Analyzing competing effects between heat transfer area and natural convection to enhance heat transfer in latent heat storage

Phase change materials (PCMs) are effective means of storing thermal energy and to balance temporal supply-demand mismatch. To enhance heat transfer within the PCM, internal fins are often employed to increase the heat exchange area, but they usually suppress simultaneously natural convection reducing the performance of the PCM storage. To better understand these two conflicting effects and to find a better trade-off between the different heat transfer means, a comprehensive numerical study of PCM storage with different fin shapes (straight and sinusoidal wavy fins) and surface areas, but with equivalent volume fraction, was accomplished. The results show that increasing the surface area of sinusoidal fins improves only marginally the heat transfer, mainly because the flow velocity of natural convection along the normal direction of the solid interface is inhibited. The velocity normal to the solid interface was found to be the most critical factors to the overall heat transfer efficiency of the system and to the time needed to complete the phase change in the PCM container. The study clearly shows that to enhance the heat transfer in PCM, the effects from natural convection (direction and strength) play a more important role than the heat exchange surface area. To improve the thermal performance and efficiency of PCM storage systems both factors need to be combined in an optimal way.