Structural evolution and thermodynamic properties of liquid-vapor transitions of molten salts in high-temperature energy storage

ZZ Rong and Y Ye and F Qiao and JF Wang and J Ding, JOURNAL OF ENERGY STORAGE, 83, 110712 (2024).

DOI: 10.1016/j.est.2024.110712

The operating temperature limit of molten chloride salts mainly depend on thermal stabilities in thermal energy storage applications. In this work, the thermal stability resulted from liquid-vapor transitions of NaCl-KCl-ZnCl2 molten salt were studied. Firstly, mass loss curves in an open testing condition were obtained by thermogravimetric analysis, and a phase change activation energy of 37.44 kJ/mol was calculated successfully using the Arrhenius equation. Secondly, molecular dynamics simulations were applied to explore the liquid-vapor interface and transition properties in the closed system. The structural evolution during phase transition was also revealed, it was found that anions and cations mainly existed in the form of molecule or ion clusters with coordination number of 2- 3 in the vapor phase. The layer thickness, stress tensors, surface tensions, density distributions, ion accumulations, and temperature distributions at liquid-vapor interface were analyzed detailly. And a thermodynamic non-equilibrium phenomenon was found at the interface on a microscale of <5 nm. A mass loss of 0.25 % was observed at 1000 degrees C when V-liquid = V-vapor in the closed system, indicating that the upper limit of operating temperature can be greatly improved to 1000 degrees C for NaCl-KCl-ZnCl2 by using a well- sealed storage tank and pipeline.

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