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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."