Analysis of charging/discharging characteristics and thermocline thickness of the packed bed thermal energy storage

During the entire charging/discharging cycle, variations in the thermocline thickness affect the performance of the thermal energy storage. Therefore, this study numerically investigates the effects of capsule diameter and phase change temperature on the thermocline thickness of a single-layer packed bed. Moreover, this study compares the variation in the thermocline thickness of cascade and single-layer packed beds to reveal the direct reason for the improved performance of the cascade packed bed. Finally, different proportions of each layer in the cascade packed bed are studied, and the relationships between the thermocline thickness, capacity ratio, charging/discharging heat power, and exergy efficiency are analyzed. The results indicate that increasing the capsule diameter increases the thermocline thickness and decreases the cycle exergy efficiency. The single-layer configuration cannot maintain high thermal performance during the charging and discharging processes. The cascade packed bed batter matches the temperature of the heat transfer fluid and exhibits high thermal performance during the entire cycle. Across configurations with different layer proportions, although the average thermocline thickness remains nearly constant, significant differences are observed in the time-dependent variations. This study provides a reference for improving the performance of packed-bed thermal energy storage systems.