Analysis of Heat Transfer Characteristics in a Latent Heat Storage Module Using Circular-Finned Tubes

Latent heat thermal energy storage (LHTES) using inorganic salt hydrates is a promising technology for buffering renewable energy fluctuations; however, phase-dependent heat transfer remains insufficiently understood for design optimization. In this study, a shell-and-tube storage module with a circular-finned tube was constructed and filled with 13.17 kg of barium hydroxide octahydrate (BHO). Discharge tests were conducted with heat transfer fluid (HTF) inlet temperatures ranging from 20 °C to 50 °C and flow rates of 10–25 L/min, while charging was performed at 90 °C. The overall heat transfer coefficient (Uo) was derived using the logarithmic mean temperature difference method, the inside coefficient (hi) was calculated by the Petukhov correlation, and the outside coefficient (ho) was obtained via thermal-resistance network. Results show that the average discharge energy was approximately 1.027 kWh (except 0.859 kWh at 50 °C inlet), with a mean utilization efficiency of 79.25%. The Uo was consistently highest in the liquid phase, followed by the latent and solid phases, with ranges of 0.257–0.863, 0.025–0.072, and 0.015–0.044 kW/m2·°C, respectively. Sensitivity analysis revealed that the HTF flow rate strongly influenced Uo across all phases, whereas inlet temperature played only a minor role. The outside coefficient ho was 0.033–0.162 kW/m2·°C in the latent regime and 0.018–0.064 kW/m2·°C in the solid regime, with a notable peak around Reynolds number 1.3 × 104 in the latent phase. These findings provide detailed phase-resolved Uo and ho data for inorganic salt hydrate storage and highlight design insights such as the diminishing returns of flow rate increase beyond a threshold, offering valuable guidelines for sizing and operation of LHTES in Power-to-Heat applications.