With the significant increase in the number of electric cars, the importance of charging infrastructure and concepts for grid security is also increasing. When electric cars are charged primarily after work or after work, they cause peak loads in the power grid at these times. These could be avoided if the entire service life was used for loading. According to a study commissioned by the Federal Ministry of Transport, the average operating time of passenger cars is only 45 minutes per day. In the SKALE project, the Karlsruhe Institute of Technology (KIT) with its partners Robert Bosch GmbH and Power Innovation Stromstechnik GmbH is therefore developing a scalable charging system with photovoltaic system, stationary lithium-ion storage and medium-voltage grid connection.
The SKALE project combines electromobility and stationary battery storage in the construction of a DC charging infrastructure for semi-public to private spaces. Intelligent charging management, the use of decentralized renewable energy generators and stationary battery storage can prevent peak loads and contribute to the stability of the power grid. The research project looks at the entire energy flow chain in order to increase charging performance and efficiency and reduce costs. All requirements from the grid-side provision of energy to demand-based caching, distribution and conversion, to vehicle battery and return to the grid are taken into account. “The new approach is intended to provide a forward-looking infrastructure solution for any parking space with a variety of charging points and to efficiently integrate decentralized energy sources,” says Professor Marc Hiller of KIT’s Electrotechnical Institute (ETI).
Local flexibility and high efficiency
Currently, electric vehicles can be charged either via alternating current (AC) or DC. When charging with alternating current, converting to direct current in the vehicle reduces the charging power and efficiency of the charging process. When charging with direct current, the charging electronics are installed in the charging stations. This allows for an increase in charging performance and efficiency, but there are significant costs on the part of the infrastructure. “The problem is that both charging concepts focus either on the vehicle or only on part of the infrastructure, but do not look at the entire energy flow chain,” explains Nina Munzke, group leader at the ETI. In contrast to conventional charging methods, the mains power electronics are to be partially centralized, a buffer tank is used, load flows are centralized and the energy is distributed in a DC grid. This should lead to cost savings, high scalability, application flexibility and high efficiency.
Within the framework of SKALE, a demonstrator of the charging infrastructure is to be built. The planned construction includes around ten charging stations, a photovoltaic system with an output of about 100 kilowatt peak and a battery storage system with a capacity of approx. 50 kilowatt hours. The aim of the demonstrator is to gain practical experience for the construction and operation of the charging infrastructure. The measurement data obtained will be used for energy system optimization and the construction of future plants.
Stable and safe operation
Concept of the scalable charging system: grid connection to the medium-voltage grid, connection of various charging stations, a photovoltaic system and stationary lithium-ion storage via a DC network. (Graphic: Starosta, KIT)
Charging points, energy storage, decentralised energy generators and grid connection points: In the SKALE project, the EIT deals with the stable and safe operation of the DC network. For this purpose, the connection of the charging infrastructure is also about the development of a suitable inverter concept for the connection to the medium-voltage grid, which has a high efficiency and can be constructed as compactly as possible, but at the same time is also economically attractive. Based on the concept, a laboratory demonstrator will be set up at the Eti, scaled in current and voltage, to provide insights into operational management and compliance with network requirements.
Based on simulations, the KIT scientists create an interpretation recommendation for the overall system in the project and develop an interpretation tool that can design and optimize the charging infrastructure, including its components, for a specific location. The measurement data from the demonstrator can be used to evaluate the efficiency of the overall system, including the efficient use of renewable energies.
The SKALE research project, with a project volume of around 4.3 million euros, is funded by the Federal Ministry of Economics and Energy and started at the turn of the year.
Details on the battery technology: www.batterietechnikum.kit.edu