Thermal energy storage can be obtained by cooling, heating, melting, solidifying, or vaporizing a material in which the energy becomes available as heat by reversing the process. Using this method provides the opportunity to mitigate environmental impacts and results in more efficient and clean energy systems.
Three main types of TES exist depending on mechanism of energy storage – (i) sensible heat, (ii) latent heat, and (iii) thermochemical reaction. Sensible heat storage involves storing thermal energy in various forms such as liquid or solid media (e.g. water, sand, molten salt, or rocks) by heating them using the heat transfer fluid .
Traditionally, heat storage has been in the form of sensible heat, raising the temperature of a medium. Examples of such energy storage include hot water storage (hydro-accumulation), underground thermal energy storage (aquifer, borehole, cavern, ducts in soil, pit) , and rock filled storage (rock, pebble, gravel).
In sensible TES, energy is stored by changing the temperature of the storage means. The amount of heat stored is proportional to the density, specific heat, volume and variation of temperature of the storage material.
Thermochemical energy storage systems have also several advantages over other types of thermal energy storage systems: Table 7. Comparison of different types of TES . Components (A and B) can usually be stored separately at ambient temperature, after cooling to ambient conditions subsequent to their formation.
Sorption processes dedicated to storage can be classified as open or closed system . In open systems, which operate at atmospheric pressure, the working fluid vapor is released to the environment, which means that only water is used for those systems.