Thermal performance analysis and optimization of a cascaded packed bed cool thermal energy storage unit using multiple phase change materials

This paper proposes a cascaded packed bed cool thermal energy storage (CTES) unit using multiple phase change materials (PCM). In terms of the solidification processes, the basic heat transfer characteristics were analyzed based on an experimentally validated simulation model. Thermal performances, including cold charging rate, the quantity of cold and exergy charged, as well as the exergy efficiency of the cascaded CTES unit were compared with those of a single-stage CTES unit. In order to further achieve better thermal performance, the materials, stages and thickness of each stage were optimized. According to the results, the 24-stage CTES unit with a phase change temperature difference between the highest and lowest stage (denoted as Td) of 6 °C has the best thermal performance, because it shows a 15.1% reduction in charging time compared with a single-stage unit, while its quality and quantity of cold charged remains almost equivalent to the single-stage unit. Moreover, 3–5 stages are recommended for an evenly distributed cascaded CTES as their thermal performances are quite close to the optimal 24-stage unit.