Thermal energy storage systems are integrated in concentrating solar power plants to improve 8 the flexibility of the electricity generation. Commonly, the liquid storage material Solar Salt, a 9 nitrate salt mixture, is applied to transport and store solar thermal energy. The lifetime and the 10 temperature range of Solar Salt in the storage units are restricted by decomposition reactions 11 of the material. Oxide ion formation is one of the fundamental issues. So far, it has not been 12 proven if oxide ion formation can be prevented by addition of gaseous reaction products to the 13 gas atmosphere. Also, a reliable reaction equation for the oxide ion formation is missing. In 14 the presented experiments, molten salt at 600 and 620 °C is purged with a gas mixture of 15 nitrogen, oxygen, and nitrous gases. Post-analysis of salt samples reveals stabilizing effects, 16 depending on the purge gas compositions. Chemical equilibrium of the oxide ion forming 17 reaction is demonstrated. It is proven that oxide ion formation can be controlled and 18 suppressed. Reaction equations are evaluated and selected in order to quantify the reaction 19 thermodynamics. The results contribute to recommendations for operating conditions and gas 20 handling in storage systems of solar thermal power plants, which finally ensure reliable and 21 constant material properties for extended lifetime and high temperatures.

The scope of our study was to examine the potential of regeneration mechanisms of an aged molten Solar Salt (nitrite, oxide impurity) by utilization of reactive gas species (nitrous gases, oxygen). Initially, aging of Solar Salt (60 wt% NaNO3, 40 wt% KNO3) was mimicked by supplementing the decomposition products, sodium nitrite and sodium peroxide, to the nitrate salt mixture. The impact of different reactive purge gas compositions on the regeneration of Solar Salt was elaborated. Purging the molten salt with a synthetic air (p(O2) = 0.2 atm) gas stream containing NO (200 ppm), the oxide ion concentration was effectively reduced. Increasing the oxygen partial pressure (p(O2) = 0.8 atm, 200 ppm NO) resulted in even lower oxide ion equilibrium concentrations. To our knowledge, this investigation is the first to present evidence of the regeneration of an oxide rich molten Solar Salt, and reveals the huge impact of reactive gases on Solar Salt reaction chemistry.

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MgCl2–KCl–NaCl is a promising thermal energy storage (TES) material and heat transfer fluid (HTF) with high operating temperatures of >700°C for next-generation concentrating solar power (CSP) plants. One major challenge for future implementation of the molten chloride TES/HTF technology arises from the presence of some corrosive impurities, especially MgOHCl, a hydrolysis product of hydrated MgCl2. Even extremely low-concentration MgOHCl (tens of ppm O in weight) can cause unneglectable corrosion of commercial Fe–Cr–Ni alloys, which limits their service time as the structural materials in the molten chloride TES/HTF system. Thus, the chemical analysis and monitoring techniques of MgOHCl at the tens of ppm O level are vital for corrosion control. In this work, a chemical analysis technique based on direct titration and a high-precision automatic titrator was developed for an exact measurement of MgOHCl at the tens of ppm O level. It shows a standard deviation below 5 ppm O and an average error below 7 ppm O when the concentration of MgOHCl is 36 ppm O. Moreover, compared to other methods available in some literature reports, it can exclude the influence of co-existing MgO on the MgOHCl concentration measurement. This chemical analysis technique was used to calibrate the previously developed electrochemical method based on cyclic voltammetry (CV) to achieve reliable in situ monitoring of MgOHCl in the MgCl2–KCl–NaCl molten salt at a concentration as low as the tens of ppm O level. The in situ monitoring technique shows a monitoring limitation of <39 ppm O. The two techniques for MgOHCl measurement developed in this work could be used to develop an in situ corrosion control system to ensure the long service time of the molten chloride TES/HTF system in next-generation CSP plants.