would always be in sync, meaning the interference is constructive and the amount of energy transferred is boosted.
They tested their theory in 2007 with great success, transmitting 60 watts across 2 metres, with 40 per cent efficiency (Science, vol 317, p 83). The team has since founded a company called WiTricity to develop the idea. Last year, the firm used two square coils 30 centimetres across, one in the receiver and one in the transmitter, to power a 50-watt TV 0.5 metres from the power supply, with an impressive 70 per cent efficiency. "In some cases, the improvement in the efficiency due to resonance can be more than 100,000 times that of non-resonant induction," says Karalis. Unlike laser-based line-of-sight energy transmission, a magnetic field is not focused and so can pass around or through obstacles between the transmitter and receiver.
The big consumer electronics companies have also been keen to investigate "resonant transfer". Sony, for example, has demonstrated a wireless TV, and Intel is investigating the technology for a range of devices. "Power transfer efficiency scales independently of power, so the same efficiency can be achieved for laptops, consumer electronics such as TVs, and smaller portable devices such as cellphones," says Emily Cooper, a research engineer at Intel's labs in Seattle. In other words, the same proportion of the total energy will be lost for a power-hungry plasma TV as for a tiny PDA
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