Preparation,Thermal Properties And Numerical Simulation Of Stearic Acid-based Composite Phase Change Energy Storage Materials

Phase change energy storage technique is widely used in solar thermal energy storage,building energy conservation,power regulation,industrial waste heat recovery and aerospace due to its advantages of high energy storage density,nearly isothermal phase change behavior,good stability and low cost.Stearic acid?SA?,as one of the most concerned organic solid-liquid phase change energy storage materials,has many advantages of high latent heat,adjustable phase change temperature,good stability,low undercooling,no corrosion and low vapor pressure.However,it also has some defects such as single value of phase change temperature,poor thermal conductivity and liquid leakage.In order to widen the phase transition temperature range of SA,and screen out the phase change energy storage materials with excellent performance and suitable for practical application.In this work,SA was mixed with three dicarboxylic acids,three alkylamides and two aromatic amides by eutectic method,respectively,and eight binary systems of SA-adipic acid?ADA?,SA-suberic acid?SUA?,SA-sebacic acid?SEA?,SA-n-butylamide?BTA?,SA-hexanamide?HA?,SA-octanamide?OTA?,SA-benzamide?BA?and SA-acetanilide?AA?were established,and many composite phase change energy storage materials with different phase transition temperatures were prepared.Phase transition behavior and thermophysical properties of these phase change energy storage materials were analyzed by Differential scanning calorimeter?DSC?.Phase diagrams and Tammann plots of the eight binary systems were drawn,and eutectic points of each system was determined by combining phase diagram,Tammann plots and Schroder's formula.Then the thermal conductivity,chemical structure and thermal stability of eutectic mixture of these systems were investigated by Fourier transform infrared spectroscopy?FTIR?,X-ray diffraction?XRD?,Synchronous thermal analyzer?TG?and thermal conductivity instrument.The results show that all the eight binary systems are eutectic systems,and the phase change temperatures and latent heat of their eutectic mixtures are in the range of341.03³42.45 K and 176.62²20.04 J·g^-1,respectively.The thermal decomposition stability of SA is improved by mixing dicarboxylic acid,alkylamide and aromatic amide,respectively.After 100 melting/freezing thermal cycles,the phase transition temperature and latent heat of eutectic have no obvious change.According to the application requirements,SA-BA and SA-AA eutectic mixture are chosen as the phase change energy storage materials of the study on enhancing thermal conductivity and overcoming the problem of liquid leakage of eutectic mixture in this paper,the molar fractions of SA in their eutectic point are 0.757 and 0.810,respectively.To enhance the thermal conductivity and overcome the problem of liquid leakage of the selected eutectic mixture,SA-BA/EG and SA-AA/EG composite phase change energy storage materials were prepared by physical adsorption method with eutectic mixture as phase change material and EG as conductive filler and carrier material.The micro-morphology,structure and liquid leakage of the composite phase change material were studied by using Scanning electron microscopy?SEM?and Thermal constant analyzer and so on,and the effect of EG addition on the thermophysical properties,thermal stability and thermal conductivity of eutectic mixtures were discussed.The results show that the rich porous structure of EG has a good adsorption for the liquid eutectic mixture and can alleviate the liquid leakage of the eutectic mixture.The saturated adsorption capacity of eutectic in EG is88 wt.%.The content of EG is inversely proportional to the latent heat of the composite phase change material,but has little impact on its phase change temperature.With the increase of EG,the thermal conductivity of the composites increases gradually.The thermal conductivity of SA-BA/EG and SA-AA/EG composite phase change energy storage materials with 12 wt.%EG is about 11 and15 times higher than their corresponding eutectic mixture,respectively.In order to further optimize the properties of SA-BA/EG composite phase change energy storage materials,A novel SA-BA/BN/EG composite phase change material was prepared by adding nano-boron nitride?BN?into the composites containing porous EG by means of the hybrid technology of various thermal conductive fillers and ultrasonic method,and their morphology,structure,thermal properties,thermal conductivity and thermal stability were demonstrated.The results show that EG adsorbed SA-BA eutectic mixture into the pore structure,and BN dispersed on its surface.The addition of BN further reduces the leakage of SA-BA/EG composite phase change energy storage materials and effectively solves the problem of liquid phase leakage of eutectic mixture.The addition of BN further improves the thermal conductivity of SA-BA/EG composite phase change energy storage materials;the thermal conductivity of SA-BA/20BN/20EG composite phase change material with BN content of 20 wt.%is 11.29%higher than that of SA-BA/20EG composite phase change material.At the same time,the addition of BN reduces the supercooling,improves the thermal cycle stability and decreases the latent heat of SA-BA/EG composite phase change energy storage materials.The prepared composite phase change energy storage materials have considerable thermal storage capacity and good thermal cycle stability.When the content of BN and EG is20 wt.%,the melting latent heat of the composite phase change material can still reach 120.75 J·g^-1.And the change of phase change temperature and latent heat of the composite phase change material is less than 1%after 100 cycles.In order to study the heat storage and release performance,heat transfer mechanism of SA-AA eutectic and SA-AA/12 wt.%EG composite phase change energy storage materials,and the effect of EG addition on the heat transfer efficiency of SA-AA eutectic,SA-AA eutectic mixture and SA-AA/12 wt.%EG composite phase change material as heat storage medium,a set of thermal energy storage system was designed and built,and the heat storage and release performance of SA-AA eutectic mixture and SA-AA/12 wt.%EG composite phase change material in the heat storage unit was studied experimentally and numerically by using Fluent software and experimental methods.The results show that there is a remarkable natural convection in the phase change process of eutectic mixture.During the charging process,the SA-AA eutectic mixture and SA-AA/12 wt.%EG composite phase change energy storage materials is dominated by the convective heat transfer and heat conduction,respectively.But during the discharging process,heat transfer of both SA-AA eutectic mixture and SA-AA/12 wt.%EG composite phase change material is mainly controlled by heat conduction.EG can significantly increases the heat transfer rate of composite phase change energy storage materials.Comparing the simulation value with the experimental measurements results,it can know that the change trend of heating/cooling curve of experiment and simulation for the eutectic mixture and the composite phase change energy storage materials is well consistent,which indicates that the numerical method can be used to guide the study of phase change heat transfer characteristics and phase change mechanism of materials during charging and discharging process.