Thermal evaluation of molten salts for solar thermal energy storage

Abstract

Molten salts are known for having good heat transfer properties to be used in thermal storage. This application has become important in the development of thermal technologies for solar energy concentration. The operating principle of these technologies is based on the concentration of sun rays by mirrors to achieve high temperatures and aims to operate turbines coupled with electric generators to produce electricity. Therefore, alkali nitrates and sodium nitrite are good candidates due to the low melting point and excellent heat capacity. These properties produce a broad molten salt operating range and enable effective thermal storage in solar power plants. But one of the critical parameters is the degradation because the cyclic process of heating / cooling. It is important to study the thermal properties in order to determine the operating conditions and lifetime during processing. The purpose of this study is to evaluate the melting point and thermal stability of conventional salt systems, such as the Solar Salt (60% NaNO3- 40% -KNO3, Tm = 220 °C) and HITEC (7% NaNO3- 53% KNO3- 40%NaNO2, Tm = 142 °C), and a novel eutectic mixture. The physical and chemical properties of the studied salts were evaluated by Differential Scanning 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 conventional salts due to the Lithium Nitrate addition in the systems.