Charge your electric car on the move

ELECTRIC cars are clean, green and rather cool these days - even if they can't compete with conventional vehicles in terms of battery life and driving range. These disadvantages could disappear, however, with a system for wirelessly charging cars as they speed down the highway.

To do this, Shanhui Fan and colleagues at the Center for Automotive Research at Stanford University in California, suggest using magnetic coils embedded in the roadway to create a weak field that resonates with a coil on the car to transfer power. This kind of power transfer works on a similar principle to an opera singer shattering a glass by singing precisely the right note. In 2007, researchers at the Massachusetts Institute of Technology demonstrated that magnetic resonance could be used to transfer power wirelessly between two stationary objects 2 metres apart.

Fan, who presented his work at the Stanford Global Climate and Energy Project Symposium in October, wondered whether magnetic power transfer would be possible at highway speeds, and at levels sufficient for vehicles. Accounting for the metal body and movement of the car, his calculations showed that a set of resonant coils and discs could transfer about 10 kilowatts with a 97 per cent efficiency within 7 microseconds - fast enough for the highway. The system, which the group has yet to build, should be safe because power can only be transferred between two objects whose resonance is very closely attuned.

There are two main benefits of highway wireless power, says Fan. A driver wouldn't have to fear getting stranded on the road with nowhere to plug in, and electric-car batteries could be smaller, cutting costs.