Review on heat transfer mechanisms and characteristics in encapsulated PCMs

Latent heat storage (LHS) is a particularly promising technique compared with the conventional sensible heat storage (SHS) as it provides a high-energy storage density with a small volume. However, there are difficulties in practical engineering applications of LHS due to the heat releasing/absorbing, which involves phase transition and moving boundary problems and the unacceptable low thermal conductivity of the phase-change material (PCM). Furthermore, the encapsulation would affect the heat transfer characteristics of PCM significantly, depending on the parameters of various encapsulations and boundary conditions. Hence, this review analyzes heat transfer mechanisms during the phase-change process and numerical analysis for heat transfer in macroencapsulated PCMs according to the shape of containment. The effective heat capacity method and the enthalpy method, two of the most widely used numerical approaches for phase-change problems, are presented in detail. Besides numerical models for different PCM containment such as spherical, rectangular, and cylindrical containment models, PCM-based heat-sink models are reviewed, including several heat transfer enchantment technologies: finned structure and porous matrix. Finally, the challenges in numerical modeling and designing an LHS unit are also summarized in this article.