New charging tech to turn e-cars into batteries on wheels

Bidirectional charging enables electric vehicles to be both charged and discharged as needed. E-cars can thus serve as mobile electricity storage units and contribute to increasing the flexibility of the energy system. To facilitate the use of bidirectional charging on a broad scale, researchers led by Fraunhofer IAF have launched a project (GaN4EmoBiL) to develop new semiconductor, device and system technologies for the 800 V class.

With bidirectional charging, electric cars can be charged with electricity from renewable sources and discharged during periods where no wind or solar energy is being produced. Consumers could use this electricity for other electrical devices or supply it to the power grid, thus contributing to energy security. However, previous technological approaches do not meet demands for cost and efficiency. There is a lack of intelligent bidirectional charging systems to connect batteries, the grid, local generators and consumers with high efficiency and high power density.

The Fraunhofer Institute for Applied Solid State Physics IAF, the University of Stuttgart, Robert Bosch and Ambibox have now launched the research project ‘GaN4EmoBiL — GaN power semiconductors for electromobility and system integration through bidirectional charging’. The researchers’ goal is to demonstrate an intelligent and cost-effective bidirectional charging system using new semiconductor devices, device concepts and system components.

Dr Stefan Mönch, a researcher at Fraunhofer IAF and project coordinator of GaN4EmoBiL, said the project aims to connect batteries, renewable energies and electrical consumers in an economical and flexible way. “Through bidirectional charging solutions, the previously unused batteries of parking electric vehicles will make a greater contribution to increasing the flexibility of the energy system and avoiding CO₂ emissions in the future,” Mönch said.

To date, initial bidirectional medium-power DC wallboxes for batteries up to 800 V use power semiconductor devices, which are not yet suitable for this application, as they are either efficient but expensive (silicon carbide) or low cost but less efficient (silicon). Available 650 V transistors made of gallium nitride on silicon (GaN-on-Si) are inexpensive and efficient, but require a complex circuit due to insufficient voltage rating.

To integrate as many batteries as possible bidirectionally, the cost, efficiency and compactness of charging solutions must be improved. The project partners of GaN4EmoBiL are researching new semiconductor solutions as a first step, to realise cost-effective GaN technology on alternative substrates (for example, sapphire), which enable low-cost and efficient 1200 V transistors. On this basis, they are developing new system components (bidirectional charging cable and charger) and investigating their reliability for increased operating hours.

At the end of the project, the research findings should fill the gap currently existing between cost, efficiency, compactness, functionality, power class and voltage class (800 V batteries). The researchers aim to promote knowledge transfer between universities, research institutions and industry in the field of electromobility.

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