Thermodynamic study on charge-discharge processes and cycle of cascaded latent heat storage in heating system

Abstract

Thermodynamic analysis can guide the application of the cascaded latent heat storage system (CLHSS) for space heating. However, in previous studies, the CLHSS charge–discharge process and the cycle are not distinguished, and the applicability of the corresponding models is not analyzed. In addition, the technical advantages of CLHSS in heat utilization are still unclear. This paper solved the above three problems to provide theoretical guidance of the CLHSS integrated into heating systems. First, the CLHSS charge–discharge cycle is the special charge–discharge processes in which the initial and final states of each PCM are consistent. The CLHSS charge–discharge processes and cycle models are established and optimized. Second, the optimization results of charge–discharge processes model cannot ensure the same initial and final states of each PCM, which leads to unstable operation and performance degradation of the CLHSS. Thus, the CLHSS charge–discharge cycle model is recommended. Third, when the total number of heat transfer unit (NTU) of CLHSS is less than that of single-stage latent heat storage system (SLHSS), the thermal performance of CLHSS may be worse than that of SLHSS. Besides, the use of CLHSS instead of SLHSS is suitable for scenes with large temperature differences between heating equipment and heating terminals. Finally, parameters effects on optimization results are analyzed. From the perspective of improving entransy efficiency, it is recommended that the total NTU of CLHSS during charge process and discharge process are between 4.5 and 6, and as close as possible.