Sodium nitrate thermal behavior in latent heat thermal energy storage: A study of the impact of sodium nitrite on melting temperature and enthalpy

Using direct steam generation in concentrated solar power plant leads to the development of new storage systems, which may include a latent heat thermal energy storage module. With this module, the behavior of the storage system matches the thermal behavior of the water used as heat transfer fluid. Sodium nitrate, which is already used in such lab-scale systems, is able to fulfill the conditions to assume the role of phase change material to store thermal energy. However, this material may evolve during its lifetime, either due to thermal decomposition or by corrosion phenomena, both leading to the reduction of nitrates to nitrites. Such evolution has been reported for the so-called "solar-salt" (NaNO3-KNO3 60:40 wt%), driving to the reduction of the melting temperature and latent heat. It is thus important to study the evolution of these properties in the case of sodium nitrate. The work proposed here aims to characterize sodium nitrate-nitrite mixture by differential scanning calorimetry for comparison with pure sodium nitrate (laboratory grade and industrial grade). Since the considered compounds are not at the eutectic composition, their melting does not occur at a fixed and single temperature. Thus, a calorimetric method using isothermal steps has been used in order to determine the evolution of involved energy during the melting of samples versus temperature for each sample. The results show the spreading of the melting over a range of temperature and the shifting of the liquidus to lower temperature when the sodium nitrite part increases. Moreover, a decrease in latent heat up to nearly 15% was observed.