Non-isothermal Crystallization Procesess Of Aqueous Composite Phase Change Materials Filled With Carbon Nanomaterials

Energy is the material basis for economic and social development, but since the industrial revolution, coal, oil and other fossil energy consumption have increased rapidly, which affecting the sustainable development of human beings. The main way to solve the energy problem is to develop new energy and to find out advanced methods to improve energy utilization efficiency. The mismatch of energy supply and demand in time and space can be solved by using phase change energy storage, as well as problems of low efficiency of energy use. Phase change cold storage is a phase change energy storage technology that cold energy of latent heat being stored during solidification, released and used when being heated. However, the main drawback of conventional phase change material is present in low thermal conductivity and the high supercooling degree, such as the thermal conductivity of water is about0.6W/mK, the undercooling is about20℃. High thermal conductivity has been added into conventional phase change working fluid to form a suspension of nanoparticles (i.e., nano-composite phase change material) to improve its thermal conductivity. Carbon nano-materials has been used as an addition to increase the coefficient of thermal conductivity of phase change material in recent years because of its high thermal conductivity, i.e., the thermal conductivity of carbon nanotubes is about3000W/mK. At the same time, high thermal conductivity of nano particles in cold storage in the working mediumcan occur as heterogeneous nucleation agent to promote thecrystallization process, reduce the degree of supercooling.Experimental research of the supercooling of water in the solidification process is carried out in this paper. Water-based carbon nano-composite phase change material is equipped with five concentration (graphene oxide suspension in water and carbon nanotube suspensions in water). Its dispersion stability is observed by gravity sedimentation and transmission electron microscopy, and the non-isothermal crystallization process is studied under three cooling rate with differential scanning calorimetry (DSC) experiments. The effect of the concentration of carbon nanotube as well as the cooling rate of DSC to the degree of supercooling of water is analyzed and discussed in this paper. The following conclusions are obtained:1) the experimental results show that with the concentration increases, the supercooling degree of suspending liquid declined gradually. At the highest concentration of1%, the supercooling degree of graphene suspensions is about5℃lower than pure water, the supercooling degree of carbon nanotube suspension in water is about7℃lower than pure water which has greater effect than ordinary metal nano particles.2) The supercooling degree of suspension increases a little when the cooling rate increases, but the concentration of the suspension almost not change.3) in the early stage of non-isothermal crystallization(namely the nucleation stage), the crystallization rate has slowed down in trend with the increase of the concentration, but increases until latter stage (i.e.,nucleation growth stage).