Enhancement of thermal performance of latent thermal energy storage systems using periodically reciprocating flows

From the perspective of increasing heat transfer rates in latent thermal energy storage systems (LTESS), less attention has been paid to the thermodynamic characteristics at the physical boundary between phase change material (PCM) and heat transfer fluid (HTF). The current numerical study analyzes a simple method of enhancing the thermal performance of a shell and tube LTESS, wherein the flow direction of HTF is periodically reversed. The periodically reciprocating flow creates higher temperature gradients across the HTF-PCM boundary which leads to higher and more uniform heat transfer rates and thereby lower average temperatures, lower maximum temperatures, and lower temperature differences across the PCM than unidirectional flow. Detailed parametric studies are performed to understand the effects of flow and heat transfer parameters of HTF, thermophysical properties of PCM, and design of PCM container on the relative thermal efficiency of the LTESS with reciprocating flow to unidirectional flow. For certain conditions, the time required for melting could be reduced by over 11.5% by using periodically reciprocating HTF flow instead of unidirectional HTF flow. Increasing the heat transfer rate by increasing the temperature or velocity of the HTF, or enhancing thermal conductivity of PCM, increases the relative effects of reciprocating flow compared to unidirectional flow.