Experimental and Numerical Study of a Laboratory-Scale Salt-Based Thermal Energy Storage (TES) System Focusing on Latent Heat

Molten salt-based thermal energy storage (TES) systems are widely deployed for large-scale energy storage; however, they currently utilize only sensible heat, limiting their energy density and overall efficiency. The utilization of latent heat in molten salts has the potential to significantly increase the extractable energy, but its practical implementation remains largely unexplored due to challenges related to solidification control, thermal stresses, and structural integrity. Addressing this gap is essential for improving the performance and economic viability of TES systems in renewable energy applications. This work presents the first combined experimental and numerical study demonstrating the controlled utilization of latent heat in a molten salt TES system at laboratory scale. A vertical shell-and-tube heat exchanger integrated into a salt tank was investigated experimentally, enabling controlled melting and solidification of solar salt under realistic operating conditions. High-resolution spatial and temporal measurements of temperature and heat flux were used to establish detailed energy balances and characterize the thermal response during phase change. In addition, a three-dimensional CFD model was developed and validated against the measured temperature histories to predict the corresponding crystallization behaviour and support system optimization and scale-up. The combined experimental–numerical analysis provides new insights into molten salt solidification. While the thermal response was validated experimentally, the spatial distribution and temporal evolution of the solid phase were inferred from the validated CFD simulations. It is shown that complete solidification can increase the thermal energy yield by approximately 10%. However, a configuration-specific recommended operating limit is identified at approximately 60% solidification, corresponding to an energy storage capacity of approximately 1300 kJ/kg (normalized to the mass of solar salt) over a temperature range from above 550 °C down to 200 °C. Beyond this point, heat transfer is significantly reduced due to the formation of insulating solid layers. These findings demonstrate the technical feasibility of controlled latent heat utilization in the investigated molten salt TES configuration and provide new insight into the mechanisms governing frozen-layer growth and discharge performance under high-temperature operating conditions.