Thermal energy storage (TES), also commonly called heat and cold storage, allows the storage of heat or cold to be used later. To be able to retrieve the heat or cold after some time, the method of storage needs to be reversible. Thermal energy storage can be implemented by storage of Sensible Heat or Latent Heat.
The most common way of thermal energy storage is as sensible heat. As shown in the figure, heat transferred to the storage medium leads to a temperature increase of the storage medium. Hot water storage for domestic heating & domestic hot water is a very common example.
The phase change solid-liquid my melting and solidification can store large amounts of heat or cold. Melting is characterized by a small volume change, usually less than 10%. If a container can fit the material while it is in it’s liquid state, the pressure is not changed significantly and consequently melting and solidification of the storage material proceed at a constant temperature. Upon melting, while heat is transferred to the storage material, the material still keeps its temperature constant at the melting temperature, also called phase change temperature. If the melting is completed, further transfer of heat results again in sensible heat storage. The heat supplied upon melting is called latent heat, and the process latent heat storage.
The latent heat storage with phase change materials is commonly used for the solid-liquid phase changes. The application is divided as a simplification into temperature control and storage and supply of heat and cold with small temperature change. Some of the most common applications are:
Inorganic PCM are engineered hydrated salt solution made from natural salts with water. The chemical composition of salts is varied in the mixture to achieve required phase-change temperature. Special nucleating agents are added to the mixture to minimize phase-change salt separation and to minimize super cooling, that are otherwise characteristic of hydrated salt PCM. Salt Hydrates are characteristic of being non-toxic, non-flammable and economical.
Bio-based PCM are organic materials that are naturally existing fatty acids such as vegetable oil. Based on their chemical composition, their phase-change temperature can vary. These products are non-toxic, non-corrosive and have infinite life cycles. They however can be expensive and flammable at high temperatures.
Organic PCM are naturally existing petroleum bi-products that have their unique phase-change temperature. These products are manufactured by major petrochemical companies so their availability could be limited. They can be toxic, flammable and expensive. They have infinite life cycles and the price varies with changes in petroleum prices globally.