Dynamic characteristics of packed bed latent heat thermal storage tank to smooth solar energy fluctuations

Thermal energy storage utilizing phase change materials (PCMs) is a crucial technology for solar energy applications. However, there is a lack of knowledge regarding investigating the influence of the solar flux intensity fluctuations during the day on the performance of water/PCM-packed bed thermocline storage systems, which typically necessitate a stable heat load output. In this context, the study examines two distinct flux intensities in separate regions of Egypt: Qena and Sinai. Subsequently, the research delves into the effects of the melting temperature of PCMs and the discharge flow rate on the performance under actual solar conditions, employing the flux data from Sinai. A comprehensive two-dimensional unsteady mathematical model is developed to effectively couple the transient temperatures of water and PCM spherical capsules within the bed and perform energy and exergy analyses. The results indicated that the system performance using Sinai's flux surpasses Qena's flux in terms of both charging duration and discharging capacity. Besides, Sinai's flux facilitates a more rapid charging of the storage tank than Qena's, demonstrating a 28 % enhancement in the charging rate and a 4 % improvement in charging power. The PCM with a lower melting temperature demonstrates the highest charging efficiency. In addition, the tank system employing RT55 has an exergy efficiency of 73.3 %, while the tank system utilizing RT65 attains an efficiency of 74.77 %. The exergy efficiency of the packed bed tank and the whole system demonstrates an inverse correlation with the discharge flow rate. A threefold increase in the discharge flow rate, from 0.3 to 0.9 m³/hr, shows a 5.3 % reduction in exergy efficiency for the tank and the whole system.