One-step synthesis of molten salt nanofluid for thermal energy storage application – a comprehensive analysis on thermophysical property, corrosion behavior, and economic benefit

Abstract

Concentrating solar power (CSP) plants have been widely commercialized for generating electricity from solar energy. Thermal energy storage (TES) systems have been deployed as a key component in CSP plants for resolving the supply-demand gap during daily operation. Nowadays, molten salts are used as both the primary heat transfer fluid (HTF) and as TES medium in many CSP plants. However, molten salts suffer from poor thermo-physical properties, e.g., specific heat capacity is typically less than 2 J/(g•K). In addition, the practical operational performance of CSP plants running on molten salt is limited by the chemical compatibility between the heat transfer fluid and the container. Many efforts have been devoted to developing molten salt material with enhanced thermal properties and reduced corrosivity. As a typical example, doping molten salts with minute quantities of nanoparticles has shown potential as a simple approach for enhancing the thermo-physical property values and mitigating corrosion due to formation of passivation on metal surface from naturally deposited nanoparticles. Nevertheless, mixing nanoparticles in solvents is not cost-effective for industrial applications and often results in agglomeration and precipitation issue. In this study, the feasibility of using molten salt nanofluid as the TES medium has been examined comprehensively based on its thermophysical property, corrosion behavior, and the economic value. Specifically, the specific heat capacity of the solar salt (NaNO3-KNO3 eutectic)-based nanofluid prepared from both two-step and one-step methods were measured at elevated temperatures (~500°C). In addition, the corrosion behavior of SS304 in molten salt nanofluids is investigated experimentally at 565 °C for 432 hours interval. Finally, the techno-economic benefit is assessed based on the energy storage capacity and material costs. The results from the study demonstrated that the one-step synthesis protocol for nanofluids involving generation of nanoparticles in-situ from cheap additives is a viable and cost-effective approach in industrial applications (e.g., CSP) for enhancing the energy storage capacity and power rating as well as for extending the life-cycle of equipment (e.g., heat exchangers).