Study On Simultaneous Optimization Of Super-cooling And Heat Transfer Performance Of Sodium Acetate Trihydrate

Thermal energy storage technology,as an effective method to improve energy use efficiency,mainly uses dielectric materials to store other energies such as waste heat and low-grade thermal energy.Improving the efficiency of energy utilization has been examined as an approach to solving the mismatch problem between energy supply and demand in time and space.Presently,the main thermal energy storage methods are latent heat storage,sensible heat storage and chemical heat storage.However,phase-change energy storage has been widely studied because of its high energy storage density,high phase-change latent heat,easy control and approximate thermostatic characteristic of the phase-change process.The core of phase-change energy storage technology is the phase-change material?PCM?.PCMs can be divided into organic,inorganic and mixed PCMs according to their composition.Among them,inorganic PCMs are widely used in various energy conservation fields,which include solar energy,industry,agriculture and building because of their low cost,wide availability,moderate operating temperatures and high latent heat.Sodium acetate trihydrate?CH₃COONa·3H₂O,SAT?,which is an excellent inorganic PCM,is characterized by a suitable phase-change temperature,high energy storage density,stable chemical performance,and relatively low cost.However,SAT also exhibits some unfavorable properties,including phase segregation and strong supercooling,which severely limit its application.Therefore,methods to effectively improve the supercooling,phase separation and thermal conductivity of SAT are a focus of current research.The specific work is as follows:?1?Firstly,the phase change characteristics of pure SAT during the melting-freezing process were studied by step-cooling curve method.The results show that after SAT was melted completely,it cannot solidify to release latent heat even it is cooled to room temperature at ambient temperature,which indicates that it has a strong undercooling tendency.Secondly,in this paper,several materials proposed in the literature were selected as nucleating agents to improve the supercooling of SAT such as Na₂HPO₄·12H₂O,Na₂SiO₃·9H₂O,Na₄P₂O₇·10H₂O and Nano-Cu.Among them,according to a large number of literature reports,CMC was selected as thickener to improve the phase separation of SAT.The obtained results show that the supercooling and phase separation problems of inorganic hydrated salt nucleating agent and the chemical properties of Nano-Cu which can easily be oxidized in air make them lose their stable nucleation effect in SAT composite phase system.?2?According to the similarity of crystal structure between SAT and nanomaterials,three kinds of nano-metal oxides?Fe₃O₄,?-Fe₂O₃,YSZ?and CMC were respectively selected as nucleating agent and thickener agent of SAT to simultaneously improve its problems of supercooling and phase separation.The obtained results indicate that the supercooling of SAT can be controlled in a small range?0-0.1??by adding three kinds of nanomaterials as nucleating agents,and all of them have certain effect on enhancing the thermal conductivity of composite PCMs?0.92%-22.5%?.Besides,the phase structure of each composite PCM was characterized by XRD.The obtained XRD results revealed the absence of a chemical reaction between SAT and nucleating agents?Fe₃O₄,?-Fe₂O₃,YSZ?.Finally,the composite PCMs still maintain good phase transition characteristics and cyclic stability after 50 melting-freezing cycles,indicating that the three nano-metal oxides are effective and stable nucleating agents.?3?Copper foam was selected as matrix of enhancing heat transfer to investigate the effect of copper foam porosities on the supercooling and thermal conductivity of copper foam/YSZ/SAT composite PCMs.The results show that the thermal conductivity of copper foam/YSZ/SAT composite PCMs increases with the increase of porosity,and it is 5.9-9.4times higher than that of pure SAT.Here,the supercooling degree of the composite PCMs is1.45-2.85?,and the volume energy storage density is as high as 448.6-473.0 MJ/m³,which is 2.09-2.20 times of water.Finally,the supercooling of composite phase change material is still within the range of 1.9-3.3?after 20 melting-freezing cycles.Therefore,the composite phase change system not only has excellent thermal conductivity,but also has stable phase change property.Firstly,the nucleation effect and stability of inorganic hydrated salt and nano-metal elemental nucleating agent were studied in this paper.Secondly,according to the similarity of crystal structure between SAT and nanomaterials,three kinds of nano-metal oxides?Fe₃O₄,?-Fe₂O₃,YSZ?were selected as nucleating agent of SAT to investigate their contents on the effect of supercooling and thermal conductivity.Finally,YSZ was selected as nucleating agent of SAT by analysis and comparison,and copper foam was selected as matrix of enhancing heat transfer to investigate the cooperative optimization effect on supercooling and thermal conductivity of SAT.Meanwhile,the great potential of copper foam/YSZ/SAT composite PCM in engineering application is obtained by comparison and analysis.