Performance investigation of single-tank thermocline storage systems for CSP plants

•Numerical analysis of single tank thermocline system with different HTFs.•Effect of porosity on effectiveness of the tank with different HTFs.•Study of charging and discharging characteristics on the performance of the tank.•Study of operation and design parameters of the tank. To address the intermittency in solar energy system due to unforeseen weather conditions and to improve dispachability, the requirement of Thermal Energy Storage (TES) system is becoming increasingly inevitable. A single tank packed bed thermocline TES system has potential to provide an effective solution. A comprehensive one-dimensional non-thermal equilibrium model is considered and is solved using method of characteristics to investigate energy storage in a single tank packed bed thermocline storage system. Thermal characteristics including temperature profiles and discharge effectiveness with different commercially available Heat Transfer Fluids (HTFs) are explored and analyzed. A tank size of 12.5m in height and 8m in diameter is considered with a capacity of 40MWht for the present study. The effect of porosity on discharge effectiveness of the storage tank with different HTFs is investigated with porosity, ε varied from 0.1 to 0.7. It is found that the discharge effectiveness consistently drops when ε value increases from 0.1 to 0.7 for all the HTFs. Two-dimensional numerical simulation is also carried out to explore the transient temperature distribution inside the tank for different HTFs with a porosity of 0.2. It is observed that as the operation time of the tank increases, filler material eventually loses heat to adjacent fluid and attains equilibrium. The thermocline moves at a faster rate in Solar Salt than the HITEC due to its higher viscosity.