Thermal Energy Storage Properties Of Polyethylene Glycol/carbon Based Composite Phase Change Materials

Organic phase change materials are widely used in thermal energy storage systems because they can absorb or release large amounts of latent heat during the phase change process.Polyethylene glycol(PEG)is an organic phase change material with high heat storage density and suitable phase change temperature.However,its disadvantages,such as the leakage problem of the liquid phase and poor thermal conductivity,have limited its practical application.Encapsulation of PEG in porous materials is an effective method to solve the above problems.Porous carbon materials obtained from waste crops exhibit unique advantages,including renewable raw materials,considerable heat transfer capability,diverse microstructures inherited from the crops,and the comprehensive resource utilization of its manufacturing process.Using these porous carbon materials as the carriers for phase change materials,composite phase change materials with excellent performance might be obtained for thermal energy storage,and good social and economic values be achieved.Calcium carbonate(CaCO₃)is a widely used construction material nowadays.However,as a carrier for phase change materials,its loading capacity and the thermal conductivity are both not satisfactory.To advance the development of green buildings,it is of great significance to study calcium carbonate based composite phase change materials with excellent thermal energy performances.This is shape-stable composite phase change materials have been synthesized using the straw-derived porous carbon scaffolds and CaCO₃-coated CNTs as carriers and PEG as the phase change material,respectively.The study is divided into the following three parts:(1)Corn straw and sunflower straw were selected as biomass carbon sources,and both the corn straw-based porous carbon(CS)and sunflower straw-based porous carbon(SS)were synthesized by solution impregnation with potassium bicarbonate followed by further high-temperature pyrolysis.The results show that the latent heats of PEG/CS and PEG/SS can reach 126.4 J/g and 110.6 J/g,respectively,and their thermal conductivities,while compared with that of pure PEG,have been improved by 60%and 84%respectively.The good shape stabilities and heat transfer rates of the two composites also imply that PEG/CS and PEG/SS might be excellent composite phase change materials.(2)To further enhance the heat storage property of the composite phase change material PEG/CS,the aerogel CS2@RGO was prepared by hydrothermal method and freeze-dried process,using CS and graphene oxide(GO)as reactants and ascorbic acid as reducing agent.The effects of the dosages of the reducing agent used and reaction time on the structure of the aerogel obtained were studied.The results show that the aerogel has mesoporous and macroporous structures.The phase change enthalpy of PEG/CS2@RGO reaches up to 158.3 J/g,and the thermal conductivity has been increased by 98.4%compared with that of pure PEG.(3)CaCO₃@CNTs porous carrier with mesoporous and macroporous pores were obtained by coating calcium carbonate on the surface of carbon nanotubes(CNTs)using PEG as an agent with multiple functions of"dispersing"and"directing",etc.The synthesis mechanism was discussed.The phase change enthalpy of PEG/CaCO₃@CNTs reaches 147.7 J/g with a loading efficiency of 77%.The thermal conductivity has been improved by 113%compared to that of PEG,and no obvious leakage occurred after 200 cycles.