Preparation And Properties Of Eutectic Phase Change Energy Storage Material Of Hydrated Salt

Hydrated salt phase change materials?PCMs?are widely applied in many fields such as solar thermal utilization,industrial waste heat recovery and building energy efficiency because of their low phase transition temperature,high latent heat,high thermal conductivity,non-toxic and flame retardancy.So far,eutectic PCM with more abundant and controllable properties become the research focus of PCM,meanwhile,hydrated salts eutectic mixture has also attracted much attention.In view of the lack of intensive research on these kind of PCMs,we study urea-SAT?sodium acetate triacetate?binary system and STP?sodium thiosulfate pentahydrate?-SAT binary system.The phase transformation process and bonding structure of each system were studied,and their thermal properties were analyzed.A phase change material with stable performance was prepared through modification,and the application performance of materials was studied by numerical simulation.The main contents and conclusions are as follows:?1?Urea-SAT systemThe solid-liquid phase diagram of the system was experimentally established by the synthetic visual method and DSC,respectively.The phase diagram exhibited a simple eutectic mixture comprised of 40%urea and 60%SAT,the corresponding phase change temperature and the latent heat of fusion were observed as 31.4 ^oC and 205.3J/g,respectively.In addition,XRD,IR,¹H-NMR and DFT calculation were carried out to further investigate the structures of mixture.These results indicated that the hydrogen bonds play a remarkable role in the formation of urea-SAT eutectic mixture.The material is modified by the uniform design method,with the addition of 3%Na₂HPO₄·12H₂O and 2.18%carboxymethylcellulose?CMC?,this mixture showed a higher stability than before.After 50 melting-freezing cycles,the melting temperature was still stable at 31 ^oC,and the latent heat of fusion was observed as 187.7 J/g.This eutectic mixture has potential for latent heat thermal energy storage,and is complementary to existing superior PCMs.Besides,the undercooling of this kind of low eutectic phase change energy storage material will decrease with the decrease of heating temperature,so the lower heating temperature should be chosen as far as possible.?2?STP-SAT systemThe solid-liquid equilibrium phase diagram of the system is studied by measurement and theoretical prediction,in which the measurement is measured by synthetic visual method and DSC.The bonding mechanism was analyzed using XRD,FT-IR and DFT calculation.Na₂CO₃·10H₂O as nucleating agent and CMC as thickening agent were used for modifying the eutectic mixture.The ratio of modifying agent was optimized using uniform design method.T-history method were used to study the thermal performance of phase change materials before and after modification.The results show that the eutectic point is located at 72%of STP and 28%of SAT.Phase change temperature and latent are 40.64 ^oC and 207.6 J/g respectively.Result of theory predicted are in agreement with the experimental data.The optimal ratio was 1.558%Na₂CO₃·10H₂O and 2.136%CMC.After 50 melting-freezing cycles,the melting temperature was still stable at 40 ^oC,and the latent heat of fusion was 184.5 J/g?maximum deviation is 2.7%?.This eutectic mixture has potential for latent heat thermal energy storage and can be used in various fields.?3?Numerical simulation of material applicationCFD software FLUENT is used to simulate the phase change process of the storage module and the energy release process of the water tank.The results show that the addition of PCM with 8.75%volume fraction can keep the temperature of the hot water tank at a high temperature.System one can prolong the time of 3 times,increase the total heat storage by 4.8%,System two respectively are 4 times and 5.0%.During the heat release process,the PCMs module plays the role of heating water and buffering,prolongs the holding time of the high temperature liquid in the water tank.