The electric vehicle is to evolve into part of an increasingly broad mobility offer and become an element of the sustainable energy transition. In the first half of 2020, renewable energies contributed more than 50 percent to the German electricity mix for the first time. However, the increasing percentage is also accompanied by a basic dilemma of wind and solar power: The generation of electricity is not always constant. On sunny days and phases with strong winds, there is often a lack of capacity to store the generated energy that the grid cannot use.
As the number of registered electric cars increases, the number of mobile energy storage units also rises. This offers great potential, provided that the storage capacity can be used intelligently. This is why Audi and Hager joined hands and developed a research and solution approach that creates financial incentives and offers greater security of supply: bidirectional charging. »Electric mobility is bringing the automotive industry and the energy sector closer together. The battery of an Audi e-tron could supply a single-family home with energy for around one week independently. Looking ahead, we want to make this potential accessible and make the electric car part of the energy transition as an energy storage device on four wheels«, said Martin Dehm, technical project manager for bidirectional charging at Audi. In the process, the high-voltage battery of the electric car not only is charged via the wall box at home but can also supply energy back to the house as a decentralized storage medium. If the customer has a photovoltaic system, the electric car serves as a temporary storage medium for the domestically generated eco-electricity. When the sun is no longer shining, the vehicle can supply the stored electricity back to the house. Bidirectional charging at home (Vehicle to Home,V2H) – has great potential to reduce the home owner’s electricity costs and increase network stability. As a further expansion stage in combination with a home storage unit, it is possible to achieve near complete energy independence and increased security of supply in the event of a blackout.
Coordinated Interaction between ifferent Technical Components
An Audi e-tron with near-series charging technology was used in the research project. In the test grid, the fully electric Audi model operated with a DC wall box, which enables a charging capacity of up to 12 kW, and a flexibly extendable home storage unit with a capacity of 9 kWh. While it could provide additional flexibility in possible series production, it is not a necessary requirement for bidirectional charging. Thanks to the DC voltage level in the overall grid, the connection between the PV system and the vehicle does not require an inverter and is thus a particularly efficient solution.
Bidirectional charging focuses mainly on use cases where home owners use their own photovoltaic system to benefit from cost-optimized charging with their domestically generated electricity. The electric car stores the excess electricity from the PV system that is not used by appliances in the house. If the customer has variable rates, the electric car can supply the entire house in phases where electricity prices are high. At night or during non-productive times of the rate, the car then uses inexpensive electricity to charge up to the desired target state of charge (SoC). Bidirectional charging also provides a security of supply that extends beyond pure cost optimization: In the event of a blackout, the system can supply the house with energy via the high-performance HV battery or it can even operate a building without a grid connection independently in what is known as stand-alone operation.