Study On Preparation And Thermal Energy Storage Properties Of High Temperature Molten Salt Phase Change Heat Storage Macro-capsules

Molten salt thermal storage materials are a promising high-temperature phase change material due to their low cost and high specific heat.However,the leakage and corrosion of molten salts limit their application.To solve the problems of molten salts in high-temperature thermal storage applications,researchers have proposed using stable inorganic materials to make shells to encapsulate molten salts to prepare capsules in the size of millimeters and centimeters.Ceramic materials are often studied for encapsulating molten salts because their excellent corrosion resistance,but they require higher sintering temperatures to achieve densification,which limits the application of ceramics in encapsulating molten salts.In order to solve the encapsulation problem of molten salt,this thesis proposes a double-shell method to encapsulate molten salt in a double-layer shell composed of an expanded graphite（EG）inner layer and a ceramic outer shell.Carbonate and chloride salts have been widely studied in thermal storage materials,due to their low cost and high thermal storage density.Therefore,in this thesis,typical carbonate and chloride salts were selected for encapsulation research.The main work of the thesis is as follows:1.Na₂CO₃-Li₂CO₃ eutectic salt（referred to as NaLiCO₃）was selected as a typical carbonate salt,and it was encapsulated into macrocapsules using a double shell method.The principle of macrocapsules preparation and phase change characteristics were studied.（1）Using NaLiCO₃ powders to make molten salt core balls,then the surface of cores is wrapped with EG inner shell and kaolin outer shell to prepare capsule precursors.The large-sized NaLiCO₃ capsules were successfully prepared by isostatic pressing and high-temperature treatment of the capsule precursors.The core balls and shell layer of the capsules are formed with powdered raw materials assisted by organic adhesives,and the gaps between the powder particles of the core raw materials can reserve space for the expansion of the molten salt phase change material.Due to the repulsive effect of the EG layer inside the capsules on the liquid salt,the liquid salt cannot penetrate the EG layer and remain inside the capsules.Therefore,the double shell of the capsules can prevent the leakage of the molten salt phase change material.On the other hand,capsules wrapped in NaLiCO₃ only with a layer of kaolin shell exhibit leakage of the phase change material in cores after sintering.（2）In the melting-freezing cycles,NaLiCO₃ double shell macrocapsules showed good cycling stability.Encapsulating NaLiCO₃ in double shell can be combined with EG composite NaLiCO₃to enhance thermal conductivity of NaLiCO₃,and those capsules also have good cycling stability.（3）The NaLiCO₃ eutectic salt phase change material exhibits a high latent heat of 313 J/g and a melting point of approximately 498°C.NaLiCO₃ double shell macrocapsules have a high thermal storage density,NaLiCO₃@EG@Kaolin capsules have a high thermal storage density of 380 J/g between 400°C and 600°C in capsules size of 10@1.5@3.2.NaCl was selected as a typical chlorinated molten salt.The principle of encapsulating NaCl into macrocapsules using the double-shell method and the phase change characteristics of the capsules were studied.The effect of isostatic pressing of the capsule core was also studied on increasing the thermal storage density of the double-shell capsules.（1）Using NaCl powders to make molten salt core balls,they were subjected to isostatic pressing treatment under different pressures,and wrapped in EG layer and kaolin shell successively.After isostatic pressing and high-temperature treatment,NaCl double-shell macrocapsules were successfully prepared.When the capsule is heated above the NaCl melting point,the EG inner layer inside the double-shell capsule can prevent NaCl leakage,while the capsule directly wrapped with Al₂O₃ceramic shell can be leakage.Because Al₂O₃ ceramics exhibit a relatively porous structure after sintering and have good wettability with liquid NaCl.（2）Pressing the NaCl cores with isostatic pressure increases the thermal storage density of the capsules.The NaCl macrocapsules can be prepared in the case of their cores pressed with isostatic pressure at 50 MPa and 200 MPa,and those capsules have good melting-freezing cycling stability.（3）According to the statistical results of the thermal storage density of different capsules,the thermal storage density of the capsules increases,as the pressure of the pressed their core increases.The NaCl double-shell macrocapsules exhibit high thermal storage density.NaCl capsules with the size of 20@2@2 and cores isostatic pressed by 200 MPa exhibit high thermal storage densities of 1024.54J/cm³ and 551.56 J/g in the temperature range of 600°C to 900°C.3.NaCl-KCl eutectic salt was selected as a typical chlorinated eutectic salt,then the principle of encapsulating it into macrocapsules using the double-shell method and the phase change characteristics of the capsules were studied.The influence of isostatic pressing of the capsule core and in-situ melting combined with the double-shell encapsulation method on the preparation of the capsules was also studied.（1）The core spheres prepared from pre-eutectic and non-eutectic（i.e.mixed）chloride salts were successfully encapsulated into capsules through double-shell encapsulation.Therefore,the double-shell method can be successfully used to encapsulate NaCl-KCl molten salt to prepare double-shell macrocapsules,and in-situ melted NaCl-KCl molten salt double-shell macrocapsules can also be successfully prepared.（2）The combination of in-situ melting method and isostatic pressing treatment with double-shell method can successfully prepare capsules,and the capsules have good melting-freezing cycling stability.（3）The NaCl-KCl eutectic salt phase change material exhibits a latent heat of190.4 J/g and a melting point of around 654.9°C.NaCl-KCl double-shell macrocapsules have a high thermal storage density.NaCl-KCl capsules with the size of20@2@2 and cores isostatic pressed by 200 MPa exhibit high thermal storage densities of 251.21 J/g and 447.57 J/cm³ in the temperature range of 600°C to 750°C.In summary,in this thesis,a double-shell method was adopted to encapsulate NaLiCO₃,NaCl,and NaCl KCl into double-shell macrocapsules.The prepared capsules have good thermal cycling stability and high thermal storage density,which can play an important role in promoting the application and development of molten salt in the high-temperature phase change thermal storage field.