Cooling process of a novel low melting point ternary mixture for thermal energy storage studied experimental and numerically in a pilot-scale tank

Abstract

The most used thermal energy storage material in concentrated solar power plants is a binary salt of 60%NaNO3 + 40%KNO3. However, the relatively high melting point of this mixture (222 °C) represents a significant risk of local solidification in the operation of these plants during stand-by periods. This research proposes the use of a novel mixture composed by 30%LiNO3 + 57%KNO3 + 13%NaNO3 with a low melting point of 127 °C. The transient cooling process of a molten salt tank working with a considerable amount of 616 kg of the proposed ternary mixture was studied experimentally in a pilot-scale tank for the first time. A computational fluid dynamics (CFD) model was then developed and validated with the experimental data with a maximum temperature relative error below 4 %. Simulation of the cooling process to obtain crystallization times for the proposed ternary salt, solar salt and Hitec molten salts were performed, and it was obtained a higher crystallization time for the proposed ternary salt 125.4 h in comparison with solar salt (56.4 h) and Hitec (124.2 h). Finally, using the model developed, new simulations were performed for a scaled-up model of a state-of-the-art cold tank of a two-tank molten salt CSP thermal storage system. Results revealed that crystallization risk is minimized with the use of the proposed ternary mixture for both considered scenarios of minimum and medium height levels, with crystallization times of 24 days and 7 h and 277 days and 5 h respectively.