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.