Evaluation of alternative eutectic salt as heat transfer fluid for solar power tower coupling a supercritical CO₂ Brayton cycle from the viewpoint of system-level analysis

Heat transfer fluids play a crucial role in solar thermal power plants. Several novel salt formulas with higher temperature limit, such as MgCl₂-KCl, have been proposed to serve as heat transfer fluids in a solar power tower system with S-CO₂ cycles. Although the thermophysical properties of these novel salt formulas have been obtained in previous studies, it is necessary to evaluate their performances from the viewpoint of system-level analysis. This paper develops an integrated model for the solar power tower plants with a recompression S-CO₂ Brayton cycle, and the performances of the MgCl₂-KCl salt are evaluated based on the thermodynamic analyses and multi-objective optimizations about the system. The results indicate that: (1) Attributed to the higher heat source temperature, the system with MgCl₂-KCl can achieve a slightly higher system efficiency and a significantly larger specific work than the conventional solar salt; (2) Reheating is not recommended for solar power tower plant with supercritical CO₂ Brayton cycles when high-temperature MgCl₂-KCl serves as heat transfer fluid; (3) The MgCl₂-KCl solar power tower system without reheating enlarges the temperature difference between hot salt and cold salt and thus needs less molten salt for the thermal storage than the solar salt system; (4) The novel MgCl₂-KCl salt has no advantages under the conditions with the operating temperature lower than 565 °C, which yields lower system efficiency and smaller specific work than the conventional solar salt due to the higher pump consumption associated with its higher viscosity.