New frontiers in thermal energy storage: An experimental analysis of thermophysical properties and thermal stability of a novel ternary chloride molten salt

Abstract

It has been always a daunting task to develop thermal systems with a wide range of thermochemical properties and high stability. It presents a significant challenge, especially when aiming to utilize them for heat storage and transfer applications. In this regard, the thermal characteristics and thermal stability behaviour of molten salt mixture as a heat transfer fluid (HTF) at high temperatures were explored. A novel ternary eutectic salt mixture (base mixture) made of cuprous chloride (CuCl), potassium chloride (KCl) and sodium chloride (NaCl) was investigated as HTF for thermal energy storage (TES) system with a range up to 653 °C in a concentrated solar power (CSP) plant. To extend the temperature span and stability, the effect of an additive was investigated. Different compositions were studied in detail concerning the properties and stability regarding the additive. The molten salt with 7 % CaCl2 additive improved thermal stability and operating temperature from 653 °C to 700 °C. The transport characteristics and thermal stability of the eutectic mixture with and without the additive were measured by high-temperature thermal analyzers. The results indicated that the melting temperature decreased and the average specific heat capacity considerably increased with the addition of CaCl2 additive compared to the ternary chloride base mixture. As the temperature increased, the eutectic salt mixture exhibited a reduction in viscosity and density. The viscosity and density of the salt mixture with and without the additive were almost uniform at 700 °C. The average thermal conductivity of the ternary salt mixture with and without additives was 0.72 W/(m°C) which is much more than that of Solar and Hitec salts. In a 100 h experiment of long-period isothermal stability, the eutectic salt mixture showed less than 4 % weight loss at 700 °C. It was defined as the highest operating temperature at which the eutectic salt mixture remained stable. The stability and recyclability of the salt mixture were confirmed through the short-period stability test. The effect of the CaCl2 additive on thermal stability of base salt mixture was analyzed by X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) techniques. According to the results, this novel eutectic salt mixture had outstanding thermal stability up to 700 °C and could be employed as a viable TES material in CSP plants.