Demonstration of the stabilization of solar salt at 620 C with a semi-closed configuration in a 100 kg-scale

Abstract

Among the variety of energy storage techniques thermal energy storage (TES), based on molten salts, is already in use for the storage of heat in a gigawatt hour scale. At the time of writing virtually all TES in CSP utilize Solar Salt (60 wt-% NaNO3 and 40 wt-% KNO3) due to its competitively low price, low vapor pressure and non-toxicity. On the downside, the operating temperature is limited to 560 °C based on its thermal stability. However, increasing the operating temperature while maintaining thermal stability of the salt using techniques that are realizable in industrial scale remains one of the main challenges. Up to now this could only be achieved in a small scale by flushing with synthetic purge gas or sealing and pressurizing the system, maintaining the necessary gas atmosphere and shifting the chemical equilibrium to the nitrate side. Both methods are hardly realizable in an industrial scale. In this work we show a new strategy to stabilize Solar Salt at 620 °C by combining the gas-purged configuration and sealed system with maximum pressure of few tens of millibars in a 100 kg scale. The formed gas phase was within the expected range in terms of oxygen and nitrous gases. Additionally, the concentration of the nitrate and nitrite ions aligned well with salt systems with gas-purged atmosphere at 620 °C. We demonstrate the first experiments on long-term thermal stabilization (4000 h) of Solar Salt at 620 °C in a 100 kg technical-scale. These findings represent an important step in the development of modern storage systems.