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Wireless Charging and Discharging for Electric Vehicles

Posted September 4, 2015

A major reason why electric cars have failed to gain widespread adoption is the lack of charging stations. This, however, may no longer be a concern for drivers with the development of a cost-effective system that allows wireless charging of car batteries.

Electric Vehicle (EV) charging at the charging station. Image credit: BP63Vincent via Wikimedia, CC-BY-SA-3.0

Electric Vehicle (EV) charging at the charging station. Image credit: BP63Vincent via Wikimedia, CC-BY-SA-3.0

The underlying technology that supports this capability is a cost-effective inductive charging system developed by researchers at the Fraunhofer Institute for Wind Energy and Energy System Technology IWES in Kassel. Inductive charging involves transmission of energy through the air, or, more precisely, through a time-varying magnetic field. This requires use of two coils–one built into a road, a parking space or a garage, and the other into the underbody of the car. Together with suitable capacitors, these coils form a sort of resonant “antenna system for energy transfer.” The closer the two coils, the more efficient the energy transfer.

The system is highly efficient across the entire power range, from 400 watts to 3.6 kilowatts, while the car and the charging coil can be placed up to 20 centimeters apart.

An EU-funded project called Unplugged which is studying the feasibility of wireless charging in public spaces considers two scenarios for use of such systems: locations where cars will be stationary for some time, such as supermarket car parks, and those where vehicles will be immobile for a few minutes, such as taxi ranks, bus stops and even traffic lights.

“Wireless charging would be very convenient for the driver, and would make it possible to increase the number of charging events per journey,” explains project coordinator Axel Barkow of the Forschungsgesellschaft für Kraftfahrwesen Aachen (fka) in Germany.

In order to make the system cost-effective, the Fraunhofer researchers used standard components that are already available on the market. In addition, they used coil systems that require fewer ferrite sheets to control and shield the magnetic field. Reducing the amount of ferrite material used further decreased the weight and cost of the coils.

Thanks to the specially designed power electronics and coil systems, “Even with an air gap of 20 centimeters, the system achieves an efficiency of between 93 and 95 per cent”–informs Dr René Marklein, project manager at Fraunhofer IWES. “Comparable systems achieve that kind of efficiency only over shorter distances, which limits their usefulness for cars with larger ground clearance.”

The charging system, containing the electronics and connections for various charging cables, is to be carried in the vehicle. Other than inductive charging, it also has a one-phase and a three-phase power connection, allowing drivers to charge their vehicle at ordinary power outlets and charge spots as well.

The system can also be used to discharge the batteries, helping to stabilize the power grid. During favourable conditions, solar cells and wind turbines often supply more energy to the power grid than is needed at the time. This surplus electricity could increase the voltage and even damage electrical devices. The inductive system allows the car batteries to be used as a sort of buffer, storing the surplus electricity temporarily and then feeding it back into the grid when needed.

The scientists plan to exhibit the prototype of their wireless inductive transmission system at the IAA International Motor Show, which will be held from September 15 to 18 in Frankfurt this year.

Written By: Uma Gupta, Contributing Author for Technology.Org

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