Molten Salts Composites For Thermal Storage In Steam Commercial Plants

Abstract:

The next generation of modified phase change material (PCM) composite is elaborated for utilizing both, a high thermal capacity of the molten salts NaNO3/KNO3 eutectics and a good thermal conductivity of the graphite matrix. The form of containment to prevent leakage of the liquid phase change material is another function of a graphite matrix. Particle porosity and larger specific surface are intended to avoid a salt clusters formation during solidification alongside the heat flow exchange improved.An extensive study on structural and thermal behavior of the modified PCM was carried out for molten salts NaNO3/KNO3/graphite composite samples, structured through different manufacturing routes with variety graphite content (10–30 wt%). Thermophysical data, collected by DSC and thermal cycling methods, include the composite phase transition temperatures from 215 to 2600C and thermal storage capacity of 75-88 [kJ/kg]. The molten salts NaNO3/KNO3/graphite crystallization process, responsible for heat released, was studied by LINKAM thermal microscopy, scanning electron microscopy (SEM). The precise imaging allowed identification of the adhesive bonding development among surfaces of the molten salts and graphite matrix.The spectral FTIR study performed before and after the composite re-crystallization, did not register additional functional groups due to chemical destruction of molten salts. The elemental characterization of the composite surface by the use of X-ray microanalysis (EDS) method specified a few areas of Na-rich spherical masses and the K-rich layered structure on the graphite plane. As the areas of inhomogeneous elemental distribution influence thermal stability of the salt/graphite composite structure, the research focus on solidification/nucleation process is obvious and planned for controlling the composite structural stability during repeatable thermal cycling process.The correlation between morphology and thermal storage capacity allowed selection of the molten salts NaNO3/KNO3/graphite composite samples with optimal thermal storage behavior. Applicability of the new generation modified PCM composite to the high temperature thermal storage process would increase energy efficiency of saturated and superheated steam commercial plants through accumulating and use the waste heat.

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