Preparation,Construction And Properties Of NaNO₃-KNO₃ Based Composite Thermal Storage Materials

With the rapid development of economic,the fossil energy will be exhausted near future,leading a serious of environment problems.Solar energy is clean and renewable which has been became the research hotspot in our society.Nitrate salt is used in solar plant,waste heat recovery and other middle-high temperature thermal storage fields for low cost and high thermal storage density.However,there are some disadvantages such as leakage and low thermal conductivity,which limit the application.Binary nitrate salt（Solar Salt,NaNO3:KNO3=6:4）is selected in the paper.The preparation methods and thermophysical properties of composite phase change materials are analysised,the leakage problem is fixed.Meanwhile,the heat capacity,thermal conductivity and thermal stability are improved,respectively.The melting temperature and enthalpy of binary nitrate salts were 211.7℃and119.1 J/g.The form-stable composite materials of expanded vermiculite-nitrate salt and expanded perlite-nitrate salt were prepared by direct physical adsorption method,respectively.The morphology,structure and thermophysical were investigated by SEM,XRD,FT-IR,DSC and other test equipments.The results indicated that expanded vermiculite and expanded perlite were three-dimensional porous structure.They have good physical compatibility and thermal stability.The saturated adsorption capacity of expanded vermiculite to nitrate salt was about 750%,and the adsorption capacity of form-stable expanded perlite was related to the curing temperature.With the curing temperature increased,the adsorption capacity first increased and then decreased,the maximum adsorption was 114.07%at 500℃curing temperature.Meanwhile,the thermal conductivity of composite materials were 0.62 and 0.76W/（m·K）,respectively.The enthalpies of expanded vermiculite-nitrate salt composite materials decreased 4.43%,and the expanded perlite-nitrate salt composite materials reduced 3.69%after 700 times thermal cycling.The form-stable composite phase change materials had god thermal stability with the porous silicate materials as adsorption medium.Nitrate salt/SiO₂ form-stable composite phase change materials were prepared by sol-gel method.The optimum parameters followed:the reaction temperature was60℃,the concentration of sodium silicate solution was 1.0mol/L,and pH=7.The results showed that main elements of the form-stable composite phase change materials were O,Si,Na,K,N and Cl,which had good thermal stability.The thermal conductivity of composite materials in the range of 1.37 and 1.94 W/（m·K）.Meanwhile,the mass loss was less than 5%below 500℃and began decomposition at 600℃.The influence of nano SiO₂ particles for binary nitrate salts was investigated in the paper.The results indicated that the heat capacity first increased and then decreased with the content of nano SiO₂.When the content of nano SiO₂ was 1 wt.%,the solid capacity and liquid capacity of nitrate salts with 30 nm were increase 7.35%and 45.92%.Further,the solid capacity and liquid capacity of nitrate salts with 50nm were increase 6.95%and 43.65%.The adsorption theory mechanism and quantum were used to analysised the phenomenon.The reason was the nano SiO₂particles existed quantity of unsaturated bonds which made the surface of Gibbs free energy increased.Therefore,the heat capacity was increased.The thermal stability of binary nitrate salts was investigated by NaCl,Na₂CO₃and NaOH,respectively.Different content of NaCl,Na₂CO₃ and NaOH were added into the binary nitrate salts.The results indicated that NaCl,Na₂CO₃ and NaOH have different effects on thermal stability of binary nitrate salts.The thermal stability was improved with the content of NaCl.Once the content exceed 5 wt.%,NaCl could improve the thermal stability obvious.Nevertheless,Na2CO3 reduced the thermal stability of binary nitrate salts.The main reason for the polarization effect and the activation energy of NaCl,Na₂CO₃ were different,behaving the different thermal stability property.Meanwhile,316L,310S and 304 stainless-steel,45 and 20 carbon-steel,cast iron,brass and copper,were selected in this paper.The compatibilities of several different metal materials were investigated.The results showed the corrosion resistant ability of metallic materials order:316L>310S>304>45>20>brass>iron>copper.