Preparation And Properties Of Hydrated Salt Phase Change Energy Storage Materials

The rapid development of society's economy has led to a sharp increase in energy consumption,while environmental problems have become increasingly serious.The development of environmentally friendly new energy sources,energy storage materials and improvement of energy efficiency have become important methods to solve these problems.In energy storage technology,Phase Change Materials(PCMs)store/release heat through phase change and adjust the ambient temperature,thereby increasing the utilization rate of heat energy and saving energy consumption.At present,PCMs are mainly used in the fields of solar energy utilization,industrial waste heat recovery,building energy saving,and cross-season heat storage and heating.At present,there are few studies on hydrated salt phase change materials are used in the field of medium and low temperature and most of them are unit systems.Therefore,this paper selects Sodium acetate trihydrate(SAT),Sodium thiosulfate pentahydrate(STP),sodium carbonate decahydrate(SCD),and disodium hydrogen phosphate dodecahydrate(DHPD)with suitable phase transition temperature and large phase change latent heat as the main raw materials to prepare binary eutectic salt phase change materials STP-SAT and SCD-DHPD.Their performance are studied.Aiming at the problems of large subcooling,phase separation and poor thermal conductivity that exist when the binary eutectic salt phase change material undergoes a solid-liquid phase change,this paper adopt the method of adding a nucleating agent and a thickening agent to prepare these.The composite phase change energy storage materials effectively solve the problems of large subcooling and obvious phase separation,and at the same time significantly improve the thermal conductivity of the material.The main contents of the research are as follows:(1)Sodium acetate trihydrate-sodium thiosulfate pentahydrate system:The STP-SAT binary system hydrated salt phase change energy storage material was prepared by melt blending method.The equilibrium phase diagram of the system was studied by the DSC and Schrader equation theoretical calculation.The thermal properties and bonding mechanism of the eutectic point of the system were discussed.Na₄P₂O₇·10H₂O was used as a nucleating agent,hydroxypropyl methylcellulose(HPMC)was used as a thickening agent to compound with binary system hydrated salt and the optimal ratio of the binary system composite material was obtained by research,which can effectively improve the phase change thermal storage property.The results showed that the system was a simple binary eutectic system with eutectic points located at 28 wt%SAT and 72 wt%STP.The phase change temperature and latent heat were 40.6? and 226 J/g respectively.The theoretically predicted eutectic point was consistent with the experimental results.The structure of the binary eutectic salt was analyzed using FT-IR and DFT calculation,and it was found that the eutectic salt was based on two pure substance salts,which was combined by the strong hydrogen bond of water molecules in the form of O-H···O.Connected them directly or indirectly were a physical combination.Modified with 1wt%Na₄P₂O7·10H₂O+2 wt%HPMC,the degree of subcooling of the modified composite PCM was 3.82?,no phase separation,and the heat release time of 70 min(base material 20 g).After 800 cycles of heat storage/release,the phase change temperature and latent heat were 41.91? and 109.5 J/g respectively.The attenuation rate of the latent heat of phase change was 36.5%.There was almost no phase separation and the thermal storage stability was great.The thermal conductivity of the modified composite PCM was 0.6405 W/m·k which was 39%higher than before.The modified STP-SAT material has a suitable phase change temperature,which can fill the gap of the hydrated salt material in this temperature range,and has great practical application potential in the fields of solar heat storage,cross-season heat storage and heating,industrial waste heat and waste heat recovery.(2)Sodium carbonate decahydrate-disodium hydrogen phosphate dodecahydrate system:SCD-DHPD binary system hydrated salt phase change energy storage material was prepared by melt blending method.The phase diagram of the system was studied by the DSC,cooling curve and Schrader equation theoretical calculation.The thermal properties of the eutectic point and the bonding mechanism were studied.Na₂Si O₃·9H₂O and Na BO₂·4H₂O as nucleating agents,HPMC and polyacrylamide as thickening agents to compound with binary system hydrated salt and the optimal ratio of the binary system composite material was obtained by research,which can effectively improve the phase change thermal storage property.The results showed that the eutectic point of the system was located at 40 wt%SCD and 60 wt%DHPD,and the phase change temperature and latent heat were28? and 243.2 J/g respectively.The theoretical calculation was consistent with the experimental results.Using FT-IR to explore the structure of the binary eutectic salt,it was found that the eutectic salt directly or indirectly connects the two salts together through water molecules.It was a simple physical mixture without the formation of new chemical bonds.Modified with 5 wt%Na₂Si O₃·9H₂O+2 wt%HPMC and 3 wt%Na BO₂·4H₂O+4 wt%polyacrylamide,the degree of subcooling of the composite phase change material 1 and composite phase change material 2 were 0.12? and2? respectively.There were no phase separation.The heat release time was 52 min and 49 min(base material 20 g).After 800 heat storage/release cycles,the phase change temperature and latent heat of phase change material 1 were 27.6? and 187.4J/g respectively.The attenuation rate of the latent heat was 18%,with almost no phase separation.CPCM1 has good phase change thermal storage stability.The phase change temperature and latent heat of phase change material 2 were 25.4? and 170.8J/g respectively.The attenuation rate of the latent heat of phase change was 23%,but after 600 cycles,there was a slight phase separation phenomenon.If using composite phase change material 2,it is recommended to limit the number of applications to 600.The thermal conductivity of the composite phase change material 1 and composite phase change material 2 were 0.7785 W/m·k and 0.8146 W/m·k which were increased by 30.73%and 36.79%respectively.The modified SCD-DHPD has great application potential,and also can be used in solar heat storage,building energy saving and other fields.