Thermal Analysis of Novel Nitrate-Chloride Salt Mixtures for Concentrated Solar Power Applications

Abstract

Nitrate salts have better thermal stability, low vapour pressure and non-toxic nature compared to synthetic heat transfer oil like Therminol VP-1®. Solar salt, a mixture of 60% NaNO3-40% KNO3 by weight, is finding increased application as a Heat Transfer Fluid (HTF) and Thermal Energy Storage (TES) material in Concentrated Solar Power (CSP) plant. In this research work, two novel salt mixtures are prepared and tested for their melting point, short and long duration thermal stability. The formulation 1 is a ternary salt comprising of 44% KNO3-32% Ca(NO3)2-24% NaNO3, referred to as Base Salt. Formulation 2 is a quinary mixture of 90% Base salt-5% NaCl-5% KCl, referred to as Base-Chloride. Solar salt, a third formulation, is also tested and compared alongside the novel mixtures. The experiments conclude that the melting point of solar salt is 223°C, Base salt is 135.8°C and Base-Chloride is 142.2°C. When heated at 5°C/min, the decomposition (3% mass loss) of solar salt is observed at 631°C, Base salt at 585°C and Base-Chloride at 589°C. Considering long duration stability, all the three mixtures remain stable till 400°C, when heated at constant temperature for 24 hours. The novel mixtures have significantly lower melting points, thus providing wider operating temperature range compared to solar salt for both heat transfer and energy storage application. Moreover, it is observed that addition of chloride salt as an impurity to a nitrate mixture does not change its operating range significantly.