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....

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Bibliographic Details
Published in:Solar energy materials and solar cells 2016-05, Vol.149, p.81-87
Main Authors: Lomonaco, Adrien, Haillot, Didier, Pernot, Eric, Franquet, Erwin, Bédécarrats, Jean-Pierre
Format: Article
Language:English
Subjects:
Chemical and Process Engineering
Differential scanning calorimetry
Engineering Sciences
Evolution
Isothermal step method
Latent heat
Latent thermal energy storage
Melting
NaNO3+NaNO2 compounds
Phase change materials
Reduction
Sodium nitrates
Sodium nitrite
Thermal energy
Thermal properties
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Summary: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 60:40wt%), 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. •Thermal characterization (DSC) of NaNO3 and NaNO3–NaNO2 mixtures.•Step method was used to determine the enthalpy function versus temperature.•Demonstration of the reduction of the liquidus temperature for mixtures.•Demonstration of the reduction of the latent heat when the NaNO2 proportion increases.
ISSN:0927-0248
1879-3398
DOI:10.1016/j.solmat.2015.12.043