Computational thermo-fluid dynamics study on the optimization of a thermocline energy storage system prototype

Single-tank thermocline thermal energy storage (TES) systems using molten salt offer significant cost advantages for concentrating solar power plants, but maintaining sharp thermal stratification remains challenging due to natural convection-induced mixing. This thesis investigates a novel thermocline TES configuration comprising a cylindrical tank filled with HITEC XL molten salt and an internal recirculation channel equipped with electrical heating resistors, representing a hybrid solar-electrical storage approach. The study employs computational fluid-dynamics (CFD) analysis using STAR-CCM+ to evaluate thermocline formation under natural and forced convection conditions. A multi-stage methodology is developed: first, a three-dimensional model characterizes the heating rod bundle across various operating conditions with pressure drop predictions and outlet temperatures values. A simplified two-dimensional axisymmetric model then simulates the complete tank charging transient over 3600 seconds, comparing baseline natural convection with enhanced forced convection using a fan interface boundary condition. The results are consequently compared to analyze if the forced convection improves the stratification of the fluid and the total efficiency of the system since the controlled circulation stabilizes buoyancy-driven flow, confining hot fluid injection and preserving sharp temperature gradients, essential for efficient storage performance. This research validates computationally efficient CFD modeling strategies combining detailed geometry with porous media approximations and demonstrates that controlled forced convection represents an effective approach for optimizing thermocline TES systems, improving both storage capacity and operational flexibility for renewable energy applications.