The cars of the future could be powered by electrified roadways. Such technology would allow electric cars to forgo their heavy batteries, which not only add to a vehicle's weight, increasing the energy needed to move it, but also force it to sit idle while recharging.
The idea has been around for decades. Previous attempts used an electrified coil in the road to create an electromagnetic field that interacts with a coil attached to the car. "Since the coils must be exactly aligned face-to-face to achieve a high energy efficiency, such schemes may be useful for [charging] vehicles in a parking lot, but never very effective for cars while running," says Masahiro Hanazawa at Toyota Central R&D Labs in Nagakute, Aichi, Japan.
Hanazawa and Takashi Ohira at Toyohashi University of Technology, also in Aichi, are developing a system that transmits electric power through steel belts placed inside two tyres and a metal plate in the road. "Our approach exploits a pair of tyres, which are always touching a road surface," says Hanazawa.
To test how much energy would be lost as electricity travelled through the tyres' rubber, Hanazawa and Ohira set up a lab experiment in which they put metal plates on the floor and inside a tyre. "Less than 20 per cent of the transmitted power is dissipated in the circuit," says Ohira.
The team presented its work in May at the International Microwave Workshop Series on Innovative Wireless Power Transmission in Kyoto, Japan.
With enough power the system could run typical passenger cars, says Ohira, and the team are now developing a small-scale prototype to prove it. He admits, however, that the system's energy loss is "much higher than regular batteries".
John Boys, an electrical engineer at the University of Auckland, New Zealand, notes that with this system, the metal pads on the road would need as much as 50,000 volts to power the car, the same voltage used to operate tasers. "You wouldn't want to step on that," he says.
What's more, at the energy levels needed, the electric plates would produce a large magnetic field that would "cause significant radio-frequency interference that might create chaos with all manner of electrical systems", says Boys.
It would be expensive to "rip up the roads and install the necessary infrastructure", says Daniel Friedman at the University of New South Wales in Sydney, Australia. But he adds that one way around that may be to limit metal plates to main highways, and then run cars on other roads using small batteries.
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