Defined purge gas composition stabilizes molten nitrate salt - Experimental prove and thermodynamic calculations

Thermal energy storage systems are integrated in concentrating solar power plants to improve the flexibility of the electricity generation. Commonly, the liquid storage material Solar Salt, a nitrate salt mixture, is applied to transport and store solar thermal energy. The lifetime and the temperature range of Solar Salt in the storage units are restricted by decomposition reactions of the material. Oxide ion formation is one of the fundamental issues. So far, it has not been proven if oxide ion formation can be prevented by addition of gaseous reaction products to the gas atmosphere. Also, a reliable reaction equation for the oxide ion formation is missing. In the presented experiments, molten salt at 600 and 620 °C is purged with a gas mixture of nitrogen, oxygen, and nitrous gases. Post-analysis of salt samples reveals stabilizing effects, depending on the purge gas compositions. Chemical equilibrium of the oxide ion forming reaction is demonstrated. It is proven that oxide ion formation can be controlled and suppressed. Reaction equations are evaluated and selected in order to quantify the reaction thermodynamics. The results contribute to recommendations for operating conditions and gas handling in storage systems of solar thermal power plants, which finally ensure reliable and constant material properties for extended lifetime and high temperatures