Study On The Thermal And Optical Properties Of Composite Phase Change Materials

The utilization of renewable energy has become an important strategy for the sustainable development of modern society.Phase change materials(PCMs)are important energy storage mediums,which have demonstrated promising potential in applications related to thermal management systems,building temperature regulation,and photothermal conversion systems.However,the application of PCMs generally faces two major problems: the low thermal conductivity and the leakage caused by the solid-liquid phase transition.When PCMs are used in the field of photothermal conversion,their lower light absorption capacity also leads to low photothermal conversion efficiency.To enhance the thermal conductivity,alleviate the leakage phenomenon,and enhance the photothermal conversion efficiency of PCMs,expanded graphite(EG),graphite powder(GP),graphene(G),carbon nanotubes(CNTs),boron nitride(BN)and cupric oxide(Cu O)are used as thermal conductivity and light absorption enhancement additives,to prepare paraffin wax(PW)-based composite PCMs.The thermal properties,stability and optical properties of composite PCMs were studied.The details are as follows.1.The effects of various additives on the thermal properties of the composite PCMs were studied.Field emission scanning electron microscopy(SEM),X-ray diffractometer(XRD),Raman spectrometer,thermal constant analyzer,and differential scanning calorimeter(DSC)were used to characterize and test the additives and the prepared composite PCMs.When using EG as the additives,the prepared composite PCMs show the highest thermal conductivity.After adding 15 wt.% of EG,the thermal conductivity of the composite PCMs was as high as 14.443 W/m·K.The latent heat of the composite PCMs is inversely proportional to the mass fraction of the additives.The enthalpy of the composite PCMs with EG as the additives is the lowest,while the enthalpy reductions of the composite PCMs with the other five additives are almost the same.2.The stability of the composite PCMs was studied.The results show that the composite PCMs with EG as the additives can maintain a stable shape when PW remains in a liquid state for a long time,while other composite PCMs leak seriously.We test the thermal properties of the composite PCMs after hundreds of melting-solidification cycles.The results show that after thermal cycles the thermal conductivity of the composite PCMs with EG as the additives increased by 16.18%,while the thermal conductivity of the composite PCMs with GP,G,CNTs,BNs,and Cu O as the additives decreased by 18.44%,4.56%,15.2%,10.61% and 5.63%,respectively.The phase change enthalpies of the composite PCMs are not affected by the thermal cycles.The in-situ observation of the phase transition process of the multi-layer material system shows that due to the three-dimensional porous structure of EG,PW can be adsorbed in the pores on the surface of EG and the pores formed by the interconnection of EG when it remains in a liquid state for a long time.However,due to the difference in density between additives and liquid PW,delamination will be formed after PW melting,leading to bad thermal performances and an unstable shape of composite PCMs.3.The effects of various additives on the optical properties of composite PCMs were studied.Carbon-based additives can significantly improve the light absorption capacity of the composite PCMs.Among all additives,CNTs is the most effective light absorption enhancing additive.The reflectance of the composite PCMs added with 5wt.% of CNTs is as low as 3.32%.The mass fraction of additives did not have an effect on the light absorption of the composite PCMs.A small amount of CNTs was added to EG-PCMs to further enhance its light absorption ability.The thermal conductivity,enthalpy and reflectance of the prepared composite PCMs with 5 wt.% of EG and 2 wt.%of CNTs are 1.98 W/m·K,159.3 J/g,and 5.83%,respectively.