| 60%NaNO3-40%KNO3 molten salt is an important heat transfer heat storage medium,which has become a good choice for high temperature heat transfer heat storage medium for solar energy and is also used as a thermal circulation medium in industrial energy saving and renewable energy utilization.Furthermore,it is a potential heat storage medium for waste heat recovery of aluminum electrolysis cells.However,when the operating temperature exceeds the upper limit of 600℃,its thermal stability will deteriorate for thermal decomposition reaction.Therefore,it is necessary to improve the heat storage performance so as to achieve more effective application.The introduction of additives into the molten salt system is an important method to improve the heat storage performance.In the present study,LiNO3,KNO2,and NaNO2 single additive and LiNO3-KNO2 and KNO2-NaNO2 with the same mass fraction of 5%,10%and 15%were added to 60%NaNO3-40%KNO3 molten salt system,respectively.The ionic structure and related thermophysical properties at different temperatures and with different compositions of molten salt were studied.The influences of different additives were analyzed and compared with each other in order to obtain the composition of the molten salt system with better thermal storage properties.First,the ionic structure of molten salt systems with different compositions was studied by Raman spectroscopy,and the transformation rules of ionic groups in the molten salt were analyzed.The results showed that in the(60%NaNO3-40%KNO3)-LiNO3 system,LiNO3 would decompose when the temperature is higher than 200℃.In the(60%NaNO3-40%KNO3)-KNO2,(60%NaNO3-40%KNO3)-NaNO2 and(60%NaNO3-40%KNO3)-KNO2-NaNO2 system,when the temperature exceeded 200℃,the equilibrium between NO2-and NO3-in the molten salt would move toward the NO3-side.Meanwhile,the thermostability of the samples refered above at 500℃,550℃,600℃ and 650℃ was analyzed by thermogravimetry experiments.Raman spectroscopy and thermogravimetric results showed that the molten salt system has better thermal stability when adding 10%KNO2,5%NaNO2 or 2.5%KNO2-2.5%NaNO2.Then,the density of mixed molten salt system at different temperatures was measured by Archimedes’ method.The experimental results showed that the density of each molten salt system was basically linear with the temperature,and the empirical equation of the relationship between the molten salt density and the temperature with each component was fitted.The experimental results showed that when different contents of LiNO3,KNO2,NaNO2,LiNO3-KNO2 or KNO2-NaNO2 were added to 60%NaNO3-40%KNO3 system,the density of molten salt increased.When the same amount of additives was added,the density of the molten salt was the maximum at the same temperature when the additive was LiNO3 or KNO2-NaNO2.Finally,the temperature and latent heat of phase transition of the molten salt system were measured by differential scanning calorimetry(DSC).The experimental results showed that there is no obvious change of phase transition temperature when adding LiNO3,KNO2,NaNO2,LiNO3-KNO2 or KNO2-NaNO2 to 60%NaNO3-40%KNO3 system.When LiNO3,KNO2 or LiNO3-KNO2 was added,the latent heat of the molten salt system decreased.With the addition of NaNO2 or KNO2-NaNO2,the latent heat of the molten salt system increased.When adding 15%NaNO2 or 7.5%KNO2-7.5%NaNO2,the mixed molten salt system showed the best improved effect from the view of the latent heat of phase transition.A comprehensive comparison of the relevant thermal properties of the above molten salts suggests that the(60%NaNO3-40%KNO3)-10KNO2,(60%NaNO3-40%KNO3)-5%NaNO2,and(60%NaNO3-40%KNO3)-2.5%KNO2-2.5%NaNO2 molten salt system is more suitable due to better high temperature thermal stability and higher density in the investigated molten salt systems if sensible heat storage is used.If using latent heat storage,(60%NaNO3-40%KNO3)-15%NaNO2 and(60%NaNO3-40%KNO3)-7.5%KNO2-7.5%NaNO2 molten salt system due to the higher phase transition latent heat is more appropriate. |
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