Design And Research On Heat Transfer And Storage Performance Of ZnCl₂-based Mult-component Molten Salt System

Solar tower power generation combined with supercritical CO₂ Brayton cycle technology has the advantages of high power and low-cost power supply,which is the key to the efficient use of solar energy.The temperature of the fluid generated by the central receiver of the tower power generation can exceed 950°C,and its operation under high temperature conditions can significantly improve the energy storage and conversion efficiency of system.However,the upper temperature limit of the thermal energy storage material NaNO₃-KNO₃?60wt%-40wt%?used in large-scale concentrating solar power plants is usually lower than 600°C.Therefore,it is necessary to search a heat storage and transfer fluid with higher operating temperature and excellent heat transfer performance.The SunShot program of the United States selected the multi-component chlorides NaCl-KCl-ZnCl₂?7.5 wt%-23.9 wt%-68.6wt%?as the potential candidate for the high-temperature thermal energy storage and transfer due to its excellent thermophysical properties and thermal stability in high temperature.However,the NaCl-KCl-ZnCl₂ ternary chlorides are limited in the large-scale commercial application since its high melting point will lead to freezing of pipelines in cold weather,and its low energy storage density will increase the design cost of the system.In light of this,this main research is as follows:?1?The novel molten materials with high energy storage density were designed by adding alkali metal chloride LiCl and alkaline earth metal chloride CaCl₂ with high specific heat capacity to NaCl-ZnCl₂ system.In this paper,the thermodynamic optimization of the LiCl-NaCl-CaCl₂-ZnCl₂ sub-binary systems were completed based on the CALPHAD method.The Kohler-Toop principle was used to predict the phase diagram and thermodynamic properties of the sub-ternary systems.Two types of novel ZnCl₂ based materials of LiCl-NaCl-ZnCl₂?5.6 wt-13.1 wt%-81.3 wt%?and NaCl-CaCl₂-ZnCl₂?39.1 wt%-15.0 wt%-46.0 wt%?were designed successfully.XRD was used to characterize the structure of the two groups of eutectic salts.The structure definitions were consistent with the predicted distribution of species,which conform the results of thermodynamic calculation.The performance of heat transfer and storage for two groups of eutectic materials was further measured,which includes the melting point,melting enthalpy,specific heat and thermal stability in high temperature and the thermodynamic and thermophysical properties of two types of eutectic salts were obtained.The melting point and liquid specific heat capacity of LiCl-NaCl-ZnCl₂ eutectic mixture are 495 K and 1.057 J/g·K,respectively.The melting point and liquid specific heat capacity of the eutectic NaCl-CaCl₂-ZnCl₂mixture are 645 K and 0.935 J/g·K.It has been seen that the alkali metal chloride LiCl can decrease the melting point more effectively than CaCl₂ since it could form the associate Li₂ZnCl₄.And the high specific heat capacity of LiCl itself can significantly enhance the specific heat capacity of the target system.Compared with the alkaline earth metal CaCl₂,the alkali metal chloride LiCl can decrease the melting point more effectively by forming the associate Li₂ZnCl₄ and increase the specific heat capacity of the target system due to the high specific heat capacity itself.Therefore,the heat transfer and storage performance of LiCl-NaCl-ZnCl₂ is significantly better than that of NaCl-CaCl₂-ZnCl₂ eutectic material.Moreover,the specific heat capacities of the two novel materials are significantly higher than that of the NaCl-KCl-ZnCl₂ system,which have the potential of being heat transfer materials.?2?In order to further reduce the melting point and increase the specific heat capacity of NaCl-KCl-ZnCl₂ eutectic material,LiCl-NaCl-KCl-ZnCl₂ quaternary molten material with low melting point is designed for energy storage by adding LiCl,which can significantly lower the melting point and increase specific heat capacity.The thermodynamic prediction and composition design of the LiCl-NaCl-KCl-ZnCl₂ quaternary system were completed based on the CALPHAD principle.Eutectic components 1^#?3.1wt%-4.8wt%-27.1wt%-65.0wt%?and 2^#?3.2wt%-5.7wt%-12.7wt%-78.5wt%?were identified by DSC technique.The heat transfer and storage properties of eutectic molten materials,including melting point,melting enthalpy,specific heat capacity,thermal conductivity and thermal stability in high temperature were further investigated.The Dulong-Petit method was used to predict the specific heat capacity,the addictive principle was used to predict the liquid density,and the average ionic weight?IW?correlation was used to evaluate the thermal conductivity of the eutectic salt.The predicted results agree well with the experimental values.In light of this,the reliable integrated database of thermodynamic and thermophysical properties of new eutectic materials was established.The melting point of eutectic salt1^#is 170°C and the specific heat capacity is 0.943 J/g·K,the melting point of salt2^#is 169°C and the specific heat capacity is 0.894 J/g·K,In comparison with NaCl-KCl-ZnCl₂ molten materials,the specific heat capacities of the investigated materials were increased by 16%and 10%and the melting points were decreased by34°C and 35°C,respectively.It can realize the effective use of energy storage materials and effectively suppress the freezing blockage of molten salt materials in industrial design.?3?Molecular size and structure are the key factors that affect the melting point and thermodynamic properties of multi-component molten systems.The previous work shows that the alkali metal chloride LiCl with a smaller molecular size effectively reduces the melting point by forming an association with ZnCl₂.Therefore,this work furthermore investigated the thermodynamic effects of the alkali metal chloride salt CsCl with a size larger than CaCl₂ on the ZnCl₂-based multicomponent molten system.The phase diagrams of RbCl-ZnCl₂ and CsCl-ZnCl₂ systems were established using DSC and XRD techniques.XRD technique was used to define the intermediate compound MZn₂Cl₅?M=Rb,Cs?with a stoichiometric ratio of 1MCl:2ZnCl₂.The thermodynamic evaluations of the RbCl-ZnCl₂,CsCl-ZnCl₂,CsCl-RbCl,CsCl-NaCl and CsCl-KCl systems were completed based on the CALPHAD principle.The Kohler-Toop rule was used to extrapolate the liquid projection of the ternary systems RbCl-CsCl-ZnCl₂,NaCl-KCl-CsCl,NaCl-KCl-ZnCl₂,KCl-CsCl-ZnCl₂ and NaCl-CsCl-ZnCl₂.Structure affects the behavior of melting.The melting points of the ZnCl₂-CsCl-MCl?M=Na,K,Rb?ternary systems are slightly higher than that of NaCl-KCl-ZnCl₂ system,but significantly lower than that of NaCl-CaCl₂-ZnCl₂system.It is inferred that the alkali metal chloride salt effectively reduces the melting point of the eutectic system by forming the association compound MCl₄^2-.Moreover,the established thermodynamic database of ZnCl₂-based multi-component system could provide theoretical support for the phase behavior and structure of molten system.