Research On Preparation Of Composite Phase Change Material And Phase Change Performance Of Energy Storage Unit

Latent heat thermal energy storage,as a thermal energy storage technology with a large energy storage density,can effectively overcome the intermittent and instability of renewable energy such as solar energy.However,the heat transfer performance of the phase change process is weak due to the low thermal conductivity of the phase change material,which will be a big limit to the application of the latent heat thermal energy storage.To improve the heat transfer performance of the phase change process,this paper conducted research based on the phase change material and the latent heat thermal energy storage unit.A paraffin/expanded graphite composite phase change material with enhanced thermal conductivity was prepared.On this basis,the effects of phase change material properties and energy storage unit design on the phase change heat transfer performance were studied through experiments and numerical simulations.The main contents and conclusions are as follows:Firstly,four expanded graphite/paraffin composite phase change materials containing 1%,2%,3% and4% expanded graphite were prepared by using paraffin as the phase change material and expanded graphite as the thermally enhancement filler.The morphology of the composite phase change material at high temperature was observed,and the thermal conductivity test and DSC analysis were carried out.The results show that from 0% to 3%,the thermal conductivity of the composite phase change material increases with the increase of the proportion of expanded graphite.When the proportion of expanded graphite is 4%,the composite phase change material contains more pores,which is not conducive to the thermal conductivity improvement.Compared with pure paraffin,the thermal conductivity of expanded graphite/paraffin composite phase change materials containing 3% expanded graphite increases by seven times,and the phase transition temperature and phase change latent heat are slightly reduced,and the reduction ratio of phase change latent heat is equivalent to the addition ratio of expanded graphite.Secondly,the melting performance of the prepared expanded graphite/paraffin composite phase change materials containing 3% expanded graphite and pure paraffin was tested,and the effect of heating temperature on the melting process of two kinds of phase change materials was studied.A phase change material melting performance test system was built.Thermocouples were arranged at different positions of the energy storage unit cross section to measure the temperature change during the melting process,and the weighting method was used to calculate the melting fraction during the phase change process.The results show that the phase change material located in the lower part of the phase change energy storage unit melts slowly during the melting of pure paraffin,while the phase change material in all directions of the phase change energy storage unit melts quickly during the melting process of expanded graphite/paraffin composite phase change materials containing 3% expanded graphite.Therefore,at a heating temperature of 80°C,the melting process of expanded graphite/paraffin composite phase change materials containing 3% expanded graphite is 78.16%shorter than that of pure paraffin.As the heating temperature decreases,the complete melting time of both phase change materials increases.However,the increase of pure paraffin is greater,which shows that the addition of expanded graphite can reduce the effect of heating temperature on the melting performance of paraffin.Then,based on the results of experimental research,the effect of natural convection and thermal conductivity on melting and solidification performance was explored by numerical simulation.According to the two heat transfer methods of convection-dominated and heat conduction-dominated during the melting processes,phase change materials are divided into convection type and heat conduction type.The numerical simulation methods of the phase change process of convection type and heat conduction type phase change materials were established.The melting/solidification processes of the two kinds of phase change materials were simulated.The results show that the existence of natural convection can shorten the complete melting time of phase change materials,but will increase the complete solidification time.When natural convection is not considered,the higher the thermal conductivity,the faster the melting/solidification rate of phase change material.But its melting/solidification form isn't changed.When the thermal conductivity increases by 29%,the problem of a longer melting process caused by the lack of natural convection can be solved.Finally,the effects of the size parameters and the number of inner tubes on the phase change performance of different phase change materials were studied.The research is based on two types of phase change materials represented by pure paraffin(the convection type phase change material)and expanded graphite/paraffin composite phase change materials containing 3% expanded graphite(the heat conduction enhanced type phase change material).The phase change process of two types of phase change materials is numerically simulated when the size and number of inner tubes of energy storage unit are increased and the amount of the phase change material is fixed.The results show that increasing the size of the energy storage unit can shorten the phase change process of the convection type and heat conduction enhanced type phase change materials.And the size change is more beneficial to the phase change process where heat conduction is dominant.The optimal number of inner tubes for the convection type phase change material during melting and solidification is different,while the optimal number of inner tubes for the heat conduction enhanced type phase change material under different conditions is the same.