Stabilization of Solar Salt at 650 °C – Thermodynamics and practical implications for thermal energy storage systems

Abstract

Thermal Energy Storage (TES) based on molten salts is thought to play a major role for the transition from fossil fuels to renewable energy carriers in the future. Solar Salt, a mixture of NaNO3-KNO3 is currently the state-of-the-art heat transfer and storage material in Concentrating Solar Power (CSP) plants which produce electricity from a Rankine cycle with steam temperatures up to 550 °C. To allow a technology transfer and adapt Solar Salt based TES systems to modern, high temperature Rankine cycles (e.g. Tsteam > 600 °C), the thermal stability of Solar Salt needs to be increased well above 615 °C. At these temperatures, the formation of nitrites, which depends on the oxygen partial pressure above the melt, needs to be suppressed effectively to prevent further decomposition into corrosive oxide ions. In this work, the thermodynamics of the nitrite-forming reaction at different oxygen partial pressure are explored in a temperature range up to 650 °C from isothermal experiments in the 100 g scale and limitations of the ideal description are revealed. The measured apparent oxide ion formation rates at 100 g scale were below previous findings. The activation energy found was 60 ± 15 kJ/mol and the preexponential factor 1*10^5 ± 0.00005 s-1. The effect of closing the storage system in terms of gas and salt phase at 645 °C are also explored to understand if and how pressure formation and oxygen release correlate. The results of this work finally contribute to an understanding of the decomposition reactions of Solar Salt at previously untouched temperatures.