With the significantly increasing number of electric cars, the importance of charging infrastructure and concepts for network security also increases. If electric cars are charged mainly after work or after work, they cause load peaks in the power grid at these times. This could be avoided if the entire idle time was used for charging. According to a study commissioned by the Federal Ministry of Transport, the average operating time of cars is only 45 minutes per day. In the SKALE project, the Karlsruhe Institute of Technology (KIT) is therefore developing a scalable charging system with a photovoltaic system, stationary lithium-ion storage and medium-voltage grid connection with its partners Robert Bosch GmbH and Power Innovation Power Supply Technology GmbH.
The SKALE project combines electromobility and stationary battery storage in the construction of a DC charging infrastructure for semi-public to private space. Through intelligent charging management, the use of decentralized renewable energy generators and stationary battery storage systems, load peaks can be prevented and a contribution made to the stability of the power grid. The research project looks at the entire energy flow chain in order to increase charging power and efficiency and reduce costs. All requirements from the grid-side supply of energy to needs-based intermediate storage, distribution and conversion to the vehicle battery and feedback into the grid are taken into account.
Local flexibility and high efficiency
Electric vehicles can currently be charged either with alternating current (AC) or direct current (DC). When charging with alternating current, the conversion to direct current in the vehicle reduces the charging power and the efficiency of the charging process. When charging with direct current, the charging electronics are built into the charging stations. This enables an increase in charging performance and efficiency, but there are significant costs on the infrastructure side. “The problem is that both charging concepts focus either only on the vehicle or only on part of the infrastructure, but do not consider the entire energy flow chain,” explains Nina Munzke, group leader at ETI. In contrast to conventional charging methods, the network-side power electronics should be partially centralized, a buffer storage used Load flows are centralized and the energy distributed in a direct voltage network. This should lead to cost savings, high scalability, flexibility of the application location and high efficiency.
A demonstrator of the charging infrastructure is to be set up as part of SKALE. The planned structure includes around ten charging stations, a photovoltaic system with an output of around 100 kilowatts peak and a battery storage system with a capacity of around 50 kilowatt hours. With the help of the demonstrator, practical experience for setting up and operating the charging infrastructure is to be gained. The measurement data obtained flow into the energy system optimization and the construction of future systems.
Stable and safe operation
Charging points, energy storage, decentralized energy producers and grid connection points: In the SKALE project, the ETI deals with the stable and safe operation of the DC grid. When connecting the charging infrastructure, it is also about developing a suitable converter concept for connecting to the medium-voltage network, which is highly efficient and can be built as compactly as possible, but is also economically attractive. Based on the concept, a laboratory demonstrator is being set up at the ETI, which is scaled in terms of current and voltage and is intended to provide information on operational management and compliance with network requirements.
On the basis of simulations, the KIT scientists create a design recommendation for the overall system and develop a design tool that can design and optimize the charging infrastructure, including its components, for a specific location. With the measurement data from the demonstrator, the efficiency of the overall system can be assessed, including the efficient use of renewable energies.