Study On Preparation And Performance Of Paraffin-modified Graphite Composite Phase Change Heat Storage Material

In order to solve the intermittence and instability of solar heating system,storing the solar energy at the peak position and releasing it at the trough position with the heat storage material is an effective technical approach to solve the winter heating problem.Researchers have done numerous studies on the structural optimization of solar phase change heat storage system and the improvement of heat storage medium.However,the phase change material(PCM)as a heat storage medium still has many shortcomings.As a promising organic PCM,paraffin has ideal phase change latent heat,non-toxic,non-corrosive,stable chemical properties,and low price,but its low thermal conductivity has always limited its application.Investigators have prepared paraffin-based composite PCM with better heat transfer performance compared with paraffin by adding high thermal conductive additives.Nevertheless,the heat storage capacity of composite PCM has been greatly reduced,and the problems of poor dispersion stability and thermal cycle stability remain to be studied.Based on the above problems,the mainly research works involved in this thesis are:(1)Using a relatively toilless modification method,two kinds of modified graphites with strong lipophilicity,rich pores and good economic efficiency were synthesized:naturally aged expanded graphite(AEG)and stearic acid modified exfoliated graphite flakes(SEG).The influence mechanism on the pore structure,surface wettability,micro morphology,phase structure and functional group composition of graphites modified by different methods were explored.The test results prove that two modification methods can improve the surface energy and organic affinity of the original expanded graphite.The prepared modified graphites can be used as thermal performance enhancement additives for paraffin-based composite PCM.(2)The integrated blending method was used to load a small amount(?4 wt.%)of AEG into the paraffin matrix to prepare a special composite PCM with improved heat transfer and heat storage comprehensive performance.The composite PCM with 4 wt.%AEG loading has the largest thermal conductivity and the best heat transfer performance,and the composite PCM with 1 wt.% AEG loading has the largest phase change heat storage capacity.The influence of the microstructure and chemical composition of AEG on the thermal performance of the composite PCM was systematically explained,and the mechanism for the improvement of the heat transfer and storage performance of the composite PCM was discussed in detail.(3)Using a small amount(?4 wt.%)of SEG as a thermal performance enhancement additive to modify paraffin,and the thermal performance of paraffin-based composite PCM including thermal conductivity,latent heat of phase change,dispersion stability and thermal cycle stability were comprehensively improved.After 500 cycles of heat storage and release,the heat transfer and heat storage capacity of PCM will hardly decrease,and it still has good dispersion stability and excellent thermal cycle stability.The influence of SEG on the thermal performance of the composite PCM was characterized,and the reasons for the improvement of the thermal performance of the composite PCM were analyzed.These two paraffin-modified graphite composite PCMs prepared in this paper have outstanding thermal properties and are expected to be widely used as heat storage materials in the field of medium-low temperature solar heating system.At the same time,it provides certain theoretical reference and experimental basis for other researchers to prepare paraffin based composite PCM with high thermal conductivity,high heat storage capacity and good thermal stability.