State of art review on the solidification and melting characteristics of phase change material in triplex-tube thermal energy storage

•Focus on application of PCM in triplex-tube thermal energy storage.•Review the solidification and melting characteristics of PCM in triplex-tube thermal energy storage.•Discuss the effect of various factors to solidification and melting characteristics of PCM. Phase change material (PCM) exhibits high latent heat but low thermal conductivity. It stores thermal energy from solar irradiation and waste heat from industry and domestic process for later use. PCM is usually placed in special containers or heat exchangers such as triplex-tube, shell and tube heat exchanger, cylindrical container, rectangular container, etc. Heat absorption and dissipation are greatly affected by PCM's low thermal conductivity. This review intends to discuss the factors affecting the solidification and melting characteristic of PCM in the triplex-tube heat exchanger (thermal energy storage). These factors include flow rate and temperature of heat transfer fluid, the shape of fins and their position, heating and cooling position, the effect of triplex-tube geometry configuration, addition of nanoparticles, the use of foam and, multiple PCMs arrangement. Triplex-tube heat exchanger is selected since this type of thermal energy storage offers a higher heat transfer area compared to the shell and tube heat exchanger. It is found that solidification and melting of PCM in triplex-tube are augmented with the combination of fins and nanoparticles. The use of optimised fins and pure PCM in triplex-tube also exhibit better phase transition characteristics compared to using only nanoparticles.