How battery storage systems are constructed

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Battery storage systems are becoming an integral pillar in the European energy revolution. 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 sophisticated technology that is always structured in a similar way. It doesn't even matter whether it is a home storage system in the basement or a stationary large storage facility. But what does a storage system actually look like? Basically, a battery storage system consists of just a few components. We present these main components in detail below.

The battery system

The battery system is at the heart of the storage system. Here, the electricity is temporarily stored and released again at a later point in time. At an elementary level, the battery system consists of battery cells, which are combined into modules. The battery management system (BMS), which monitors and protects the cells, takes care of the necessary coordination during charging and discharging. In the case of large storage, these are in turn combined in several cabinets and, if necessary, stored 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 optimal temperature control of the system, which has a significant influence on the aging process of the cells. A large stationary battery storage system has an expected lifespan of 10-20 years, depending on the application and cycle. The extinguishing system is installed as a precautionary measure to quickly and specifically extinguish a fire in an emergency and to prevent it from spreading to other cells.

Lithium-ion cells are mainly used for storage today. However, lithium-ion cells are not the same as lithium-ion cells. Depending on cell chemistry, there can be significant differences in cell behavior. The electrode active material is decisive for this. There are currently three types of lithium-ion cells. Lithium cobalt oxide (LCO) is mainly found in electronic devices, such as our mobile phones or laptops. Lithium nickel-manganese cobalt (NMC), on the other hand, is mainly used for electric mobility, but is also still used in stationary storage. This is mainly due to the effect of scale, which has significantly reduced the prices of NMC technology in the last decade. The cells also have a very high energy density. This is a significant advantage for the automotive industry, because its application must contain maximum energy in a limited space.
Lithium iron phosphate (LFP) is now becoming established for stationary storage systems. LFP has key advantages over NMC. Iron and phosphate are available in larger quantities, which means that the prices are already cheaper today than those for NMC. In addition, LFP cells are safer than NMC or LCO, which plays an important role in thermal runaway in particular. In addition, LFP 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.

The inverter

Batteries are powered by direct current, but the power grid is powered by alternating current. Therefore, every battery storage device requires an inverter for the conversion. In order for the battery system to be able to both charge and discharge, the inverters must be bidirectional — i.e. both convert alternating current into direct current and vice versa. The inverters (and the battery system) are usually operated in the low voltage range (<1000 V).

The inverter and battery system can be scaled independently of each other. The ratio of the power from the inverters and the capacity of the battery system results in the C-rate of the battery storage system. The C rate indicates how quickly a battery can be discharged. 1C means that the memory can operate at maximum power output for one hour before it is empty. Stationary storage systems in Germany are in the range of 0.5-1C. They can therefore 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 stationary battery storage systems feed in or out large amounts of electricity. This amount of energy can no longer be absorbed at the low voltage level, which is why a conversion to higher voltages must take place. Transformers are used for this purpose, which, just like inverters, must be able to be used bidirectionally. Together with a switchgear, which distributes power and shuts down the system, the transformer station forms the hub to the grid level. The station thus marks the physical grid connection point to which the battery storage device is connected.

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