One of the most widely used methods is based on the form of energy stored in the system as shown in , which can be categorized into mechanical (pumped hydroelectric storage, compressed air energy storage and flywheels), electrochemical (conventional rechargeable batteries and flow batteries), electrical (capacitors,supercapacitors and superconducting magnetic energy storage),thermochemical (solar fuels), chemical (hydrogen storage with fuel cells) and thermal energy storage (sensible heat storage and latent heat storage). A detailed description and discussion of each type of EES technology will be given in the next section following the above order of category.