**2.2 Thermal storage systems**

The requirement of a thermal storage system arises in many applications. These include harnessing intermittent sources of energy like solar, wind, etc. and heat recovery systems. Thus, developing efficient and reliable thermal storage devices is an important requirement for both conventional and non-conventional thermal energy systems. Thermal storage system includes sensible heat storage (i.e. solid media storage, hot- and cold-water storage, underground thermal storage), latent heat storage (i.e., ice storage, molten salts), and thermochemical storage.

### *2.2.1 Sensible heat storage*

Practical Heat Storage (SHS) is the process of storing heat by increasing the surrounding temperature. Increasing the temperature of a substance, whether it be a solid or a liquid, serves as a means of storing thermal energy in this system. In SHS systems, the amount of stored heat depends on the storage material, the specific heat of the substance, and the change in temperature. The need for a high volume of material is one of the most significant drawbacks of SHS, particularly when the acceptable temperature range is narrow.

#### *2.2.2 Latent heat storage*

Latent heat storage units (LHSUs) are used to store energy in a substance that changes phase when heat is added or removed. When a medium goes from one state to another—solid, liquid, or gas—it is said to have undergone a phase change. Whether energy is being absorbed or discharged determines the direction of this transition (i.e., liquid to solid or solid to liquid).

In comparison to SHS systems, LHSU is a more appealing option. This is due to its high energy density and very narrow temperature range. LHSU also offers consistent energy storage and delivery in a compact design. In the case of water, for instance, the latent heat of fusion is eighty times that of the sensible heat. As a result, at a constant temperature of 0 degrees Celsius, latent heat-based storage systems that utilise water may extract 80 times as much energy as a sensible heat-based storage system. This implies that the quantity of material for a specific amount of energy may be drastically reduced in both weight and size. It is to be noted that the energy storage capacity of a LHSU is generally 5–10 times higher than that of an SHS system [2–5].

Among the several energy storage technologies discussed above, some technologies have undergone significant research and demonstrations and have matured for commercialization. These include pumped storage technologies, sensible heating technologies etc. Some technologies like thermochemical, supercapacitor, flywheel (high speed), flow batteries, etc., are still in the preliminary stage of research. Technologies like latent heat storage units (LHSU), flywheel (low speed), compressed air energy storage, etc., however fall in the category where there need more research and demonstrations before they can be brought to commercialization. More work has to be done on these systems before they can reach their full potential.
