New Charging Technology Turns E-Cars Into Batteries on Wheels

Bidirectional charging turns e-cars into mobile electricity storage devices and thus contributes to making the energy system more flexible. To make such charging suitable for mass use, a consortium led by the Fraunhofer IAF is researching innovative charging technologies in the »GaN4EmoBiL« project.

Bidirectional charging allows electric cars to be charged with electricity from renewable sources and discharged as needed during periods when no wind or solar energy is being produced.Consumers could use this electricity for other electrical devices or deliver it to the power grid, contributing to energy security.

However, current technological approaches do not meet the demands of cost and efficiency. There is a lack of intelligent and cost-effective bidirectional charging systems to connect batteries, the grid, local generators and consumers with high efficiency and high power density.

Research project »GaN4EmoBiL« launched

This challenge has now been addressed by the Fraunhofer Institute for Applied Solid State Physics IAF, the University of Stuttgart, Robert Bosch and Ambibox in the recently launched research project »GaN4EmoBiL - GaN Power Semiconductors for Electromobility and System Integration through Bidirectional Charging«. The consortium's goal is to demonstrate an intelligent and cost-effective bidirectional charging system using new semiconductor devices, device concepts and system components.

»Our project aims to connect batteries, renewable energies and electrical consumers economically and flexibly. Through bidirectional charging solutions, the previously unused batteries of parked electric vehicles will make a greater contribution to increasing the flexibility of the energy system and avoiding CO₂ emissions in the future,« explains Dr. Stefan Mönch, researcher in the field of power electronics at Fraunhofer IAF and project coordinator of »GaN4EmoBiL«. Previous solutions are expensive, inefficient or too complex

Previous solutions are expensive, inefficient or too complex

First bidirectional DC wallboxes of medium power for batteries up to 800 V have so far used power semiconductor devices, which are not yet optimal for this application: They are either efficient but expensive (silicon carbide) or inexpensive but less efficient (silicon). Currently available 650 V transistors made of gallium nitride on silicon (GaN-on-Si) are inexpensive and efficient, but require complex circuitry because the dielectric strength is not sufficient.

To integrate as many batteries as possible bidirectionally, the cost, efficiency and compactness of the charging solutions must be significantly improved. To this end, the project partners of »GaN4EmoBiL« are researching new semiconductor solutions as a first step. They want to realize a new low-cost GaN technology on alternative substrates (for example sapphire) that enables inexpensive and efficient 1,200 V transistors. Building on this, they will develop new system components (bidirectional charging cable and charger) and investigate their reliability for greatly increased operating durations.

At the end of the project, demonstrators should fill the research and development gap that currently exists in the area of tension between cost, efficiency, compactness, functionality, power class and voltage class (800 V batteries). In addition, the consortium aims to promote knowledge transfer between universities, research institutions and industry, train young scientists and secure national know-how in the field of electromobility.