Thermal Energy Storage using Solar Salt at 620 °C: How a reactive gas atmosphere mitigates corrosion of structural materials

Kurzfassung

Molten nitrate salts play a crucial role as storage and heat transfer media in Thermal Energy Storage (TES) systems. In Concentrating Solar Power (CSP) plants molten salt storage allows for generation of dispatchable electricity and in the future, may assist in decarbonizing and prolonging the life-time of coal-fired power plants. The operating temperature range of commonly used Solar Salt, a binary mixture of sodium- and potassium nitrate, is set by the freezing temperature (plus a safety margin) on the lower-, as well as the decomposition temperature on the higher end, which is typically agreed upon as 290-565 °C. At DLR facilities research has been aiming at pushing the high temperature limit of Solar Salt up to, and even above, bulk temperatures of 600 °C. [1, 2] Stabilization of Solar Salt at 650 °C, in terms of a nitrate-nitrite equilibrium has been demonstrated on last year's SolarPaces conference. We demonstrated that the decomposition of Solar Salt depends on the partial pressure of oxygen and/or nitrous gases in the gas phase above the melt, at a certain temperature. In the last months, we have performed long-term exposure experiments to assess the corrosion performance of structural materials in Solar Salt at elevated temperatures. In an attempt to assess the impact of gas phase and temperature, three sets of experiments were performed: 1: Under state-of-the-art conditions of 570 °C in air, 2: At elevated temperatures of 620 °C in air, resembling a case where the salt is not stabilized, and 3: at 620 °C under a purge gas flow of 80 % O2 and 400 ppm NO (rest N2), resembling a case where the salt is chemically stabilized . The metal specimens were analyzed in terms of corrosion rate by descaling and cross-section analysis to evaluate the long-term corrosion performance. Our work constitutes a major contribution to the implementation of Solar Salt storage at higher operating temperatures and supports material and system engineering in the field of TES.