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 supply it to the power grid, thus contributing to energy security. However, previous technological approaches do not meet the demands for 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

The Fraunhofer Institute for Applied Solid State Physics IAF, the University of Stuttgart, Robert Bosch GmbH and Ambibox GmbH have now taken up this challenge 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. The three-year project is funded by the German Federal Ministry of Economic Affairs and Climate Action (BMWK) as part of the "Elektro-Mobil" program.

“Our 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,” says Dr. Stefan Mönch, researcher in the field of power electronics at Fraunhofer IAF and project coordinator of GaN4EmoBiL.

“In future, efficient, small-scale and intelligent charging infrastructures in electromobility will contribute to overcoming social challenges”, says Dr. Etienne Tchonla, R&D Director at Ambibox. 

Previous solutions are expensive, inefficient or too complex

To date, initial bidirectional medium-power DC wallboxes for batteries up to 800 V use power semiconductor devices, which are not yet optimal for this application: They are either efficient but expensive (silicon carbide) or low-cost but less efficient (silicon). Today 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 significantly improved. For this purpose, the project partners of GaN4EmoBiL are researching new semiconductor solutions as a first step. They want to realize a new cost-effective GaN technology on alternative substrates (for example sapphire), which enables 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 significantly increased operating hours.

At the end of the project, demonstrators should fill the research and development gap currently existing between cost, efficiency, compactness, functionality, power class and voltage class (800 V batteries). Furthermore, 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.

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