Charging behavior of packed-bed thermal energy storage systems in medium and low temperature applications

Thermal energy storage (TES) technology is crucial for addressing the intermittent and cyclical drawbacks of renewable energy. Packed bed thermal energy storage (PBTES) using phase change material (PCM) capsules is an important form of TES. However, the thermal storage performance of PBTES systems at medium and low temperatures, especially the PCM microcapsule-filled system, is still unclear. In this paper, we use the finite element method to study the charging process and thermal storage performance of multi-stage and cascaded PBTES systems with different structures. Parameters such as heat storage capacity, heat storage rate, and exergy efficiency are evaluated. For single-layer PBTES systems, optimal comprehensive heat storage performance is observed when the capsule diameter is between 2 and 5 mm. The PBTES system with d = 2 mm owns the highest exergy efficiency, reaching 80%. The system's behavior is influenced by the volume flow rate, inlet temperature of heat transfer fluid (HTF), and filling order of PCM capsules. The two-stage structure significantly improves the performance of macrocapsule-filled PBTES systems. Their thermal storage rate can be increased up to 27% compared to the corresponding single-layer system, and their heat storage efficiency is all over 94%. However, the multi-stage structure has little effect on microcapsule-filled PBTES systems. Cascaded systems also enhance the overall thermal storage performance of PBTES systems at medium and low temperatures. Economical and efficient options include systems with different PCM cascades or one rock layer sandwiched by two identical PCM capsule layers.