Scientists at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany, have developed a highly efficient system for inductively charging electric vehicles. As well as being efficient, the system is also fast – it is possible to charge a conventional electrical vehicle battery to 80 % of its nominal capacity in less than one hour.
The wireless energy transfer to the car battery is carried out by means of an electromagnetic field between two coils: one installed in the car and a second coil integrated into the street or the parking place. A cable connecting the charging station and the electric vehicle is no longer necessary with this system.
In the first prototypes an efficiency of 97.4 % has been achieved for the inductive power transmission at a coil distance of 13 cm, with charging rates for transmitted power of 22 kW being achieved.
All of the necessary power electronics for energy transfer, the coil system and the control technology were developed and constructed at Fraunhofer ISE within the framework of the Fraunhofer project "Shared use of e-mobility: vehicles, data and infrastructure", or GeMo for short in German. Among other things, the scientists in Freiburg have developed a resonant power converter. With a resonance circuit and a stationary coil, the device creates a high frequency magnetic field for the power transfer process. A second converter reshapes the high frequency induced current into DC for charging the battery.
The focus of research was to optimise all stages in the inductive charging process chain. By employing new semiconductor devices made from silicon carbide (SiC), the efficiency of the various power converters used in the charging system could be increased greatly. Owing to these low switching losses, SiC transistors allow high switching frequencies of 100 kHz which makes the mechanical construction very compact and much lighter compared to conventional devices. Other losses could be minimised by optimising the coil and the resonant circuit. With special capacitors on the stationary and the mobile side, the reactive power requirement and scatter field of the coil were compensated. This also eliminates reactive power exchange between the power electronics and the coil.
To make the charging system complete, the researchers worked both on a bidirectional inverter for connecting the stationary part of the system to the grid and on a bidirectional converter for connecting the mobile part of the system to the battery. These allow continuous, bidirectional operation of the charging system, which can not only take power from the grid and feed it into the battery but also feed the power stored in car battery back into the grid. For the complete system, the research team achieved an efficiency of up to 95 %.