2.1 Classifi cation of EES systems A widely-used approach for classifying EES systems is the determination according to the form of energy used.
In Figure 2-1 EES systems are classified into mechanical, electrochemical, chemical, electrical
and thermal energy storage systems.
Hydrogen and synthetic natural gas (SNG) are secondary energy carriers and can be used to store electrical energy via electrolysis of water to produce hydrogen and,
in an additional step, methane if required. In fuel Section 2 Types and features of energy storage systems cells electricity is generated by oxidizing hydrogen or methane.
This combined electrolysis-fuel cell process is an electrochemical EES. However, both gases are multi-purpose energy carriers.
For example, electricity can be generated in a gas or steam turbine. Consequently, they are classified as chemical energy storage systems.
In Figure 2-1 thermal energy storage systems are included as well, although in most cases electricity is not the direct input to such storage systems.
But with the help of thermal energy storage the energy from renewable energy sources can be buffered and thus electricity can be produced on demand.
Examples are hot molten salts in concentrated solar power plants and the storage of heat in
compressed air plants using an adiabatic process to gain effi ciency.
Electrical Energy Storage