Asymmetrical melting and solidification processes of phase change material and the challenges for thermal energy storage systems

The melting and solidification processes of the organic phase change material – lauric acid exposed to air were experi-mentally studied to investigate the heat exchange and its effect on the heat transfer behaviour inside a shell as well as its phase-change characteristics. Lauric acid was placed in spherical shells made of polyvinyl chloride with diameters of 44, 63, and 74 mm. This study was based on analyses of the surface temperature and vertical temperature distribution data inside the shells. We found that the phase change characteristics were strongly related to the dominant heat transfer mech-anism. In this case, melting was dominated by convection, whereas solidification was dominated by conduction. The convection intensity increased as the shell diameter increased. Further analysis revealed the melting and solidification periods. In contrast to latent heat release accompanying solidification, latent heat absorption accompanied by melting does not occur at a constant temperature, although it has a smaller temperature gradient than does sensible heat absorption. Based on the asymmetry between the melting and solidification processes, we discuss various possible strategies by which to control the charging and discharging of the phase change material by restraining the heat transfer rate to optimise its performance as a latent thermal energy storage material.