Computational Fluid Dynamics Analysis of a Thermocline Thermal Storage Unit for Solar Thermal Applications

Solar power is an important instrument to contribute against world energy demand. The intermittency of solar energy is an issue, but it is possible to constrain intermittency either with direct electricity storage for photovoltaic systems or with thermal energy storage for concentrating solar systems. In this study, a single-tank thermocline storage system filled with solid materials was considered. A computational fluid dynamics analysis of a discharge process was simulated. Streamlines and temperature distributions in the tank due to different porosity, filler material sphericity, and fluid type were investigated during discharge process. The simulation results showed that when the fluid has high volumetric heat capacity, the initial energy stored in the tank increases. In addition, if volumetric heat capacity of the fluid is lower than that for the filler material, initial energy stored in the tank is mainly stored in the filler material which is the optimum operation case. Furthermore, for fluids with high volumetric heat capacity, when the porosity of the tank increases, the amount of energy retained in the tank during discharge cycle increases. On the other hand, the situation is reverse if fluid has lower volumetric heat capacity value. Temperature profiles and streamlines of the three hours of discharge showed that for low values of porosity, higher sphericity value prevents mixing of hot and cold fluid and results in better discharge performance. In addition, sectioning of the tank interior with the walls improved tank performance around 30% and diminished the effect of porosity and sphericity.