Preparation And Study On Thermal Conduction Mechanism Of Hollow Fiber Composite Phase Change Energy Storage Material

Hollow fiber phase change energy storage materials(HF-PCESMs)have good development prospects in energy storage and thermal management duo to dual storage channels,large storage density and strong weavability.However,the disadvantages of easy leakage and low thermal conductivity largely limit their application.The morphology instability of phase change materials during the phase transition process may cause leakage problems,and the low thermal conductivity seriously reduces the speed of energy charging and discharging.For the purpose of solving above mentioned problems,the polyvinylidene fluoride hollow fiber membrane(PVDF HFM)was used as a carrier,and paraffin was used as an energy storage medium.The pore structures on the inner and outer surfaces of PVDF HFM were effectively controlled to prevent the leakage of paraffin.The thermal conductivity of PCESMs were increased by two approaches.First,the thermal conductivity of the carrier(PVDF HFM)was improved,and the mechanism of different types of reduced graphene oxide(RGO)in enhancing the heat transfer of PVDF HFM was revealed.Second,the thermal conductivity of the energy storage medium was improved due to a heat transfer network of multi-walled carbon nanotubes(MWCNTs)in a paraffin matrix was constructed.The bottleneck of single mode enhanced thermal conductivity was broken by double enhancement thermal conductivity of carrier material and energy storage medium.The PVDF HFMencapsulated paraffin technology is a new and promising research direction.This technology not only enriches the carrier selection of phase change materials,but also provides new ideas for the design of future-PCESMs.The main research contents are as follows:(1)For the leakage problem,the effects of factors such as polymer content,porogen content,and coagulation bath temperature on membrane structure and performance were studied by regulating the microstructure of PVDF HFM.A carrier membrane with large inside-small outside pore structure and excellent performance was successfully prepared.The large holes on the inner surface are beneficial for the paraffin to enter the membrane wall,and the small holes on the outer surface(dense structure)can prevent the leakage of the phase change material.For the formulation parameters and preparation process of the casting solution,the optimal system was finally determined to be 17% PVDF,5% PVP,5% Tween-80,73% DMAc and coagulation bath(10℃).In this system,the average pore diameter is about 440 nm,the porosity is maintained at 80%,the tensile strength exceeds 6 MPa,the elongation at break exceeds 135%,and the thermal conductivity is approximately 0.05 W/m·K.The high porosity(severe air thermal resistance)of PVDF HFM affects the heat flow transfer,and the thermal conductivity of the virgin membrane is at a low level.(2)For the low thermal conductivity of the carrier material issue,doping high thermal conductivity nanoparticles can efficiently improve the thermal conductivity of the system.By doping reduced graphene oxide(RGO)with high thermal conductivity into PVDF HFM,the influence-mechanism of RGO on the thermal conductivity of virgin membrane was explored.For the same RGO content system,medium-diameter RGO contacted with each other in the PVDF matrix,causing fewer “matrix-filler” interfaces in the same heat conduction path,thus the heat transfer efficiency is higher.For the same RGO lamellar diameters system,the high content of RGO breaks the wrapping of the PVDF matrix because of the quantity advantage,and thermal vibration(phonon)loses less thermal vibration energy during conduction.For the RGO / MWCNTs compound system,the agglomeration phenomenon was caused by the increase in the concentration of MWCNTs,while the individually doped RGO dispersed better in the PVDF matrix,and the heat conduction network was relatively complete.The test results show that different types of RGO improve the thermal conductivity of virgin membrane without destroying the PVDF HFM large inside-small outside hole structure.The pure RGO system with a lamellar diameter of 1-3μm and a content of 0.5% has the most significant contribution to the thermal conductivity of virgin membrane.Thermal conductivity reaches 0.17 W/m·K,which is 3.5 times higher than virgin membrane.The RGO acts as a bridge between the crystals inside the PVDF.(3)For the problem of low thermal conductivity of the energy storage medium,MWCNTs were doped in paraffin to simulate the heat flow transfer of MWCNTs in paraffin.For the same MWCNTs content system,medium-length MWCNTs break through the constraints of finite size effects,and a large number of contact points appear between the particle units to transfer heat flow.For the same MWCNTs length system,the MWCNTs of medium content has no obvious agglomeration in the paraffin matrix,and can also contact the new heat transfer carrier at the axial end of the fiber.When the MWCNTs length is 30-50μm and the content is 5%,the MWCNTs shows a more significant contribution to the heat transfer of the paraffin.At this time,the heat transfer network and path constructed are relatively perfect,which can improve the heat transfer efficiency and reduce the interface thermal resistance.(4)The MWCNTs-thermally conductive paraffin was encapsulated with the optimized RGO-thermally conductive membrane,which constructed an efficient and high-speed three-dimensional thermal conductive network.The interfacial thermal resistance between the PVDF matrix and the paraffin matrix was limited with the aid of two thermally conductive fillers.The thermal conductivity of the composite phase change energy storage material prepared by thermally conductive membraneencapsulation-thermally conductive paraffin reached a peak(0.65 W/m·K),which is 13 times higher than the virgin membrane thermal conductivity.The heat transfer enhancement effect is remarkable,which endows the system with excellent heat conduction performance.