| Solar energy is inexhaustible,but it is unstable and discontinuous under the influence of time and space,so it is necessary to store solar energy to improve its utilization efficiency.Phase change energy storage technology is an ideal solar energy storage technology with phase change materials as the core,which has the characteristics of high energy storage density and small temperature change during phase change.However,the phase change material has low photothermal conversion,low thermal conductivity and easy leakage,which limit its practical application.In view of the shortcomings of the above-mentioned phase change materials,in this study,we prepared carbon-based composite phase change materials(CPCM)using expanded graphite and biomass porous carbon to adsorb paraffin,and studied their thermal properties,shape stability and photothermal conversion properties.At the same time,in order to explore the practical application of carbon-based CPCM,a Trombe-like model with solar energy utilization and thermal insulation was designed.The main results were as follows:(1)To improve the thermal conductivity,shape stability and photothermal conversion characteristics of paraffin as a phase change material.Expanded graphite composite phase change materials(PA/EG)having different mass fractions were prepared with paraffin as phase change material and expanded graphite as porous carbon material.The results showed that the latent heat of PA/EG-30% reached 198.4 J/g,and the thermal conductivity reached 5.21W/(m?K),which was approximately 20 times that of pure paraffin.After 100 cycles,the change rate of latent heat of the composite phase change material was less than 8%,which exhibited good thermal and cyclic stability.Additionally,PA/EG-30% demonstrated strong photothermal conversion performance,and the photothermal conversion efficiency was as high as 91.24%.(2)To reduce the cost of expanded graphite as a carbon-based porous material,the widely distributed,cheap and readily available biomass corn straws and sunflower straws were used to prepare carbon-based porous materials instead of expanded graphite,and paraffin was impregnated into the carbon-based porous materials using vacuum impregnation method to prepare bio-based CPCM.The results showed that compared with expanded graphite,biomass porous carbon had larger pore volume and could adsorb more phase change materials.The loading rates of carbonized corn straw and sunflower straw were 97.04% and 98.52%,respectively.Compared with biomass porous carbon CPCM,expanded graphite CPCM had better thermal conductivity and shape stability.However,due to the pore structure of carbonized straw has great inclusion and aspect ratio,the reflected sunlight could be effectively recovered and recycled.The photothermal conversion efficiency of carbonized sunflower straw and corn straw CPCM was 94.31% and 92.96%,respectively,which were higher than that of expanded graphite CPCM(87.43%)at the same mass fraction.(3)To verify the photothermal conversion performance and thermal insulation performance of carbon-based CPCM in building applications,a Trombe-like model with solar energy utilization and thermal insulation was designed.The carbonized sunflower straw CPCM with high photothermal conversion efficiency was used as the heat collection wall of Trombelike model,which was mixed with cement mortar as the surrounding heat storage wall,and the pure cement mortar heat storage wall was used as the control.The results showed that the mass loss rate of phase change wall was only 1.4% after heating for 8 h,and the latent heat was reduced from 73.86 J/g to 69.81 J/g after 500 cycles,and the latent heat was only reduced by5.48%.As the addition of CPCM,the thermal conductivity of cement mortar wall increased from 0.837 W/(m?K)to 0.911 W/(m?K).When the light intensity was 144 m W/cm2,the average temperature in the model with cement mortar and phase change wall as heat storage walls increased by 26.89℃ and 24.16℃ respectively after the heat collection wall was exposed to sunlight for 90 min.When the temperature in the model was reduced to 36℃,the time delay of phase change wall was more than 11 min compared with cement mortar wall.Therefore,it was proved that the carbonized sunflower straw CPCM had good photothermal conversion performance when used as a heat collection wall,and the phase change wall could effectively alleviate indoor temperature fluctuation and had thermal insulation performance when used as a heat storage wall. |
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