Energy storage systems differ in terms of the technology-related reversible conversion of electrical energy during the storing in electrical, mechanical, thermal and chemical energy storage systems.

Mechanical Storage

Mechanical storage systems use kinetic energy for storage and retrieval, which is buffered as potential, kinetic or pneumatic energy.

• Pumped storage plants are the reference technology for energy storage systems. The potential for expansion, however, is almost exhausted. They are cost-effective, but not installable decentralized and near the centers of consumption.
• Flywheel technology is representative of the group of technical implementation in which electrical energy is converted into rotational kinetic energy and stored that way temporarily. Due to the friction in the bearings of the moving mass, the self-discharge rate is very high, so the flywheels can only maintain their high initial efficiency of 80-95% within short time cycles.
• In compressed air energy storage systems a compressible medium is condensed in a defined storage capacity. Nonadiabatic compressed air storage has low efficiency. However, it is installable near the centers of consumption and is generally also suitable for large storage amounts due to the separation of power and work. 

Electric Storage

Electric storage actually stores the power and doesn’t convert it. This can take place in capacitors (dual-layer capacitors) or magnetic fields (superconducting magnetic energy systems). The high efficiency excludes the high level of self-discharge. The costs of the storage are prohibitively high, so economic application is not feasible at present.

Temperature Storage

Analogous to the use as temporary storage in solar thermal plants, heat can be stored well and converted into electricity using established turbine technology. For cases of application in which the energy is obtained not directly as heat but as electricity, the temporary storage results in a high level of efficiency loss. A comparison of costs is not possible due to the current technology status and lack of implementation.

Chemical Storage

Chemical Storage systems represent the class of the currently most commonly used energy storage for electrical energy. What they have in common is the use of a chemical compound which is dissociated in ions in solid, liquid or dissolved states and which moves in a directed manner under the influence of an electric field. The term electrolyte is often used to describe the solid or liquid material, which contains the mobile ions.

The distinction between internal and external energy storage is based on the dependence of power and work. When it comes to internal storage, both variables are directly linked. More storage accordingly always means an increase of power and work. This restriction does not apply to external storage in which the pumpability of the (liquid or gaseous) electrolytes enable a separation of (working) storage and power unit. External energy storage can be divided into liquid or gaseous after the phase of the pumpable medium. Internal energy storage is divided into high and low temperature storage.

• In hydrogen storage the electrolysis in fuel cells enables the reversible separation of water into its molecular components, whereby it is only necessary to store the hydrogen component separately and to keep it available for the power generation process. In Power-to-Gas technology, the separate spatial production and use of the storage medium and its flexible employment for power and heat generation is interesting from an economic point of view. The storage capacity of the existing gas network of 400 TWh especially indicate the significance of this technology for the base load supply of an economy, which is supplied with renewable energy to 100%.
• Redox flow storage is characterized by the pumpability of two storage mediums, which interact through an ion exchange in the reduction and oxidation process. The two energy storing electrolytes circulate in separate circuits. This results in a rather complex construction with mechanical stress, which is untypical for chemical storage. This entails a loss of efficiency due to the necessary pumping on the one hand and causes unavoidable wear on the moving components on the other.
• The class of high-temperature storage is characterized by the verified high operating temperatures of the electrodes. This is due to the use of solid electrolytes in combination with liquid electrodes. Concerning material combinations, nickel and sodium have predominantly been found to be suitable as electrode material. The disadvantages are the high operating temperature and the need for protection against mechanical damage in order to prevent the release of the aggressive constituents sodium and sulfur.
• Lead-acid batteries are manufactured in large quantities and are used worldwide. This is mainly due to their use as starter batteries for vehicles. But also the use in control reserve markets, as the Upside Group successfully shows in its Alt Daber project or as a system for uninterruptible power supply (UPS) are proof of the numerous application possibilities. The 17MW/14MWh storage system operated by the former BEWAG since 1986 is an example of its use as a grid storage system. The input materials are inexpensive, even if the heavy metal lead and the electrolyte sulphuric acid require disposal precautions. One of the advantages of the lead-acid technology is the easy determination of the state of charge, since the cell voltage is proportional to the acid concentration and thus to the state of charge (SOC).
• Lithium-ion batteries are produced in large quantities and are therefore very suitable for commercial use in the control energy market. Lithium iron phosphate (LiFePO4) shows very good properties from the selection of different lithium types to ensure the usability during the entire project duration. With Li-Ion batteries, the determination of the SOC by complex mathematical algorithms is almost possible. The relationship between cell voltage and state of charge is much more complex. Therefore, continuous monitoring of the cell voltage and a technically mature battery management system (BMS) are necessary to ensure operational reliability. On the other hand, the BMS also offers the possibility of an optimized timetable within the cell network for stationary large storage facilities. On the one hand, the loss from injection and withdrawal cycles can be minimized, and on the other hand, the service life can be maximized. A balancing BMS, which evenly loads all cells in the network, can considerably slow down the ageing process. Lithium iron phosphate combines high intrinsic safety with a long service life. Due to the materials used, the batteries can be manufactured relatively cost-effective. Numerous manufacturers have specialized in LiFePO4, so that the technology can be described as pretty much advanced.
• The lead-carbon technology is one of the market leaders in the field of stationary cells and has been in use worldwide for decades similar to the standard lead-acid cell. It combines the advantages of both worlds, the economy and safety of lead-acid technology with the low maintenance requirements of lithium cells. The lower energy density compared to lithium cells does not represent a disadvantage when used in stationary storage systems. Most lead-carbon battery cells are dry accumulators, so-called VRLA-AGM (Valve Regulated Lead Acid - Absorbent Glass Mat). The electrolyte is bound in a glass fibre fleece so that there are no internal liquids. Although these cells must also be equipped with a one-way valve for possible outgassing (in the event of overcharging), they can be tilted or even flooded without any problems, without the possibility of substances such as sulphuric acid escaping. The production processes are standardized so that the quality is considered to be assured. In addition, a very high proportion of the batteries can be recycled.