Charging down Electric Avenue

Swagat Chopra demonstrates - Photo: Tomas van Dijk

Range anxiety holds people back from buying electric vehicles. But what if the road itself would keep the batteries charged?

In the laboratory, MSc student Swagat Chopra places his model car above a square coil of electric wire. The car carries a similar but smaller coil below its chassis. Chopra flicks some switches and the voltage over the car’s batteries starts rising. This is magic: power transfer through the air.

The physics is not so different from what happens in any transformer, says
Chopra’s supervisor, Professor Pavol Bauer (Electrical Engineering, Mathematics and Computer Science). The only difference is that the primary and secondary coils are not linked by a chunk of iron, but rather by a gap of air. To overcome this obstacle, high frequencies are used (typically 100 kilohertz) and the coils are tuned to the same resonance frequency.

Contactless Power Transfer (CPT) has many advantages over socket charging, Prof. Bauer explains. Not only is the technology more convenient and safer, CPT also offers the opportunity of charging vehicles wait for the traffic lights by means of embedded coils in the street. Equip motorways with such power antennas and cars could be charged while driving down ‘Electric Avenue’.

What’s more: it would be better for the batteries as well. The Li-ion battery model that MSc student Long Lam developed shows that short charging and discharging cycles prolong battery life. Lam’s model for Li-ions batteries reveals that it would be better to recharge the batteries at every traffic light than to fast-charge it to the brim and drive home without recharging. Clearly, CPT offers the opportunity to provide specially adapted vehicles with easy small recharges.

So what’s holding back the introduction? Apart from the obviously large investments needed to electrify the roads, there are also doubts as to the system’s efficiency. Although the energy transfer between the coils reached 91 percent efficiency, the overall efficiency got stuck at 83.2 percent, due to losses in the power electronics, which transfer direct currents to high frequency AC and back. Prof. Bauer’s group however is determined to improve the CPT’s overall efficiency.