With a view to elevated operating temperatures in thermal energy storage - Reaction chemistry of Solar Salt up to 630°C

Abstract

Sensible heat storage is a cost-efficient and scalable technology for energy storage. The state-of-the-art storage systems in concentrating solar power (CSP) plants use the storage material Solar Salt, which is a nitrate salt mixture. Chemical stability of this salt material is crucial for lifetime reliance, and for development of the storage technology towards higher temperatures. High temperatures enhance the storage capacity, but also promote decomposition reactions. For instance, harmful gases can evolve, and oxide ions are produced, which aggravate corrosion. Up to now, it is unclear how to describe the salt chemistry, and how to quantitatively predict the problematic decomposition products. The experimental method in this study is chosen with regard to the exclusion of mass transport limitations. Thin films of salt are heated to 560–630 °C. The salt composition is analyzed by ion chromatography and acid-base titration. The ratio of nitrite to nitrate ions stabilizes, which indicates chemical equilibrium of the nitrite forming reaction. The oxide content increases continuously over time, and is interpreted in terms of a kinetic rate law. A consistent mathematical description of Solar Salt chemistry at high temperatures (≥560 °C) in contact with air is presented. It includes thermodynamic parameters, in particular the reaction enthalpy of 95 ± 4 kJ⋅mol−1 and entropy of 86 ± 5 J⋅mol−1⋅K−1 for the nitrate-nitrite reaction. The microkinetics of the oxide ion formation are characterized by an activation energy of 42 ± 3 kJ⋅mol−1. The work presented finally contributes to a forecast of material stability at and above 560 °C.