Study On Preparation, Properties And Phase Change Heat Transfer Process Of Polyethylene Glycol-based Composite Thermal Energy Storage Materials

Phase change thermal energy storage technique has great potential in many fields such as solar thermal application, thermal management of electronic equipment and building energy conservation owing to its superior advantages of large heat storage capacity and nearly isothermal phase change behavior. Polyethylene glycol (PEG) is one of the most preferential phase change materials during the current research and applications due to its superior advantages such as high latent heat, wide range of phase change temperatures, stable in physical and chemical property, security and environmental protection. In this work the technology of materials compositization was proposed to overcome the disadvantages of PEG including low thermal conductivity and liquid leakage during the phase change process. Expanded graphite (EG) and graphite nanoplatelets (GnPs), with different types of structural characteristics, were employed as thermal conductive filler, and the PEG/EG and PEG/GnPs composite phase change materials were prepared, respectively. The relationship between the component, structure and thermo-physical property of composite phase change material and its kinetic mechanism were investigated. On this basis, a new type of composite form-stable phase change material was designed and prepared. Moreover, the phase change heat transfer process of prepared composite phase change material was studied experimentally, and the result was employed to guidance the components optimization design of composite phase change materials.Using the method of vacuum infiltration, the porous structure EG serving as conductive filler was combined with PEG to obtain the PEG/EG composite phase change material. The morphology, structure and thermo-physical properties of composite phase change materials were investigated by several means such as FE-SEM, XRD, FT-IR, POM and DSC. The obtained results show that, EG with porous structure can effectively absorb and embed the liquid PEG, and the composite phase change material with EG content of8wt.%can maintain its shape during the phase change process. With the increase of EG content, the thermal conductivity network is gradually formed inside the PEG matrix, and the thermal conductivity of the composite phase change material increased gradually. The thermal conductivity of composite phase change material with EG content of10wt.%changes up to19.4times over that of pure PEG. When the EG content exceeds6wt.%, the diffusion of PEG molecular chain segments is obviously inhibited by the porous structure of EG, which leads to the obvious decrease of phase change temperature and phase change enthalpy of composite phase change material. The melting point (Tm) and the solidification point (Tf) of composite phase change material with EG content of10wt.%decrease from50.9℃and35.7℃of pure PEG to41.8℃and21.5℃, respectively. Moreover, the melting enthalpy (ΔHm) and the solidification enthalpy (ΔHf) are only67.4%and73.6%of their theoretical values, respectively.GnPs with large aspect ratios (300～800times) were obtained by sonicating the EG. Using the method of vacuum infiltration, GnPs serving as conductive filler was combined with PEG to obtain the PEG/GnPs composite phase change material, and the morphology, structure and thermo-physical properties of composite were investigated. Results show that, the ultrasonic fragmentation nearly does not impact the phase and chemical surfactant of graphite. The GnPs with large aspect ratio possess advantage of easier to be dispersed in polymer matrix to form conducting network. The thermal conductivity of the composite phase change material with GnPs content of10wt.%changs up to10.8times over that of pure PEG. Compared with PEG/EG composite phase change material, the PEG/GnPs composite possesses more stable phase change temperature, and its phase change enthalpy is more closer to the theoretical value.Aim at the difference between the impacts of graphite structures on the phase change parameters of PEG, the non-isothermal phase change kinetics of pure PEG and composite phase change materials were studied by means of DSC. Results show that, the apparent activation energy of composite phase change material is higher than that of pure PEG, which indicates that the EG and GnPs are both inhibiting the diffusion of PEG molecular chain segments at certain limitations. Compared with GnPs, the same mass fraction of EG leads to the greater increase of apparent activation energy of the composite phase change material, which indicates that the inhibition of lamellar structure of GnPs on the diffusion of PEG molecular chain segments is relatively smaller than the porous structure of EG.Based on the phase change kinetics results, GnPs selecting as conductive fillers and polymethyl methacrylate (PMMA) acting as supporting material were combined with PEG to obtain a new-type of PEG/PMMA/GnPs composite form-stable phase change material by using the method of in situ polymerization upon ultrasonic irradiation. XRD and FT-IR results indicated that all the components are physically combined with each other during polymerization process. FE-SEM and POM results show that the PEG is uniformly dispersed and embedded inside the micro-level network structure of PMMA, which contributed to the well package and self-supporting properties of composite form-stable phase change material. Ultrasonic-assisted in situ polymerization process could effectively disperse the GnPs into the polymer matrix to build the thermal conductivity network. The thermal conductivity of composite form-stable phase change material with content of8wt.%changs up to8.4times over that of PEG/PMMA composite. It is also confirmed that all the prepared specimens possess available thermal storage density and form-stable performance. When the content of EG is8wt.%, the ΔHm, ΔHf, compressive strength at55℃and mass loss rate after75cycles of composite form-stable phase change material are114.7kJ-kg-1,97.0kJ-kg-1,3.7MPa and2.5%, respectively.The phase change heat transfer processes of pure PEG and composite phase change materials were experimentally studied by time-temperature method. According to the experimental results, the influence of composite modification on the usability of phase change material was evaluated. Results show that there is a significant natural convection effect during the phase change heat transfer process of pure PEG. The porous structure of the EG is able to effectively absorb and embed the liquid PEG to limit natural convection effect. The EG is able to significantly increase the thermal conductivity of the composite phase change material and reducing the thermal resistance of phase change heat transfer process. Then the phase change rate of composite phase change material is able to be increased effectively. The optimize content of EG among PEG/EG composite phase change material is approximately6～8wt.%.In this paper, the design and preparation of PEG based composite phase change materials, the microstructure, the thermo-physical property and the kinetic mechanism of composite phase change materials are together investigated. The research results may be helpful to improve the usability of PEG and promote its practical application. One point deserved mentioning is that the research on phase change kinetics of PEG/graphite composite phase change materials may be helpful to understand the unconventional phase transition mechanism inside the composite system. Moreover, the experimental measurement of phase change heat transfer processes is able to effectively evaluate the influence of composite modification on the usability of phase change material, and it has a great guiding significance on the performance control and optimum design of thermal energy storage materials.