Experimental study of thermophysical properties and thermal stability of quaternary nitrate molten salts for thermal energy storage

Abstract

Renewable energy has been rapidly developing because of the increasing severity of pollution and energy crises. In particular, solar thermal power generation technology has been attracting considerable attention globally due to its large scale, low unit cost, and environment-friendliness. Thus, it has been quickly progressing. The intermittence and instability of solar energy can be solved by combination of solar thermal power with heat storage technology. Mixed molten salts stand out as heat transfer and storage materials due to their wide temperature range, low vapor pressure, high heat capacity, low viscosity, and economical environmental protection. In the present study, a new kind of quaternary nitrate molten salt was presented. The salt was Hitec salt with Ca(NO3)2 additive and has a low melting point, high decomposition point, and low cost. The thermophysical properties of the proposed salt, such as melting point, decomposition point, crystallization point, specific heat, density, thermal conductivity, and thermal stability, were measured. Results showed that the melting, crystallization, and decomposition points of the Hitec salt with Ca(NO3)2 additive were 83.1 °C, 163.1 °C, and 628.5 °C, respectively. The proper working temperature range of the new salt was 200–565 °C, which was wider than those of Hitec salt (200–450 °C) and solar salt (290–565 °C). The average specific heat and thermal conductivity of the Hitec salt with Ca(NO3)2 additive were approximately 1.52 J/(g K) and 0.655 W/(m K), respectively, which showed better heat storage and heat transfer performance than did Hitec salt (1.40 J/(g K) specific heat, 0.350 W/(m K) thermal conductivity) and solar salt (1.50 J/(g K) specific heat, 0.520 W/(m K) thermal conductivity). An experimental study was conducted on the stability of mixed molten salts at 565 °C for 1200 h and 120 times for quenching/heating at 200–500 °C, and the Hitec salt with Ca(NO3)2 additive exhibited good thermal stability.