The Preparation,Thermal Performances,and Functionalization Of Composite Macro-encapsulated Phase Change Materials Based On Eutectic Hydrated Mixtures

To improve energy utilization efficiency,balance power load,utilize green solar energy efficiently,and reduce carbon emissions,many scholars have carried out in-depth research on the integration of phase change materials（PCM）into radiant floor heating systems（RFHS）or solar-coupled RFHS.However,it is found that the following problems existing in PCM need to be solved urgently:（a）the limitation of PCM selection（preparing the eutectic PCM to meet the requirement of phase-transition temperature）;（b）the leakage problem of PCM in the solid-liquid phase transition（achieving permanent shape-stabilization of PCM）;（c）the enhancement of heat transfer and photothermal conversion abilities of PCM（enhancing heat transfer ability and efficiently utilizing solar energy of PCM）.Given the above problems and taking the electric heating-typed RFHS or coupled solar floor RFHS as the background,this paper aims to develop a new type of inorganic-inorganic or inorganic-organic eutectic hydrated PCM,to carry out research on the permanent shape-stabilization and photothermal conversion enhancement of novel composite PCMs,and to discuss their application possibility in RFHS.The specific research work and conclusions are as follows:（1）Firstly,the optimal SAT-STP eutectic hydrated salt system with high phase-transition enthalpy（ΔH）and suitable phase-transition temperature（T_m）was obtained by adjusting the ratios of sodium acetate trihydrate（CH₃COONa·3H₂O,SAT）and sodium thiosulfate pentahydrate（Na₂S₂O₃·5H₂O,STP）as the main components.After that,the optimal SAT-STP PCM with excellent thermal properties and low super-cooling degree（ΔT）was prepared by adding different contents of strontium chloride hexahydrate（Sr Cl₂·6H₂O）as the nucleating agent.Secondly,the carbonized Melamine sponge（CMS）was selected as the carrier of PCM whose carbonization temperature was optimized based on its morphology and mechanical strength.Then,the melted SAT-STP PCM was adsorbed into CMS to prepare semi-shape-stabilized SAT-STP PCM/CMS.Finally,the PCM/CMS@MPU macro-encapsulated PCM was prepared by using respectively PCM/CMS and Si C modified Polyurethane resin（MPU）as core and shell.The permanent shape-stabilization of PCM/CMS@MPU was verified by its heat transfer performance,thermal stability,and thermal reliability.DSC results showed that,with the optimal ratio of 28%SAT-72%STP,the SAT-STP binary eutectic salt had an appropriate T_m（41.45℃）and highΔH(211.9 J?g^-1).The step-cooling curves indicated that theΔT of PCM decreased to 0.462℃by adding 2%Sr Cl₂·6H₂O nucleating agent.Under the optimal carbonation temperature of 400℃,the CMS displayed high thermal conductivity and excellent adsorption capacity for PCM.DSC,FT-IR,and XRD results indicated that CMS had little influence on the thermal performance of PCM/CMS,and CMS composited with PCM physically.Adding Si C increased the thermal conductivity of Polyurethane resin（PU）greatly,and the obtained MPU showed better compressive strength,tensile modulus,and elongation at break.The thermal conductivity of PCM/CMS and MPU in PCM/CMS@MPU increased to0.8077 and 0.8699 W?m^-1?K^-1,and PCM/CMS@MPU displayed superior heat transfer performance during heating.Leakage test found that PCM/CMS@MPU with 2.5 mm thickness MPU did not leak after heating at 60℃for 24 h,furthermore,XRD and DSC results indicated that PCM/CMS in macro-encapsulated PCM had consistent phase-transition peaks,ΔT andΔH compared with those before heating.After 200 heating-cooling thermal cycles,the results of step-cooling and DSC curves showed that PCM/CMS@MPU had good thermal reliability because the thermal properties of PCM/CMS inside it had little fluctuation,indicating that the novel PCM/CMS@MPU can be well applied as a phase-transition energy storage medium to the electric heating-typed RFHS.（2）Firstly,the optimal APSD-OAD eutectic hydrated system with highΔH and suitable T_m was obtained by adjusting the ratios of aluminum potassium sulfate dodecahydrate（KAl（SO₄）₂·12H₂O,APSD）and oxalic acid dihydrate（C₂H₂O₄?2H₂O,OAD）as the main components.After that,the optimal APSD-OAD PCM with excellent thermal properties and lowΔT was prepared by adding different contents of boric acid（H₃BO₃）as the nucleating agent.Secondly,the modified PUS（MPUS）was prepared by in-situ growth and deposition of Cu S particles on Polyurethane sponges（PUS）,and its microstructure,crystal structure,element composition,and distribution were analyzed.Then,the melted APSD-OAD PCM was physically adsorbed into MPUS to prepare semi-shape-stabilized APSD-OAD PCM/MPUS.The thermal properties,microstructure,crystal structure,and element distribution of PCM/MPUS were analyzed.Finally,the modified silicon resin（MSR）with excellent mechanical properties,thermal conductivity and photothermal conversion ability was prepared by adding Si C and Cu S particles as modifiers into silicon resin（SR）.Then,the PCM/MPUS@MSR macro-encapsulated PCM was prepared by respectively selecting PCM/MPUS and MSR as core and shell,and its photothermal conversion performance,heat transfer performance,thermal stability,and thermal reliability were investigated.DSC results showed that,with the optimal ratio of 75%APSD-25%OAD,the APSD-OAD binary eutectic mixture had an appropriate T_m（49.43℃）and highΔH(232.6 J?g^-1).The step-cooling curves showed that theΔT of APSD-OAD PCM decreased to 1.082℃by adding 2.5%H₃BO₃nucleating agent.XRD and XPS spectra illustrated that Cu S particles in situ grew on PUS,and the Cu S clusters could be observed to deposit on the skeleton of PUS in SEM images.DSC curves showed that PCM/MPUS had higherΔH(166.4 J?g^-1)and suitable T_m（46.98℃）.The result of the thermal conductivity test showed that the thermal conductivity of PCM/MPUS and MSR inside PCM/MPUS@MSR increased to 0.6682 and 0.5334 W?m^-1?K^-1,respectively,and PCM/MPUS@MSR displayed superior heat transfer performance during heating.The result of UV-vis-NIR light absorption spectra showed that the addition of Cu S particles enhanced the absorption ability of PCM/MPUS to the light at 500-1700 nm wavelength,and the photothermal conversion efficiency of PCM/MPUS@MSR was 51.25%.Due to the addition of Cu S and Si C,MSR had excellent mechanical properties.Leakage test found that PCM/MPUS@MSR with2.0 mm thickness MSR did not leak after heating at 60℃for 24 h,furthermore,XRD and DSC results indicated that APSD-OAD PCM/MPUS in macro-encapsulated PCM had consistent phase-transition peaks,ΔT andΔH compared with those before heating.After 200 heating-cooling thermal cycles,the results of step-cooling and DSC curves showed that PCM/MPUS@MSR had good thermal reliability because the thermal properties of PCM/MPUS inside it had little fluctuation,indicating the novel PCM/MPUS@MSR can be well applied as an energy storage medium on solar-coupled RFHS.（3）The prepared SAT-STP PCM/CMS@MPU macro-encapsulated PCM was applied as the energy storage medium for the electric heating-typed RFHS,and the heat transfer condition of the system was analyzed by experiment and simulation.Firstly,taking two rooms with（Room 1）and without（Room 2）macro-encapsulated PCM as comparison rooms,the thermal comfort duration ratios in the two rooms were explored experimentally under the same heating conditions.Secondly,a 3D unsteady heat transfer model matching Room 1 was established and modified by experimental data.Thirdly,the thermal conductivity of the core-shell structure in macro-encapsulated PCM was orthogonally optimized,taking economy and comfort as indexes.Finally,the optimal heating power of the electric heater under different ambient temperatures was carried out in the room（Room 3）with optimal thermal conductivities of the core-shell structure in macro-encapsulated PCM.The experiment results showed that,after heating for 3h,the temperatures of the floor surface and air in Room 1 reached 31.51 and 26.86℃,while those in Room 2 reached 47.45 and 39.79℃,respectively,far from the human thermal comfort requirements in winter.In the cooling stage,the temperatures in Room 1 kept in thermal comfort requirements for 17 h while those in Room 2 kept only for 9 h,indicating that Room 1 had a longer thermal comfort time and higher thermal inertia.The result of the simulation showed that Room 1 had a longer thermal comfort time（80.88%）and a lower daily electricity cost(0.2595 CNY·day^-1)compared with Room 2.The result of optimization on thermal conductivity indicated that Room 3 had a lowest c?(0.2480 CNY·day^-1)and longer thermal comfort time（85.47%）when thermal conductivities of its core and shell structure were respectively 1.0 and0.25 W·m^-1·K^-1.When the outside temperatures were respectively-10、-5、0、5、10℃,Room3 with 0.15 k W heating power had the best economy.The above results show that the newly prepared macro-encapsulated PCM as a heat storage medium in RFHS has excellent economy and human thermal comfort.