Thermal evaluation of molten salts for solar thermal energy storage

Abstract

Molten salts are known for having good heat transfe r properties to be used in thermal storage. This application has become important in the development of thermal technologie s for solar energy concentration. The operating principle of th ese technologies is based on the concentration of sun r ays by mirrors to achieve high temperatures and aims to operate tu rbines coupled with electric generators to produce electri city. Therefore, alkali nitrates and sodium nitrite are g ood candidates due to the low melting point and excellent heat cap acity. These properties produce a broad molten salt operating ra nge and enable effective thermal storage in solar power pla nts. But one of the critical parameters is the degradation because the cyclic process of heating / cooling. It is important to st udy the thermal properties in order to determine the operating cond itions and lifetime during processing. The purpose of this study is to evaluate the meltin g point and thermal stability of conventional salt systems, suc h as the Solar Salt (60% NaNO 3- 40% -KNO 3, Tm = 220 °C) and HITEC (7% NaNO 3- 53% KNO 3- 40%NaNO 2, Tm = 142 °C), and a novel eutectic mixture. The physical and chemical propert ies of the studied salts were evaluated by Differential Scanni ng Calorimetry (DSC), Thermogravimetric Analysis (TGA) and morphology was observed in SEM images. In order to compare experimental results with theoretical calculations, the eutectic temperature and heat capacity were calculated with Factsage® thermodynamic software. Melting point was found to be lower than that of co nventional salts due to the Lithium Nitrate addition in the sy stems.