| Stearic acid（SA）is employed in phase change thermal storage systems to solve the problem of intermittent and unstable solar energy utilization due to its advantages of high latent heat and suitable phase change temperature.However,SA has disadvantages such as low thermal conductivity and easy liquid leakage.Therefore,in this paper,materials with porous structure and high thermal conductivity are selected for modification.In addition,to address the problem of low heat transfer efficiency of the current heat exchanger,this paper strengthens the heat transfer by adding fins to increase the heat transfer area between the phase change material and the thermal boundary.Firstly,SA/BN composite phase change materials were prepared by a melt blending method using boron nitride（BN）as the support material.The resulting materials were characterised for their thermal properties,thermal conductivity,crystal structure,chemical stability,thermal stability,cyclic stability,thermal storage/exhaust properties and temperature response characteristics.The results show that the scaled BN absorbs part of the SA,but the composite loses up to 5.03%of its mass after 30 minutes of heat treatment.Further,the SA/EG composite phase change material was prepared by the same method using expanded graphite（EG）as the support filler.It was found that the rich porous structure of EG had a good adsorption effect on liquid SA,which alleviated the leakage problem of the phase change material,and the mass loss was only 0.79%after 5 h of heat treatment.At an EG mass fraction of 12 wt.%,the thermal conductivity was as high as 6.54 W·m-1·K-1.The latent heat value of the composite at this point was negatively correlated with the EG mass fraction,at 163.75 J·g-1,and the crystallinity was 90.48%.The analysis concludes that the two composite phase change materials are suitable for low temperature phase change applications（<100°C）.Secondly,due to the limited ability of a single filler to encapsulate SA,SA was encapsulated by synergistic interaction between BN and EG,and the thermal properties of the composites were investigated.The latent heat of phase change of 154.20 J·g-1 was obtained,and the mass loss of the SA/BN/EG composite after 5 h of heating was only 0.20%,indicating that the synergistic encapsulation further inhibited the leakage of the material.In addition,the melt temperature and latent heat of melting of the SA/BN/EG composite varied by 0.37°C and 0.21 J·g-1 respectively after 50 thermal cycles,indicating the good cycling stability of the composite phase change material.Finally,in order to study the heat transfer performance of the phase change heat storage unit,a series of new V-shaped fin type heat storage units are established in this paper,and the structure of the phase change heat storage unit is simulated and optimised by Fluent software.The effects of the angle,length,number and arrangement of the V-shaped fins,the angle of the branch fins and the length of the branch fins on the heat transfer performance are investigated respectively.It is shown that the heat transfer of phase change materials in the phase change heat storage unit is divided into three stages,with the first and third stages dominated by heat conduction.The second stage is dominated by natural convection,and natural convection enhances heat transfer.The optimal values of the fin geometrical parameters areα=55°,L1=7 mm,N=3,L3=4 mm andβ=55°,respectively,as optimised by response surface analysis.The optimum arrangement for enhanced heat transfer should be one where the lower half of the fins are longer than the upper half,namely L1=7 mm&L2=9mm. |
