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Thermal characterization of sodium nitrate-sodium nitrite compounds for thermal storage applications

Abstract : Thermal energy storage seems to be the most promising way to improve concentrated solar power plant efficiency. In the specific case of direct steam generation, where the working fluid changes of phase (liquid-vapor), a latent thermal energy storage system, fitting the working fluid behavior, appears to be the most suitable. The sodium nitrate is a phase change material already used for latent thermal energy storage to store a high amount of energy at a fixed temperature: its melting temperature. However the fixed and single nature of this temperature is a direct consequence of the purity of the phase change material. Two steps are critical for this parameter: the manufacturing of the phase change material, where the purity depends on process, and the "life" of this material in industrial conditions where it may be altered through many processes, like thermal degradation or corrosion of metallic materials in contact with the phase change material. These mechanisms lead to the production of impurities, mainly sodium nitrite. It then becomes essential to determine the influence of this impurity on the thermal behavior of the sodium nitrate. That is why the thermal characterization of pure NaNO3 and of NaNO3-NaNO2 compounds has been done using the differential scanning calorimetry. Since the considered compounds are not at the eutectic composition, their melting does not occur at a fixed and single temperature. A calorimetric method using isothermal steps has been chosen in order to determine the involved energy during the melting of samples versus temperature. Pure sodium nitrate (laboratory grade) and different compounds of sodium nitrate-sodium nitrite (2%, 4% and 6% molar fraction), were experimentally studied. This work confirmed the spreading of the melting over a range of temperature and the shifting of the melting peak to lower temperature when the sodium nitrite part increases. A decrease in latent heat in a range of 12-14 % for the worst case, i.e. the 6 % molar compounds, was determined.
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Submitted on : Monday, June 10, 2019 - 4:56:49 PM
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Adrien Lomonaco, Didier Haillot, Eric Pernot, Erwin Franquet, Jean-Pierre Bedecarrats. Thermal characterization of sodium nitrate-sodium nitrite compounds for thermal storage applications. ECOS 2015 - the 28th international conference on Efficiency, Cost, Optimization, Simulation And Environmental Impact Of Energy Systems, Jun 2015, Pau, France. ⟨hal-02151911⟩

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