Energy storage systems can be spitted up into high and small capacity systems. High capacity systems are typically used in two different configurations for supporting the electrical grid. The first one can be seen as power configuration and the second is the energy configuration. For power configuration applications batteries are used to provide a large amount of power for injection to the electrical grid in a respectively short time. In the energy configuration, batteries are inserted to provide a steady amount of power into the grid for a longer time.

The nowadays used electrical energy storage systems can be classified into four different groups which are actually offered on the marked.

• The mechanical storage systems are using pumped hydro, compresses air and flywheel technologies.
• The chemical group consists of secondary batteries, flow batteries and hydrogen fuel cells.
• Also, a very important part for energy storage systems is the electrical group. This group is using capacitors, supercapacitors, ultracapacitors and superconducting magnetic coils.
• The smallest group is the thermal group which is operating with heat storage systems.
• The biggest development is actually ongoing in the electrochemical and in the electrical group.

The most important components of the electrochemical group are the secondary batteries which are implemented in stationary and mobile applications.

The most important battery types are currently based on lithium technology. In general lithium ion batteries has a relatively high efficiency range of 95 % – 98 %. This technology also has the benefit to provide a high number of charging and discharging cycles, no memory effect, high energy density, high discharge rates and a flat discharge curve. By usage of a balancer circuit the lithium-based batteries are prevented from overcharging and deep discharging. This balancer circuit is monitoring the voltages of each battery cell separately16. Also, very important for short time energy storage are supercapacitors and ultracapacitors. These are capacitor types which have a very high energy density in comparison to commonly used capacitors. These special types of capacitors are often used for high power applications to deliver power in a relatively short time. The advantages of these ultracapacitors are the high energy storage possibility in comparison to commonly used capacitors. The very low equivalent series resistance of this capacitors leads to a very good current handling. This leads further on to very fast charging and discharging of the capacitor. They can be implemented together with batteries in an electric vehicle to improve the performance of the entire system17

Also, Redox cells are belonging to the group of chemical energy storages which offers a huge storage capacity. They are operating with the principle of oxidation and reduction. There are several types of flow cells available on the marked. The vanadium-vanadium flow cell offers the highest efficiency range from 70% up to 90%. A flow cell can be also seen as a battery. During the charging process the electrical energy from an external source is converted into chemical energy. During the discharging process the stored chemical energy will be converted into electrical energy. The electrical input and output of such a cell is always DC. Therefore, an AC/DC converter is needed for such systems. The big advantages of flow cells in comparison to batteries are that the storage capacity and the output power can be controlled. The lifetime of such systems is around 40 years but they are very complex and cost intensive in construction.