The thermal stability of molten nitrite/nitrates salt for solar thermal energy storage in different atmospheres

The study of the thermal decomposition of molten nitrite/nitrates salt used for thermal energy storage (TES) in concentrating solar power (CSP) was carried-out with a HITEC (Reg. U.S. Patent – Coastal Chemical Company) type salt. This salt is the commercial mixture of NaNO3–KNO3–NaNO2 in the proportions 7–53–40 wt.% (NO2/NO3 weight ratio of 0.7). The study was done by simultaneous DSC/TG-MS analysis between room temperature and 1000 °C in gas atmospheres of argon, nitrogen, air and oxygen. It was found that:•The thermal stability of the salt can be significantly enhanced by controlling the atmosphere.•By two assessment criteria, TG and DSC, the salt operated in an inert atmosphere could be used at temperature of at least 610 °C and when operated in an oxidising atmosphere up to between 650 °C and 700 °C.Oxidising atmosphere was found to change the chemistry of the salt by converting some nitrite to nitrate, and although this may have a bearing on increasing the melting point, it has the benefit of rising the thermal decomposition temperature.

► Three criteria are used to discern the thermal decomposition of molten salt mixtures under different atmospheres. ► Combining these three criteria allows identifying the maximum temperature at which the salt can be used. ► The salt under a blanket of inert gas offers thermal stability up to 610 °C. ► Oxidising atmosphere has the benefit of higher thermal stability up to ∼700 °C.