| Phase change thermal storage technology can overcome the problems of time intermittence,spatial dispersion and strength instability in the conversion and utilization of different energy forms.It has a wide application prospect in the fields of medium and low temperature thermal energy,such as renewable energy,industrial waste heat recovery and building energy conservation.The application of this technology is mainly affected by the thermal properties of the phase change material(PCM)and the heat transfer performance of the heat storage device.In order to improve the thermal performance of phase change heat storage technology,this paper uses expanded graphite(EG)as a porous matrix to prepare a new hybrid carbon-based composite phase change material(CPCM).In addition,theoretical and experimental studies were carried out on the changes of the thermal properties of CPCM,the thermal properties of the packed bed with spherical packaging heat storage materials,and the optimization design method of the thermal properties in practice.Firstly,in this paper,natural flake graphite was used as raw material to prepare EG by chemical oxidation and microwave expansion.The pore structure of EG was characterized.The influence of preparation conditions on the expansion characteristics was analyzed.The effect of EG on the adsorption properties and heat storage properties of palmitic acid was studied.The results showed that the vermicular EG had abundant multistage pore structure,and the average cumulative specific surface area and pore volume were 32.43m2/g and 0.14m3/g respectively,and the pore body was mesoporous structure.The influence of the main preparation conditions on the expansion volume was revealed by single factor experiment,and the optimal preparation conditions were obtained by orthogonal experiment.The experimental results of adsorption characteristics and charging/discharging performance show that the pore structure of EG can effectively limit the fluidity of liquid PCM,and shorten the charging/discharging time according to the filling ratio.The comprehensive comparison shows that it is a suitable composite ratio when the content of EG is 10%.Secondly,a new type of CPCM based on CF was proposed to enhance thermal conductivity.The microstructure and compatibility of CPCM were analyzed,and the phase change characteristics,thermal decomposition stability and thermal conductivity were characterized and tested.The results show that EG and CF construct a new network structure,which is a key factor affecting the change of thermal properties.As the proportion of CF increases,the phase change enthalpy of CPCM decreases and the melting/solidification peak temperature decreases and increases,respectively.The results of thermal decomposition stability analysis show that CF can inhibit the thermal decomposition of PCM in the pores of EG.In terms of thermal conductivity,the increase of CF can further strengthen the thermal conductivity,which increases by 27.3%when the content is 10%.At the same time,the charging/discharging time is shortened by obviously.Finally,the thermal cycle experiment proves that the new CPCM has good thermal cycling stability.Thirdly,in order to explore the heat storage characteristics of CPCM in the packed bed,this chapter designed a packed bed heat storage device based on the spherical encapsulated unit,and built a heat storage performance experimental platform.The results show that when the carbon content is low,CPCM still has the characteristics of three-stage temperature change in the process of charging/discharging.When the fluid temperature increases by 10?in the low-temperature heating zone,the charging time is shortened,the effective charging heat capacity is increased,the average charging rate and the charging efficiency are improved,and the initial temperature increase in the discharging process extends the discharging time,and the total efficiency is increased by 7.8%.When the flow rate increases from 60L/h to 120L/h,the average charging/discharging rate and charging/discharging efficiency both increases,and the total efficiency increases by 13.9%.By comparing the changes of the heat storage characteristics of the packed bed filled with the three kinds of heat storage materials,it is shown that the total efficiency of filling the mixed carbon-based materials increased by 3.4%and 11.4%respectively compared with filling the other two materials.Fourth,based on the porous media method,a mathematical model was established for the heat transfer process of the packed bed heat storage device and the finite difference method was used to discrete control equation.The reliability of the model was verified by comparison with the experimental data,and the model is used to predict the evolution law of the internal temperature of the packed bed.The results show that Reynolds improves the heat storage density mainly by affecting the convective heat resistance outside the heat storage sphere,meanwhile,friction resistance loss and environmental heat loss caused by the fluid flow reduce the exergy efficiency.Stefan number has the greatest influence on various indicators.As Stefan number increases,the heat storage density increases but the exergy efficiency decreases,and the increasing effect of the exergy charging rate is higher than the charging rate;The increase of keff,PCM can significantly increase the charging rate by reducing the heat resistance inside the ball,but when keff,pcM>4.5W/m.K,the change rate of the charging rate decreases,and the increase of keff,pcm has a more obvious effect on the charging rate than the exergy charging rate.The increase of Dc/D has a negative impact on the improvement of all indexes.Finally,taking the heat storage device in the actual solar heating system as the optimization object,using the established numerical model,the response surface method is used to carry out multi-objective optimization analysis on the research object.Taking effective charging time,effective charging capacity and exergy efficiency as the joint optimization goals,and 9 parameters of material thermal property,structural parameters and operational parameters were taken as influencing factors to establish a regression model.The accuracy of the model was verified by ANOVA,and the parameter combination with the strongest interaction influence on the evaluation index and its influence rule were obtained by comparing the significance coefficient.The design parameters were optimized,and the diameter of the encapsulated ball and porosity increased by 12.0%and 22.5%,respectively.HTF specific heat capacity and thermal conductivity are 13.3%and 5.6 times higher than the baseline value respectively.The thermal conductivity of PCM was increased by 5.7 times,and the latent heat of PCM decreased by 3.4%.The method and results provide theoretical support for the optimal design of heat storage device. |
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