Material aspects of Solar Salt for sensible heat storage

Abstract

For sensible thermal energy storage (TES) in liquids in the temperature range from 250°C to 550°C, a mixture of 60wt% sodium nitrate (NaNO3) and 40wt% potassium nitrate (KNO3), known as Solar Salt, is commonly utilized. At the time of writing, TES technology for concentrating solar power is the major application. Although commercial systems have been demonstrated, there are still several material aspects to be investigated. In this paper we address thermophysical properties and metallic corrosion, as well as thermal decomposition processes. The paper reviews temperature dependent thermophysical properties of Solar Salt. Deviations among the authors of these properties were small for the density (±1.5%), medium for the heat capacity (±7%) and large for thermal diffusivity and thermal conductivity values (±15%). The paper gives an overview of the various aspects of steel corrosion in molten alkali nitrate salts. From literature data, four steel type categories mainly depending on the temperature range are defined. The paper presents thermal stability examinations of Solar Salt and NaNO3 by isothermal lab-scale tests and thermal analysis measurements. Salt analysis in the isothermal test showed a steadily increasing oxide level at a constant nitrite to nitrate ratio. The result shows that there are kinetic differences in the first decomposition process with nitrite formation and the second decomposition process with oxide formation. The impact of the partial oxygen pressure on the decomposition temperature was examined by thermogravimetric measurements. Measurements show an improved stability limit for higher partial oxygen pressures.