A simplified method for exergy assessment of thermal energy storage tanks: Comparative performance of tanks containing a phase-change material and water

The integration of thermal energy storage (TES) units into thermal systems can be strategic to increase the renewable energy contribution and overall system performance as a result of the greater operational flexibility provided. On the other hand, the integration of a TES unit also involves an intermediary step between the heat source and sink that adds some irreversibility to the system operation for a number of reasons, such as the increased heat losses to surroundings and the pressure drop, the influence of inner heat exchangers (thus, involving a finite temperature difference), or associated fluid mixing. To properly model these complex phenomena, designers should incorporate a Second-Law approach in their designs to assess the exergy losses (degraded useful work) that a TES unit integration entails. This work assesses the entropy generated by four different TES units: three different water tanks (most typical configurations) and an experimentally validated TES tank containing a phase-change material (PCM), when they undergo a complete heat storage and recovery cycle, using a simplified and computationally efficient method in the TRNSYS simulation program to quantify the entropies generated. The TRNSYS simulation results revealed that the greater temperature difference required between the fluid and the PCM, due to its low thermal conductivity, was the main reason for the entropy generation of the PCM tank, while heat losses to the surroundings represented no less than one-half of the total entropy generated by all water tank configurations.