Study On Energy Storage Characteristics And Application Of Functional Composite Phase Change Materials With Electrothermal Conversion Function

With the development of economy and the acceleration of human industrialization,the global energy consumption increases year by year.Building energy consumption accounts for about 1/3 of the total social energy consumption,and air conditioning accounts for about 60%of the total building energy consumption.Therefore,energy conservation and emission reduction are urgent,but it also brings new opportunities for the development of energy storage industry.Energy storage technology has been paid more and more attention at home and abroad because it can adjust the imbalance of energy supply and demand and promote the rational application of energy.Among different thermal energy storage methods,phase change energy storage has been widely studied and become a research hotspot in the field of new energy because of the high energy storage density,keeping the temperature constant in the heat storage/exothermic process.Solar energy,geothermal energy and industrial waste heat are limited by external factors such as time,geography and climate,so the versatility of thermal energy storage as phase change materials(PCMs)is poor.As clean,easily controlled and converted energy form,electric energy has become efficient heat source for thermal energy storage of PCMs.In addition,the use of phase change energy storage materials can also solve the contradiction between peak and valley power supply.In order to better realize the management of electric energy and thermal energy conversion,it is urgent for energy storage industry to develop high energy storage density,shaping and electrothermal transformation PCMs.Polyethylene glycol polymer(PEG)is a kind of representative organic PCMs,which has been widely studied for its high energy storage density(150?200 k J/kg),adjustable phase change temperature(20?70?),strong renewability,high chemical/thermal stability,low price and environmental friendliness.However,PEG are solid-liquid PCMs,which involves liquid leakage in the application process.It is difficult to be applied in the field of electrothermal conversion for the poor of electronic conductivity(10^-12?10^-7 S/cm)and thermal conductivity(0.1?0.5W/m?K).In view of these inherent defects of PEG,PEG-CaCl₂ shaped composite PCMs are successfully prepared by using the coordination properties of Ca²⁺and hydroxy(-OH)or ether(-O-).Secondly,carbon materials(CNTs,EG)are used as the conductive matrix to improve the conductivity and thermal conductivity of PEG-CaCl₂ composites.Finally,a new temperature control method of phase change device for office building put forward by using the self-made electrothermal transformation formalized composite PCMs.The temperature control performance of phase change unit to the indoor environment of office building is studied.Based on the established model of phase change temperature control system,the influence of different factors on the temperature control process is analyzed.The optimal results for office building projects are proposed.(1)Research on thermophysical properties of pure PEG:Based the law of material thermodynamics,the thermodynamic parameters and thermal storage properties of PEG PCMs are studied.The experiment results show that PEG only melt during the heating process,accompanied by heat storage in the test temperature range,which is typical first-order phase transition;otherwise,the heat is released.For the PEG PCMs(H(OCH₂CH₂)_nOH),the phase change temperature and latent heat increase with the degree of polymerization(n).In order to widen the phase change temperature of single PEG,PEG2000/PEG400 binary eutectic was studied.(2)Study on the structure shape of PEG:A novel shaped composite PCMs(PEG-CaCl₂)was synthesized by coordination chemistry and coordination substitution technology.The basic principle of shaped composite is explored.During the in-situ generation of PEG-CaCl₂complexes,they constructed each other to form a three-dimensional porous inert skeleton.The complex skeleton wraps the excess PEG PCMs,and finally formed the gel structure,which can shape the PEG.During the synthesis process,the complex is not in contact with air,and the complex is surrounded by liquid,so the air will not enter the composite.It overcomes the shortcomings of the traditional injection method to prepare shaped composite PCMs.(3)Study on the enhancement of thermal conductivity of PEG:A novel functional composite PCMs is synthesized by one-step ligand replacement method with PEG as PCMs,high thermal conductivity carbon nanotubes(CNTs)as thermal conductivity framework material and CaCl₂ as coordination body.The mechanism of thermal conductivity enhancement of CNTs is discussed.The effect of CNTs content on the thermophysical properties and thermal conductivity of PEG-CaCl₂ complex is studied.The results show that the phase change temperature of the composite PCMs is shifted due to the addition of CNTs,the phase change latent heat decreases with the increase of CNTs content,and the thermal conductivity increases with the increase of CNTs content.(4)Study on electrical conductivity of PEG:Based on the low conductivity PEG,a novel functional composite PCMs(PEG-CaCl₂/EG)is synthesized by one-step synthesis of EG,PEG and CaCl₂.The interaction mechanism between the phase change behavior of the functional composite PCMs and the thermal energy storage heating was discussed,and the reproducibility of conductive and heat-conducting PCMs in long-term use was evaluated.The experimental results show that the addition of EG improves the conductivity and thermal conductivity of PEG-CaCl₂ complex;the resistivity of functional composite PCMs decreases with the increase of EG content,and the heating rate increases with the increase of EG content.With the increase of the operating voltage,the temperature of the functional composite phase change material increases faster and faster.(5)Study on the electrothermal conversion performance of PEG:The resistivity,conductivity and stability of the phase change element are measured by experiments.Based on the analysis of the influencing factors of the heat storage rate of phase change plate(PCP),the temperature rise characteristics of the phase change unit were studied.The resistance value and the heat storage time are different for different faces of the same phase change element.PCP structure optimized resistance(4.7?)than the maximum resistance increased 1.8?before optimization,the heat storage time is saved by 3.7 min,and the electrothermal conversion efficiency is about 20%higher than before optimization.(6)Research on temperature control characteristics of electrothermal conversion functional PCMs:The temperature control characteristics of phase change devices applied in office buildings are explored.Based on the temperature control system of PCPs,the influence of the number,the structure,the phase change temperature,the placement of PCPs and the heating power on the room temperature control is studied.In order to reduce the cost,the size,quantity and phase change temperature of PCPs are optimized according to the requirements of environment and temperature control.On this basis,the paper puts forward the configuration strategy of phase change temperature control system for office buildings,which provides reference and basis for the application of temperature control methods and system establishment in practical projects.