Novel high specific heat capacity ternary nitrate/nitrite eutectic salt for solar thermal energy storage

Abstract

A novel ternary eutectic salt KNO3-NaNO2-KNO2 (KNK) was designed and prepared for thermal energy storage (TES) in a concentrating solar power system (CSP). The thermo-physical properties of KNK such as melting point, decomposition temperature, fusion enthalpy, density, viscosity, thermal conductivity and specific heat capacity were determined respectively. Furthermore the thermal energy storage capacity was evaluated and discussed. Then a comparison of the thermo-physical properties between commercial Hitec and KNK was carried out. KNK showed smaller density, lower viscosity, slightly smaller thermal conductivity, much higher specific heat capacity and bigger thermal energy storage capacity. Raman spectrum displayed that the N-O of NaNO2 frequency peak at about 820 cm−1 in KNK had more significant shift, which could be attributed to a stronger interaction between Na+ and NO2−. Scanning electron microscope (SEM) showed K and Na elements of Hitec were uniformly distributed, while K and Na elements of KNK were in complementary distribution which could be caused by stronger repulsion between Na+ and K+. It was concluded that the stronger interaction and repulsion were correlated to the higher specific heat capacity. Molecular dynamics (MD) simulation was performed to explore the different macroscopic properties of two molten salts from the perspective at molecular level. The characterizations of radial distribution functions (RDF), coordination number and angular distribution functions (ADF) suggested that the interaction between Na+ and NO2− is stronger and that the arrangement of ion clusters was looser in KNK. The results were in good agreement with the analysis of Raman spectrum and SEM.