Binary and ternary alkali polyphosphates (MPO3, M = Li, Na, K) for thermal energy storage

Up-to-date, solar salt (a mixture of 60 mass% NaNO3 and 40 mass% KNO3) is practically the only media for thermal energy storage (TES) in concentrated solar power (CSP). This commercial product's utilization is limited below 550 degrees C to avoid an irreversible thermal decomposition. The development of new performing TES materials is decisive for the deployment of CSP technology. Our recent work reported promising results obtained with mono-alkali polyphosphate (M-PO3) as TES materials. These materials can work up to around 900 degrees C, but their melting point is still high, which is at least 628 degrees C. In order to lower their melting point, binary and ternary mixtures of alkali polyphosphates are investigated in this work. Different mixtures made of two or three alkali polyphosphates were prepared and studied using thermal analysis methods. The most promising mixture found in this work was the ternary Li-Na-K-PO3, which contained 33.3% (mol%) of each alkali metal. This mixture can be used as a liquid TES material in the temperature range of 398 to 900 degrees C. The results open new prospects for the development of the thermal energy storage field.