Design And Properties Of Heat Transfer And Storage Materials For Chloride Molten Salt

The energy crisis and environmental problems need to be solved urgently,and the energy transformation is imminent.The development and utilization of sustainable energy and industrial energy conservation are important solutions.Heat transfer and storage system is an indispensable part of solar thermal power generation and energy conservation in industry.The research and development of thermal medium energy storage and heat transfer fluid is one of the key technologies.In all kinds of heat transfer and storage media,chloride molten salt is the idea heat storage material with the characteristics of of large reserves,low cost,large latent heat of phase change,wide working temperature range and large heat storage density.Therefore,it is of great significance to develop new and efficient chloride molten salt materials for heat transfer and storage.Phase diagram calculation is a time-saving and efficient method for the design and development of heat transfer and heat storage materials.In this paper,the method of phase diagram calculation was used to determine the eutectic point temperature and composition of the system.The early stage was the preliminary exploration of phase diagram calculation and verification,in which the hard sphere ion model was used to calculate the phase diagrams of four binary systems:Na Cl-KCl,Na Cl-Ca Cl₂,Li Cl-KCl and Li Cl-Na Cl;and the effective charge model was used to calculate the phase diagrams of six binary systems:Na Cl-Ca Cl₂,Ca Cl₂-Mg Cl₂,Na Cl-Mg Cl₂,KCl-Li Cl,Li Cl-Na Cl,KCl-Na Cl and Na Cl-Ca Cl₂-Mg Cl₂ ternary systems.The cooling curve method was used to verify the ternary calculated phase diagram of Na Cl-Ca Cl₂-Mg Cl₂.To overcome the problem of large deviation in phase diagram calculated by the above model,a new idea of calculating different branches in the same phase diagram with different interaction coefficients?was proposed,and the normal solution model calculation method was adopted to calculate the binary phase diagrams of Na Cl-KCl,Na Cl-Ca Cl₂,Li Cl-KCl,Li Cl-Na Cl,Ca Cl₂-Mg Cl₂,Na Cl-Mg Cl₂,KCl-Ca Cl₂,KCl-Mg Cl₂,Li Cl-Ca Cl₂ and Li Cl-Mg Cl₂systems.For the three binary systems of KCl-Ca Cl₂,KCl-Mg Cl₂ and Na Cl-Mg Cl₂,the Gibbs free energy of the compound formation region was modified.The calculated results of the temperature and composition of the eutectic point were in good agreement with the values of the phase diagram database data.At the same time,four binary phase diagrams of Na Cl-KCl,Na Cl-Ca Cl₂,Ca Cl₂-Mg Cl₂ and Na Cl-Mg Cl₂ were also experimentally verified.Five kinds of chloride molten salt materials,i.e.Li Cl,KCl,Na Cl,Ca Cl₂ and Mg Cl₂,were selected as components to calculate the phase diagram of ternary chloride molten salt materials.The ternary phase diagram was calculated by the interaction coefficient?calculated by the binary boundary system,and the ternary phase diagram was calculated by using the developed normal solution model.The phase diagrams of six ternary systems were calculated,among which Li Cl-Ca Cl₂-Mg Cl₂,Li Cl-Na Cl-Ca Cl₂ and Li Cl-Na Cl-KCl systems have no compound formation,but only one eutectic point.And compounds were formed in three systems:Na Cl-KCl-Ca Cl₂,KCl-Ca Cl₂-Mg Cl₂ and Na Cl-Ca Cl₂-Mg Cl₂,the system of Na Cl-KCl-Ca Cl₂ has two eutectic points,the systems of Na Cl-Ca Cl₂-Mg Cl₂ and KCl-Ca Cl₂-Mg Cl₂ have three eutectic points.At the same time,samples were taken near the eutectic point of the phase diagrams calculated by Na Cl-KCl-Ca Cl₂,KCl-Ca Cl₂-Mg Cl₂ and Na Cl-Ca Cl₂-Mg Cl₂,and the temperature and composition of the eutectic point of the above three systems were determined by DSC.Six chloride molten salt materials were prepared as a high-temperature heat storage and heat transfer media,including Na Cl-Ca Cl₂,Na Cl-KCl-Ca Cl₂,Na Cl-Ca Cl₂-Mg Cl₂,KCl-Ca Cl₂-Mg Cl₂,Na Cl-KCl-Mg Cl₂ and Na Cl-KCl-Ca Cl₂-Mg Cl₂.Then the DSC method was used to determine the eutectic temperature and composition of those molten salts,and the other thermophysical properties were also measured,which were heat capacity,density and viscosity.Also,the mass loss of molten salt materials was investigated to determine the upper limit of working temperature.Correspondingly,energy storage density was calculated for each salt.The results show that Na Cl-KCl-Ca Cl₂-Mg Cl₂ has an eutectic temperature of 380.3?with good fluidity.The operating temperature range is 430?-700?,and the energy storage density is 625.1 J?cm^-3.By comparison,Na Cl-KCl-Ca Cl₂-Mg Cl₂ has the lowest eutectic temperature and the highest energy storage density,which is suitable to be used as heat transfer and heat storage material.The eutectic temperature of Na Cl-KCl-Ca Cl₂ is 503.8?,and the working temperature range is 550?-850?.The energy storage density is 559.9J?cm^-3,that means Na Cl-KCl-Ca Cl₂ is suitable for high-temperature heat storage material.