Preparation And Characterization Of Expanded Graphite/Paraffin Shape Stabilized Phase Change Thermal Energy Storage Material

As a kind of latent thermal energy storage material,phase change material(PCM)has a high thermal energy storage density and the working temperature of which is nearly constant.PCMs have been widely used in the thermal energy storage field.By selecting PCMs with suitable melting point(usually~25°C)and applying it into the building envelope,the thermal mass of the buildings can be increased and the indoor temperature fluctuation can be reduced.In this way,the building energy consumption will be reduced.The common way of applying PCMs into buildings was focused on incorporating PCM into building materials.The composite PCMs prepared by this method have a relatively low thermal heat storage density and a low thermal conductivity,and the long-term use stability of this kind of composite PCMs have hardly been studied.In this thesis,a form-stable composite PCM which can be independently used in buildings was prepared,and the properties of which were studied.Firstly,the expanded graphite(EG)/paraffin composite PCM was prepared by vacuum impregnation method.The SEM results showed that paraffin was uniformly adsorbed on the surface of the“worm”-like EG.The leakage-proof test showed that the paraffin mass fraction in this composite PCM could reach 94%without the leakage of liquid paraffin.The DSC results demonstrated that the melting point of the composite PCM was~26°C,and the latent heat of the composite PCM increased with the increase of the paraffin mass fraction in the composite PCM.And the latent heat of the composite PCM with paraffin mass fraction of 94%can reach 136.2 J/g.The thermal conductivity analysis showed that the thermal conductivity of the composite PCM increased with the increase of the compact density and decreased with the increase of paraffin mass fraction.When the paraffin mass fraction was 94%and the compact density was~0.83 g/cm~3,the corresponding thermal conductivity was 2.14 W/(m·K),which was much higher than pure paraffin(~0.2 W/(m·K)).The thermal conductivity of the composite PCM was further enhanced by carbon nanotubes(CNTs)doping.The thermal conductivity analysis showed that the doping of CNTs in the composite PCM could significantly increase the thermal conductivity of the composite PCM when the paraffin mass fraction was high.Also,the thermal conductivity of the CNTs modified composite PCM increased with the CNTs doping amount.However,the growth trend of the thermal conductivity could be slow when the doping amount of CNTs exceeded 0.8wt%.By doping 0.8wt%CNTs,thermal conductivity of the composite PCM with paraffin mass fraction of 94%was further increased to 4.11 W/(m·K).Finally,the compacted composite PCM was conformally sealed by epoxy resin and the hard epoxy resin shell could provide support for the composite PCM,which made it possible to applying in buildings independently.The FTIR analysis showed that the epoxy resin had a good chemical compatibility with the internal composite PCM,so it won't have any effect on the internal composite PCM during the curing process.The numerical simulation results showed that the duration of the thermal storage and release process was proportional to the thickness of the composite PCM.When the duration of the thermal storage process was 8 h,the corresponding thickness of the composite PCM was~0.76 cm.And the optimized thickness of this composite PCM was 0.76 cm for building applications since the high-temperature duration in a day was probably 8 h.It was also found that there was almost no temperature gradient in the composite PCM,which means that the thermal conductivity of the composite PCM is high enough to meet the requirements for building applications.