| In view of unreasonable energy consumption structure and energy waste at present,the development of renewable energy and the improvement of energy efficiency are the key to solving the energy shortage problem.However,most renewable energy have intermittent and dispersion characteristics,such as solar,wind and tidal energy.It is difficult to recycle waste heat in industrial production,resulting in serious energy waste.Therefore,the development of energy storage technology is an important link to balance energy supply and energy consumption,and is also an important measure to improve energy efficiency.Thermal energy storage technology is one of the best forms of large-scale energy storage.In this paper,innovation high temperature and stability molten salt system and multi-component system with low melting point were designed and developed to solve low energy storage efficiency and frozen plugging problems in CSP system.The traditional Trial and Error method is time-consuming and high-cost,and is not suitable for the composition design of multi-component molten salt.Based on calculation of phase diagram?CALPHAD?method,phase diagram of molten salt systems were optimized and calculated in this paper.A self-consistent thermodynamic database was successfully constructed and it can be used to complete the composition design of multi-component molten salt.Moreover,thermal-physical properties were studied using thermal analysis method,and an integrated database of phase diagram thermodynamic and thermal-physical properties was established to provide guidance for the TES materials in CSP and waste heat recovery in industrial.The main research is as follows:Phase diagram of molten salt systems were calculated by the CALPHAD method.However,the Gibbs energy function of liquid Ca?NO3?2 and mixing enthalpy of Ca?NO3?2-CsNO3 system are hitherto not reported,which severely restrict the thermodynamic description of the Ca?NO3?2-base molten salt systems.Limiting slope of liquidus and ionic parameter function model were established to predict the Gibbs energy function of liquid and mixing enthalpy,and the predicted fusion enthalpy of Ca?NO3?2 and mixing enthalpy of Ca?NO3?2-CsNO3?50-50 mol%?were 23849 J·mol-1and-4334 J·mol-1,respectively.Based on phase equilibria and thermochemical data,the Ca?NO3?2-MNO3?M:alkali metal?binary systems were calculated using the CALPHAD approach.The substitutional solution model was used to describe liquid phase,and all the intermediate compounds were treated to be stoichiometric ratio.A set of self-consistent thermodynamic database is eventually obtained.Moreover,some mixture molten salt systems were successfully optimized by the CALPHAD approach.Thus,a multi-component thermodynamic database with the model parameters was constructed,and this database will be used to choose and design thermal energy storage materials of molten salt systems.Based on the thermodynamic database of multi-component molten salt,the phase diagram of the NaF-NaCl-Na2CO3 ternary was firstly obtained.The calculated temperature of eutectic point was 576?,and eutectic compositions were NaF=21.66mol%,NaCl=41.87 mol%and Na2CO3=36.47 mol%,respectively.By Differential Scanning Calorimetry?DSC?,the calculated results were verified experimentally and the agreement between the measured and predicted values was satisfactory.The thermal-physical properties of the NaF-NaCl-Na2CO3 ternary system were measured.The fusion enthalpy was 328.3 J·mol-1,and the average values of heat capacity of solid and liquid states were 1.450 J·g-1·K-11 and 2.006 J·g-1·K-1,respectively.The density of molten salt fit linearly as a function of temperature:?=2.000-6.07×10-4T?g/cm3?,and the density of molten salt was 1.575 g/cm3 at 750?.Thermal stability manifest that molten salt weight change was 0.2%at range from ambient temperature to 750?,and the weight change was measured to be 0.4%at 800?.By measuring thermal-physical properties and economical analysis,the NaF-NaCl-Na2CO3 ternary salt with excellent heat transfer property and energy storage efficiency was designed and developed using the CALPHAD method.This salt can be used to solve energy storage efficiency problem,and it will be developed as innovative TES materials for next generation CSP plants.The LiNO3-NaNO3-KNO3-NaNO2-KNO2 five-component system with low melting point was systematically investigated,and an eutectic point was obtained and the eutectic composition were LiNO3=33.5 mol%,NaNO3=1.2 mol%,KNO3=1.2mol%,NaNO2=17.4 mol%and KNO2=46.7 mol%.By means of the DSC measurement,the melting point and fusion enthalpy of eutectic salt were 74.4?and168.55 J·g-1,respectively.Likewise,specific heat capacity of eutectic salt based on heat capacity ratio method was also determined.Results demonstrated that the average values of heat capacity of solid and liquid states were 1.302 J·g-1·K-11 and1.685 J·g-1·K-1,respectively.The density of eutectic salt based on Archimedean principle was experimentally measured as a function of temperature and expressed as?=1.982-5.898×10-4T?g/cm3?.The viscosity of eutectic salt was experimentally measured using a rotational coaxial cylinder viscometer and expressed as?=0.09981exp?1988.54/T??cP?.Based on the additive principle,the density and viscosity of eutectic salt were empirically predicted and the predicted values were in good agreement with the experimental results.For the thermal stability,the upper limit of operating temperature of eutectic salt was verified at 508?.Results demonstrated that the weight loss of eutectic salt was 5.45%at 600?.By laser flash technique,the thermal diffusivity of eutectic salt was determined and expressed as?=0.00179-5.25786×10-6T+2.43571×10-8T2?cm2/s?,and thermal conductivity was calculated based on heat capacity and density measured.By the thermal-physical properties measured,the database of five-component reciprocal system with low melting point was established,and the eutectic salt of low melting point and excellent thermal-physical properties can be used to solve frozen plugging problem in CSP. |
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