Research On Optimization And Application Of Heat Storage And Release Performance Of Sodium Acetate Trihydrate Composite Phase Change Material

Nowadays,energy storage technology is one of the most demanded technologies responding to China's policy of"electric energy substitution"and the power grid operation of“shift electric demand from peak period to valley period”,for solving the mismatch of power supply and demand in time and space.Here it's aim is to develop one kind of phase change heat storage material for heating in winter in northern cities.The valley and low-cost electricity in night was converted into heat energy and stored in the phase change material,and then used in the peak period of electricity and heat demanding in the daytime.It's not only benefit to balance the electricity supply of peak and valley throughout the day,but conducive to heat economically in winter.Also,The phase change material can be appropriate to collect the heat from solar energy during the day.Sodium acetate trihydrate(SAT)is extensively studied among the inorganic phase change materials,with the advantages of high latent heat,chemical stability,no corrosion and low-cost.But it has few obvious problems,including the insurmountable super-cooling,phase separation,and low thermal conductivity,thus the heat storage and heat release performance of SAT are limited.Therefore,the primary task in this work is to prepare different heat storage systems by adding nucleating agents and suspending agents into SAT,then the best additive formula was determined comparatively.Based on that,the brine mixture of SAT was absorbed into the micropore of the porous medium expanded vermiculite(EV)to consist the compound phase material,then the storage and release heat performance of compound SAT was optimized.The main research contents and conclusions are as follows:Firstly,the borax and disodium Hydrogen Phosphate dodecahydrate were selected as nucleating agents,sodium carboxymethyl cellulose and polyacrylamide as two suspending agents as well.They were in pairs added into SAT to prepare four heat storage systems.Then the thermal performance and thermal cycling stability of these four systems were analyzed.The results showed that it was effectual to promote the nucleation and crystallization of SAT by added 2wt%borax and 1wt%polyacrylamide into SAT,and the supercooling was stabilized to about 3?.The thermal performance test presented that the solidification curves of multiple thermal cycles showed good thermal cycle stability.With the thermal cycles increasing,the latent heat had a significant attenuation though the initial value was high.Secondly,the material was based on the brine mixture of SAT adsorbed in micro-porous EV.SAT was mechanically mixed with 2wt%borax and vacuum impregnated was applied to load the SAT mixture in different proportions of EV.It was determined that the absorptive capacity of the brine mixture of SAT in EV is up to600wt%,to form a new composite phase change material(CPCM).The components and structure of CPCM were characterized by BET,SEM,XRD and microcomputer differential thermal balance.And the stability of CPCM in multiple thermal cycles was verified by a thermal cycle test system in lab.The serious test results showed that the BET surface area of EV was 17.092 m~2/g and the pore volume was 0.073 cm~3/g.The SEM images showed that numerous micro-structure in EV provided space for phase change of SAT,which played a decisive role in improving the phase separation problem.Concurrently,the SAT was filled in the layered micro-pores of EV uniformly and the filling rate was over 95%.XRD tests indicated good chemical compatibility among the different components of the CPCM.DSC-TG tests showed that the phase change temperature and latent heat of the CPCM were 57.6°C and 238.84 kJ/kg respectively,and the weight loss was very little during the melting-solidification process.All these made clear that CPCM still had a high heat storage capacity and stable phase change temperature.Finally,it was found that the specific heat capacity of CPCM was increased after SAT loading into EV,thereby greatly improving the sensible heat storage capacity and total heat storage capacity of CPCM.And the thermal conductivity was higher than that of each component in the compound.It was worth noting that the super-cooling degree was stable at 2°C after several melting-solidification cycles.The thermal performance test showed that the CPCM was stable over 150 cycles.Then the XRD pattern of the diffraction peaks after 150 cycles were consistent with that of first cycle,so the physical properties did not change.It is fully proved that CPCM has great stability in long-term melting-solidification cycles.It will provid experimental support and theoretical guidance for the practical application of heat storage technology by this study.