Wireless, on-the-go charging for electric cars a step closer

The researchers demonstrate their wireless charging model. The bright led light shows that current is flowing.
The researchers demonstrate their wireless charging model. The bright LED light shows that current is flowing.
Stanford University

Two commercially available components could be the key to one day rolling out electric cars that can be continuously recharged just by driving along the road.

A study published in Nature reveals a modest proof-of-concept experiment that shows how electricity can be transmitted wirelessly. Deployed on a much larger scale, the research opens the door to having transmitter coils embedded into roads, which would then charge up cars as they drove over them.

The study also goes on to show how incorporating a voltage amplifier and feedback resistor into electric car design might overcome the main hurdle any such scheme would face – the need to continually retune the car’s magnetic resonance frequency as it moves over the coils.

The study was led by Shanhui Fan of Stanford University in the US. He and colleagues began their work by building on 2007 Massachusetts Institute of Technology research that demonstrated the wireless transmission of electricity over a short distance.

Fan and his colleagues refined that work and succeeded in transmitting power to a moving LED lightbulb. It was a mere one-milliwatt trickle, and electric cars will require a flow many orders of magnitude higher, but until now the base concept had not been demonstrated.

Interestingly, though, the distance over which the power was transmitted – less than a metre – might already be enough.

“We still need to significantly increase the amount of electricity being transferred to charge electric cars, but we may not need to push the distance too much more,” says Fan.

Fan’s concept relies on a technique called magnetic resonance coupling. In essence, coils of wire held between magnets would be embedded into roads – and also on the undercarriage of cars. {%recommended 4355%}

Electricity passing through the wires of the road-based coils creates an oscillating magnetic field. This, in turn, stimulates the electrons in any nearby coil – such as the ones in the cars – to also start to oscillate, thus transferring energy.

This transference, however, only works at optimum levels if the two magnetic fields are oscillating at the same frequency, which only happens if the angle between them doesn’t change. In other words, both coils have to be stationary.

For the process to work with a moving vehicle, its onboard transmission coil would have to be constantly recalibrated to take account of the rapidly changing angle between it and the coil in the road – a complex and power-hungry process.

However, Fan and his team worked out that if the car was equipped with a voltage amplifier and feedback resistor – both standard items of electrical engineering kit – the problem can be easily resolved.

“Adding the amplifier allows power to be very efficiently transferred across most of the [one-metre] range and despite the changing orientation of the receiving coil,” says team member Sid Assawaworrarit. 

“This eliminates the need for automatic and continuous tuning of any aspect of the circuits.”

Assawaworrarit tested the set-up using the LED lightbulb model. Without the extra components the LED’s intensity rose and fell as it approached, travelled over and went past the power source. With them, the light remained constant.

For Fan, the implications of the wireless charging model are huge.

“We can rethink how to deliver electricity not only to our cars, but to smaller devices on or in our bodies,” he says. 

“For anything that could benefit from dynamic, wireless charging, this is potentially very important.”

In an opinion piece in the same issue of Nature, Geoffroy Lerosey of the Langevin Institute in Paris, France, calls the work of Fan’s team a “beautiful concept” that “can have real-life applications” and “builds an inspiring bridge between the worlds of quantum physics and engineering”.

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