One idea for improving the energy independence of electric vehicles is to recharge their energy supply while they are driving along the road. This can be done using resonant electromagnetic circuits.

How can we improve the energy autonomy of electric vehicles? By enabling the cars to recharge their power supply while driving along the motorway, say a group of researchers at Stanford University’s Automotive Research Centre. Their project is based on installing reciprocally resonant circuits. This was already developed in 2007 by scientists at MIT who succeeded in transferring energy wirelessly between two stationary objects two metres apart. In this latest research the Californian engineers have tried to extend the method to a moving car body. Small magnetically-coupled resonating coils located in the roadbed would serve as electricity emitters, creating a small electromagnetic field. This would then be captured by metallic disks located under the vehicle that are set to the same resonance, which thus act as receivers, and are connected to the battery.

A system of electromagnetic receivers

The two circuits resonate with each other when programmed to the same frequency, and energy can thus be transferred directly to the vehicle without any cabling. The researchers have calculated that the power transfer generated will be around 10kW, transmitted in 7 microseconds from the road to the vehicle with 97% transfer efficiency. The coils therefore need to be set in the roadbed in places where drivers have to slow down, to allow time for the energy transfer to take place. This system opens up new horizons, given that in general vehicle consumption is today around 30 kW per 100 km.

Easier to drive and potential savings

"The main advantage of this approach using magnetic resonance to generate electricity is that a driver would no longer risk finding himself stuck out on the road with nowhere to recharge his vehicle", explains Shanhui Fan, Associate Professor of Electrical Engineering at Stanford and co-author of the report. "In addition it also enables us to reduce the size of the batteries and thus bring down the cost of the vehicles." Given that the range of the magnetic field is small and that power transfer only takes place when both circuits are in resonance with each other, the system is unlikely to interfere with the vehicle’s other electronic instruments.