September 1, 2015

Wireless charging and discharging for electric vehicles

This coil system for the inductive charging of electric cars is built into the road.
© Fraunhofer IWES

(September 1, 2015)  In the future, a wireless charging system will allow electric cars not only to charge their batteries, but also to feed energy back into the power grid, helping to stabilize it. The cost-effective charging system achieves high levels of efficiency across the whole power range, from 400 watts to 3.6 kilowatts, while the car and the charging coil can be up to 20 centimeters apart. Fraunhofer researchers are presenting their prototype from September 15 to 18, 2015 at the IAA International Motor Show in Frankfurt (Hall 4, Booth D33).

When it’s pouring rain, a driver who has to connect a thick, unwieldy cable between their electric car and a charge spot is sure to get soaked to the skin. But sometimes there’s no alternative – the battery is empty. Using wireless inductive systems to charge the car is much more convenient. This involves transmitting energy through the air, or, more precisely, through a time-varying magnetic field. The technology is essentially based around two coils, with one built into a road, a parking space or a garage, and the other fitted to the underbody of the car. In conjunction with suitable capacitors, these coils form a sort of resonant "antenna system for energy transfer." The nearer the two coils are to each other, the more efficiently the energy is transferred.

Highly efficient, bidirectional charging

Researchers at the Fraunhofer Institute for Wind Energy and Energy System Technology IWES in Kassel have come up with a more cost-effective design for such inductive charging systems. "We deliberately use standard components that are already available on the mass market," explains Marco Jung, deputy head of the converter technology department at Fraunhofer IWES. In addition, the researchers use coil systems that require fewer ferrite sheets. The sheets are needed to control and shield the magnetic field and are very heavy on account of the iron oxide they contain. They are also expensive. Reducing the amount of ferrite material used further decreases the weight and cost of the coils.

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