Battery storage is becoming a soli pillar in the European energy transition. With unique features such as response speed, flexibility and reliability, they can stabilize the power grid, prevent overloads and integrate more renewable energy at the same time. It is a mature technology that is always similar in design. It doesn't even matter whether it's the home storage unit in the basement or a stationary large-scale storage unit. But what does a storage system actually look like? A battery storage system consists of only a few components. In the following, we present these main components in detail.
The battery system
The battery system is the heart of the storage system. This is where the electricity is temporarily stored and released at a later point in time. At the elementary level, the battery system consists of battery cells that are combined into modules. The necessary coordination during charging and discharging is handled by the battery management system (BMS), which monitors and protects the cells. In the case of the large-scale storage system, these are in turn grouped together in several cabinets and, if necessary, placed in containers. This protects the system from dust and water.
Each cabinet or container also has its own cooling and extinguishing system. The cooling system is important for the optimal temperature control of the system, which has a significant influence on the aging process of the cells. A large-scale battery storage system has an expected lifetime of 10-20 years, depending on the application and cycling. The extinguishing system is installed as a precautionary measure to quickly and selectively extinguish a fire in case of emergency and to prevent it from spreading to other cells.
Today, lithium-ion cells are mainly used for storage. However, not all lithium-ion cells are the same. Depending on the cell chemistry, there can be significant differences in the behavior of the cells. The decisive factor is the electrode active material. Currently, three types of lithium-ion cells can be distinguished. Lithium cobalt oxide (LCO) is mainly found in electronic devices, such as our cell phone or laptop. Lithium-nickel-manganese-cobalt (NMC), on the other hand, is mainly used for electromobility, but is also still present in the stationary storage sector. This is mainly due to the economy of scale, which has greatly reduced the price of NMC technology over the last decade. The cells also have a very high energy density. This is a significant advantage for the automotive industry, because in its application maximum energy has to be accommodated in limited space.
Lithium iron phosphate (LIP) is now gaining acceptance for stationary storage. LIP has key advantages over NMC. Iron and phosphate are available in larger quantities, which means that prices are already cheaper than those for NMC. In addition, LIP cells are safer than NMC or LCO, which is particularly important for thermal runaway issues. Furthermore, LIP cells offer an opportunity to remove cobalt from the supply chain, change the dependence of current resources, and thus better meet the sustainability obligation for companies.
Batterien werden mit Gleichstrom betrieben, das Stromnetz aber mit Wechselstrom. Daher benötigt jeder Batteriespeicher für die Konvertierung einen Wechselrichter. Damit das Batteriesystem sowohl Laden als auch Entladen kann, müssen die Wechselrichter bidirektional sein – also sowohl Wechselstrom in Gleichstrom wandeln als auch umgekehrt. Die Wechselrichter (und das Batteriesystem) werden dabei üblicherweise im Niederspannungsbereich (<1000V) betrieben.
Inverters and battery system can be scaled independently. The ratio of the power from the inverters and the capacity of the battery system results in the C-rate of the battery storage. The C-rate indicates how fast a battery can be discharged. 1C means that the storage system can be operated at maximum power output for one hour before it is empty. Stationary storage units in Germany range from 0.5-1C. This means that they can be operated for at least one to two hours. Internationally, there are already storage systems that achieve C rates of 0.25C or less.
The transformer station
In contrast to home storage systems, large-scale battery storage systems feed in or output high quantities of electricity. This amount of energy can no longer be absorbed at the low-voltage level, so conversion to higher voltages must take place. For this purpose, transformers are used which, just like the inverters, must be bidirectional. Together with a switchgear, which handles power distribution and disconnection, the transformer station forms the node to the grid level. The station thus marks the physical grid connection point to which the battery storage system is connected.