Molten salts: Potential candidates for thermal energy storage applications

Molten salts as thermal energy storage (TES) materials are gaining the attention of researchers worldwide due to their attributes like low vapor pressure, non-toxic nature, low cost and flexibility, high thermal stability, wide range of applications etc. This review presents potential applications of molten salts in solar and nuclear TES and the factors influencing their performance. Ternary salts (Hitec salt, Hitec XL) are found to be best suited for concentrated solar plants due to their lower melting point and higher efficiency. Two-tank direct energy storage system is found to be more economical due to the inexpensive salts (KCl-MgCl2), while thermoclines are found to be more thermally efficient due to the power cycles involved and the high volumetric heat capacity of the salts involved (LiF-NaF-KF). Heat storage density has been given special focus in this review and methods to increase the same in terms of salt composition changes are discussed in the paper. Methods of concatenating energy storage systems with nuclear power plants are also discussed with different types of nuclear reactors like MHTGR, PAHTR, VHTR, etc. Nanomodifications of molten salts are done to improve heat transfer properties and efficiency of the transfer. The best dopants for such modifications were found to be TiO2, SiO2, MWCNTs, etc. Future challenges for large scale deployment of molten salts viz., high volume expansion ratio, low thermal conductivity, incongruent melting, corrosion, etc., are listed and discussed. Corrosion of molten salts and its mitigation has been discussed in detail for developing next-generation storage systems and its remedies are also discussed.