Numerical and Experimental study on the performance of a new two-layered high-temperature packed-bed thermal energy storage system with changed-diameter macro-encapsulation capsule

In this paper, a new two-layered high-temperature packed-bed thermal energy storage system (PBTES) with changed-diameter macro-encapsulation capsule has been established to improve the thermal performance of PBTES. The effects of different diameters on thermal energy storage (TES) charging rate and TES density are numerically analyzed primarily. Second, the optimized hierarchical structure of PBTES is further constructed based on the proposed optimal comprehensive index called TES rate density. The optimal diameter pair of capsules exists when one of the diameters is fixed to obtain the optimized TES charging rate and TES rate density. Moreover, the thermal performance of diameter-changed two-layered PBTES is obtained compared to that of single-layered PBTES system. Finally, thermal performance of two-layered PBTES is experimentally examined. The influence of different inlet temperature and mass flow on the thermal performance of the system are investigated. The results are concluded as follows. (1) The phase change material of the lower-layer is melted faster by adopting the two-layered PBTES system. The improvement of heat transfer performance and enhancement of uneven heat transfer temperature can further increase the thermal performance of the system. (2) When the air inlet temperature increases from 425 °C to 465 °C, the TES charging rate can be increased by 60.5% and the heat storage efficiency rises from 84.8% to 91.1%. The TES charging rate is raised by 23.5% when the mass flow rate increases from 180 kg·h−1 to 260 kg·h−1 and the heat storage efficiency is decreased slightly. (3) With the same inlet temperature and mass flow, the TES charging rate of diameter-changed two-layered PBTES is better than that of single-layered PBTES. The maximum TES charging rate of the former system can be increased by 12.4% compared to that of the latter system, and TES rate density of two-layered PBTES can be improved by 13% as well. In summary, the new diameter-changed two-layered PBTES is an optimized system of heat storage. The study provides a design of such the PBTES for a first step implementation of the technology and the improvement of thermal performance optimization.