Long-Term Compatibility Testing of Solar Salt and Solid Particles at High Temperatures: A Thermal and Chemical Characterization

Abstract

Thermal energy storage offers a viable solution to address the global energy problem of balancing the gap between the energy demand and the energy supply. One of the most advanced and mature thermal energy storage technologies in solar power technologies is a Concentrating Solar Power plant with a tower configuration and molten salts as thermal energy storage. Despite their advantages, molten salts also have limitations that include their corrosive nature, solidification at temperatures below 240°C, and high cost. Therefore, alternative thermal energy storage materials, such as solid-state thermal storage using concrete blocks or ceramic particles, are under research. Solid particles have a high thermal energy storage density, comparable to molten salts, and can withstand higher temperatures, making them well-suited for use in Concentrating Solar Power systems. The use of alternative materials for thermal energy storage is an important aspect of the circular economy concept, which aims to extract the maximum value from resources and reduce greenhouse gas emissions. This work aims to test the compatibility of Solar Salt with several alternative materials for use as thermal energy storage media, including silica sand, commercially sintered bauxite, and two different waste materials from the mining and steel industries. The study compares the thermal and chemical properties of these solid-molten salt mixtures with those of Solar Salt and quantifies the formation of nitrites in Solar Salt as a direct measurement of Solar Salt degradation. Additionally, a rheology study was conducted on the Solar Salt samples, revealing slight changes in viscosity attributed to the nitrite content. Although the thermal properties of the materials remained almost identical and natural and inert ceramic materials exhibited good compatibility, Solar Salt in contact with the waste materials exhibited the formation of nitrites, indicating an expected further degradation of the Solar Salt within these compounds.