Experimental study of the effect of cyclic and stepped temperature and flow rate change operations on the solid particle/thermal oil fin-tube heat exchanger

This study delves into the thermal performance of various cycles and operational strategies within the magnetite/heat transfer oil (HTO) tube-fin thermal energy storage (TES) system. In the initial cycle, longer charging times and higher charging energy/exergy rates are evident due to lower average temperatures. However, this performance gap diminishes in subsequent cycles. As cycles progress, temperature evolution within the TES unit (except bottom temperature) becomes more predictable, yet a thermal performance gap persists. Although these enhancements are irregular, energy and exergy efficiency improve each cycle. Specifically, energy efficiency increases by 15.0 % and 18.0 %, and exergy efficiency increases by 11.4 % and 13.4 % for Cycles 2 and 3, respectively, relative to Cycle 1. Adopting the stepped temperature change strategy results in extended charging/discharging times but yields a more gradual temperature variation and a stable energy/exergy rate. While this method amplifies energy dissipation and irreversible losses during charging, it minimizes entropy increase during discharging, enhancing exergy efficiency. The energy/exergy efficiencies for Cases 1, 2, and 3 are 83.7/62.8 %, 82.5/71.1 %, and 80.9/67.9 %, respectively. Similarly, implementing the stepped flow rate change strategy, particularly with a lower flow rate (Case 4), leads to slightly prolonged charging and discharging but results in smoother temperature changes and more stable energy/exergy rates. The energy/exergy efficiencies for Cases 4 and 5 are 85.7/65.2 % and 83.7/63.1 %, respectively. Although there exists a slight gap in the energy and exergy efficiencies among the three cases, the stepped flow rate change strategy ensures stable energy transfer and high efficiency. Overall, strategic thermal management is crucial for TES systems, and the findings can potentially advance the development of energy storage solutions for dynamic and flexible energy systems.