Study On The Effect Of Component Modulation On The Properties Of Heat Storage Materials

With the rapid development of economy,the demand for energy is increasing.It has become an important topic for contemporary research to accelerate the pace of developing and utilizing new energy sources and improve the efficiency of energy utilization that has been developed and put into use.In China,building energy consumption is the largest of all energy consumption types,and heating energy consumption occupies a large proportion of building energy consumption.Therefore,it is an important way to reduce building energy consumption and realize energy saving and emission reduction to make rational use of building heat storage materials and improve their performance.Molten salt is an easy and efficient heat transfer and storage material,which has been widely used in commercial solar thermal power generation,energy storage electric heating and so on.At the same time,latent heat storage with phase change materials as the core is also an important form of energy storage electric heating,which has a very broad application prospects.There are two research objects in this paper:one is molten salt used in solar thermal power generation;the other is the building commonly used energy storage materials paraffin.The influence of composition addition and ratio change on its heat storage performance was studied.The research methods include the numerical simulation and the final experimental verification.Finally,the parameter change rule and the best combination of each kind of research object are obtained,and the numerical prediction method which can be used to guide the follow-up experimental research is also given.In this paper,firstly,the eutectic points of nine kinds of binary mixed nitrates of Mg?NO₃?₂-NaNO₃ are predicted by programming with MATLAB software.The predicted results show that only when the mass ratio of Mg?NO₃?₂:NaNO₃ is 5:5,the solid-liquid two-phase points almost coincide and the temperature difference is about 1k.It is presumed that the proportion is eutectic molten salt,which is most suitable for practical operation system.The simulation results show that the eutectic temperature is about 340?.Then nine binary mixed nitrates with different mass ratios were prepared by adding Mg?NO₃?₂ of different proportions to NaNO₃.The initial melting temperature,primary crystallization point,decomposition temperature,specific heat and latent heat of mixed nitrates were studied by DSC and TGA,respectively,in order to compare the molten salts of this component with those of molten salts that have been put into operation on a large scale.Comparatively,is it more suitable for solar thermal power generation system.The experimental results show that three of the nine mixtures of molten salts do not eutectic,and the mass ratios of Mg?NO₃?₂ to NaNO₃ are 1:9,2:8 and 3:7,respectively.At the same time,the melting point and the initial crystallization point of the mixed molten salt with the same mass ratio are the closest,the melting point is 357.8?,the initial crystallization temperature is 368.5?and the decomposition temperature is 440.0?.The melting point temperature of the other eutectic binary nitrates is maintained at 340?,the initial crystallization point temperature is maintained at 240?,and the decomposition temperature is maintained in the range of 430-500?.Comparing the experimental results with the simulation results,as far as the solid phase is concerned,there is a slight difference between the theoretical and experimental values,but the difference is not significant;as far as the liquid phase is concerned,except for the mass ratio of 5:5,the experimental values of the other five proportions have the same trend as the simulation values,the temperature is 5K-10K higher than the simulation values,and the accuracy is higher within the allowable error range.For the mass ratio of 5:5,the simulation results show that the molten salt can achieve eutectic at the eutectic ratio.The experimental results show that the solid phase point and the liquid phase point of the ratio do not intersect,and there is no eutectic formation.Compared with other proportions,the solid phase temperature and liquid phase temperature have an upward trend.For the latent heat of melting,with the decrease of Mg?NO₃?₂ concentration,the latent heat of melting increases gradually and varies greatly.When the concentration of Mg?NO₃?₂ is50 wt.%,the maximum latent heat of melting is 154.2 J/g.The experimental results show that the average specific calorific value of molten salt with equal mass ratio is 1.034 J/?g K?.On this basis,the specific heat curve of eutectic molten salt was fitted by least square method.Origin software was used to fit the specific heat curve formula of binary nitrate with equal mass ratio.The results show that for each system,the simulated values of melting point are in good agreement with the experimental values,and the specific heat of molten salts is in high agreement.The simulation results are theoretically guided and verified,which greatly reduces the experimental workload and provides a reference for theoretical prediction of thermal properties of other new heat transfer and storage materials.In this paper,by adding expanded graphite powder or metal oxide into paraffin,16 kinds of composite phase change materials are prepared and the method of differential scanning calorimetry and laser heat conduction are used by adding expanded graphite powder or metal oxide to paraffin,the composite phase change material is formed,and the changes of thermal conductivity,specific heat and latent heat of the composite phase change materials of paraffin and different proportions are compared by experiments.The experiments have proved that the melting curve of the composite phase change heat storage material below 150?is similar to that of pure paraffin,and the latent heat of phase change is equal to the product of the latent heat and mass fraction of paraffin phase change.The specific heat value of 5 preferred proportions was higher than that of pure paraffin samples,and the specific heat of each proportion increased with the increase of temperature.The thermal conductivity of composite phase change materials was higher than that of pure paraffin samples,and the thermal conductivity of ternary composite phase change materials was improved obviously.