Thermal performance of the packed bed thermal energy storage system with encapsulated phase change material

The thermal performance of the packed bed thermal energy storage (PBTES) system used in waste heat recovery and utilization is studied experimentally and theoretically. The experiments are conducted to test the thermal performances of the PBTES system at various charging temperatures and mass flow rates. In addition, a one-dimensional concentric dispersion model considering heat loss is established and validated by the experimental results. Furthermore, the non-dimensionalized model is developed, and the heat transfer processes inside the PBTES system at various Pe, Nuloss,vol and Ste numbers are simulated. The results show that the time durations of the charging and discharging processes are significantly decreased with the increases of the Pe and Ste numbers, e.g., reduced by 44.7% of charging duration as the Pe number increases from 188 to 338, and the time durations are indistinctively affected by the Nuloss,vol number, i.e., increased by 7.1% of charging duration as the Nuloss,vol number increases from 19.9 to 46.5. The thermal efficiency is significantly increased from 65.6% to 80.1% as the Pe number increases from 188 to 338, and from 59.2% to 77.6% as the Ste number increases from 0.479 to 0.799. The novel dimensionless parameter Nuloss,vol number shows great effect on the thermal performance and the thermal efficiency is significantly decreased from 91.3% to 72.6% when the Nuloss,vol number increases from 19.9 to 46.5. Besides, a new dimensionless parameter Nr¯ number which is a monotonic function of instantaneous thermal efficiency is proposed and can be used for estimating the heat transfer performance of the PBTES system.