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The universe has shifted its focus from non-renewable to renewable energy sources from the last decade. The limited availability of fossil fuels and pollution is the reason behind the utilization of renewable sources. A major renewable energy source, solar energy, requires efficient thermal energy storage due to its intermittent nature. Phase change material (PCM) can store a large amount of heat by changing its phases. However, the low thermal conductivity of PCM makes its usage very limited. Hence, PCM's heat transfer augmentation technique using eccentricity and fin is studied in the present research. First of all, positive (lower displacement of the inner tube) and negative eccentricity (upper displacement of the inner tube) are studied in triplex type latent heat thermal energy storage (TTLHTES). Among the eccentricities of-15 mm to +15 mm, the eccentricity of +10 mm is found most effective in melting with a 27.63% reduction in melting time, while-3 mm eccentricity is found most effective in solidification with a 12.82% reduction in solidification time. The combination of fins with eccentricity is examined for melting and solidification enhancement. The combination of fin and eccentricity efficiently reduces melting time, with a maximum decrement of 67.37% with +3 mm eccentric TTLHTES with four fins. However, the only fin without any eccentricity is found more effective in solidification enhancement. The solidification time is reduced maximum by 46.15% using four longitudinal fins and no eccentricity. It can be concluded that fin and eccentricity must be coupled when melting time is required to enhance. However, only fin must be utilized when solidification is more critical.

Along with the heat transfer mechanism for the development of a latent heat storage unit (LHSU), the choice of the phase change material (PCM) plays an important role. The enviable thermo-physical, kinetic, and chemical properties of PCM with the economy is an essential criterion for efficient thermo-economical LHSU. The most important criteria that have limited widespread use of LHSU are the useful life of phase change materials. For long term performance of LHSU, the PCM used in the system should be thermally stable and reliable. It does not deteriorate its own properties, especially latent heat and melting point after a repeated number of thermal cycles. Thus an exhaustive literature survey is carried out for different types of PCMs used. The primary objective of this chapter is to carry out a critical review of thermal stability of different group of PCM especially for low temperature applications. Further, an extensive list of different PCMs which are undergone thermal cyclic tests by different researchers is prepared. This information is towards the selection of reliable PCM for latent heat storage unit.