Study On Preparation And Performance Of Composite Phase Change Materials Based On Sodium Acetate Trihydrate For Thermal Energy Storage

With the increasing demand for energy and the exhaustion of nonrenewable resources,the development and utilization of renewable energy has become a research focus.Solar energy,owing to its environmental friendliness and sustainable utilization,has received widespread attention.However,most renewable energy sources,such as solar are unstable,which cannot guarantee an all-weather energy supply.The phase change thermal storage technology can realize the "peak load shaving" of energy,and effectively solve the contradiction between the mismatch between energy supply and demand.Phase change material(PCM)is an actuating medium for latent heat storage technology,sodium acetate trihydrate(SAT)is regarded as the most promising candidate for middle-low temperature solar thermal energy storage owing to its large latent heat of phase transformation and suitable phase change temperature.However,SAT also has a series of obstacles such as supercooling,phase separation,and low thermal conductivity.In this paper,we aim to solve the above-mentioned defects and prepare composite phase change thermal storage materials with good performance by adding nucleating agents,porous materials and thickeners.The research contents are as follows:The supercooling degree of SAT was measured by temperature cycling curves and differential scanning calorimetry,and the temperature was reduced to-80 °C without energy release.The improvement effect of a large number of nucleating agents on SAT supercooling was tested,and it was found that the nucleating agent nano-SiC can effectively reduce the supercooling degree of SAT.In order to improve the phase separation phenomenon,the improvement effect of various thickeners on the phase separation of SAT was tested through the storage/ release cycle test.It was found that sodium carboxymethyl cellulose(CMC)could effectively inhibit the phase separation phenomenon of SAT,but could not improve the supercooling of SAT.By adding expanded graphite(EG)to the SAT/SiC composite PCM,a composite PCM with good heat transfer-heat storage performance was prepared.The thermal conductivity of composite PCM increased with the increase of EG addition.The composite PCM was subjected to 200 heat storage/ release cycles,and the composite PCM with 2 wt.% EG always had a small degree of supercooling,no phase separation and a large latent heat of melting and solidification.The effects of SiC and EG on the supercooling and phase separation phenomena of composite PCM were analyzed,and the principle of SiC promoting SAT nucleation is explained according to the principle of heterogeneous nucleation.Some nucleating agents have poor compatibility with PCMs,and a series of experiments are designed to improve the stability of composite PCMs.First,the hydrophilicity of the nucleating agent SiC was improved,and its dispersibility,surface wettability and improvement effect on the supercooling phenomenon of SAT were tested.Then,the optimized modified SiC,high-quality thickener and SAT were compounded by a comprehensive blending method to prepare a composite PCM with good comprehensive properties,and its phase separation,supercooling,microstructure,chemical composition,Chemical compatibility and thermal storage properties were characterized in detail.In this paper,two SAT-based composite phase change thermal storage materials were prepared,which not only effectively solved the problems of supercooling and phase separation,but also ensured that the composite PCM had good thermal conductivity,heat storage capacity,cycle stability and economy,so they have extensive application prospects in middle-low temperature solar thermal energy storage.