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NaCl–CaCl 2 molten salt is considered as a promising high-temperature heat transfer and storage fluid for advanced nuclear power plants and concentrating solar power plants in the field of renewable energy utilization. However, the comprehensive physical properties and their microscopic mechanisms for the molten NaCl–CaCl 2 are failed to be measured accurately due to the extremely measuring condition. In this work, the ab-initio molecular dynamics simulation is used to investigate its microstructures and thermophysical properties for entire operating temperatures. It reveals that ion clusters are formed in terms of three for face-sharing, two for edge-sharing, and one for corner-sharing Cl− ions between the coordination shells of two neighboring cations. The coordination numbers of Na+-Cl- and Ca2+-Cl- ion pairs decrease from 5.88 to 6.46 at 783 K to 5.33 and 6.02 at 1173 K respectively. Meanwhile, the reliable and meaningful values of densities, ion self-diffusion coefficients, viscosities, and thermal conductivities were evaluated from 783 to 1173 K. It suggests that the distances and interactions between ions pairs determine thermodynamic properties directly. The ab-initio molecular dynamics simulation is proved to be an effective way to obtain the essential data for the designs of heat transfer and thermal energy storage systems in practical applications. • Ab-initio molecular dynamics simulations are applied for NaCl–CaCl 2. • Approach-to-equilibrium molecular dynamics method is used for thermal conductivity. • The correlation between microstructures and thermophysical properties are studied. • The distances and interactions between ion pairs determine its properties. [ABSTRACT FROM AUTHOR]

Highlights • Transport properties were simulated and agreed with experimental value. • Local structures of carbonates were calculated and analyzed. • Macroscopic properties’ variation with temperature were clarified. • The BMH potential plus harmonic bond, angle, improper force was suitable. Abstract Molten alkali carbonates has been researched as one of the most promising thermal energy storage (TES) materials in Concentrating Solar Power (CSP) and received extensive attentions. Some attractive properties must be determined accurately, such as thermal conductivity, and viscosity. over a wide temperature range. However, these significant thermal and transport properties are difficult to be obtained for experiments on account of high-temperature extreme conditions. Molecular dynamics (MD) is an alternative way to predict these properties for molten salts. Systematic results including density, thermal conductivity and shear viscosity as a function of temperature from molecular dynamics simulations of molten alkali carbonates are presented in detail in this paper. Both reverse nonequilibrium molecular dynamics (RNEMD) and nonequilibrium molecular dynamics (NEMD) methods are tried for thermal conductivity and viscosity, and then the results are compared to experimental values. The temperature dependence are investigated and analyzed by correlating transport properties with local structures. The results show that the Tosi-Fumi potential predicts negative temperature dependences for both viscosity and thermal conductivity of the alkali carbonates. The simulation results are in good agreement with the experimental data available in the literature. [ABSTRACT FROM AUTHOR]

Molten carbonate salts as phase change materials have received particular attention for high-temperature thermal energy storage and heat transfer applications due to desirable thermal characteristics such as wide operating temperature range, low causticity and excellent thermal stability. In this study, molecular dynamics (MD) simulations were performed on the binary carbonate salt Na 2 CO 3 -K 2 CO 3 (58-42 mol%) based on an effective pair potential model, a Born-Mayer type combined with a Coulomb term. The temperature dependences of thermodynamic properties including the density, sheer viscosity and thermal conductivity were simulated in detail from 1000 to 1400 K, which were all difficult to achieve from experiments on account of high-temperature extreme conditions. Moreover, the radial distribution functions (RDF) and coordination number curves of the binary carbonate salt were characterized to explore the mechanisms of their temperature dependences from microscopic view. The simulation results suggested that the changes of thermodynamic properties with temperature were induced by the distance changes between ions. Besides, it can be concluded that the high bond energy in the strong covalent bond C-O of molten carbonate salts must contribute to the large specific heat capacity in comparison with that of molten un-oxyacid salts. [ABSTRACT FROM AUTHOR]

Comprehensive molecular simulations have been carried out to compute local structures and transport properties of different components of binary NaCl-KCl over a wide operating temperature range. The partial radial distribution functions, coordination number curves and angular distribution functions were calculated to analyze the influence of temperature and component on local structures of molten Alkali Chlorides. Transport properties were calculated by using reverse non-equilibrium molecular dynamics (RNEMD) simulations including densities, shear viscosity and thermal conductivity. The results show that ion clusters are considered to be formed and the distance of ion clusters become larger with increasing temperature which has great influence on macro-properties. The calculated properties have a good agreement with the experimental data, and similar method could be used to computationally calculate the properties of various molten salts and their mixtures. [ABSTRACT FROM AUTHOR]

To develop a potential salt in a solar power generation system, some important properties must be determined, such as thermal conductivity, viscosity etc. over the entire operating temperature range. However, due to high-temperature extreme conditions, these thermal and transport properties are hard to be obtained for experiments. It is required to find an alternative way to predict these properties accurately for molten salts. Systematic results including density, shear viscosity and thermal conductivity as a function of temperature from molecular dynamics simulations of molten alkali carbonates are presented in detail in this paper. Of which, thermal conductivity and viscosity are computed from reserve nonequilibirum molecular dynamics methods. The effects of temperature have been investigated and analyzed. The results show that the Tosi-Fumi potential predicts negative temperature dependences for both viscosity and thermal conductivity of the alkali carbonates. The simulation results are in agreement with the experimental data available in the literature. [ABSTRACT FROM AUTHOR]

It is of great importance to obtain the accurate thermal properties of molten salts over the entire operating temperature for system design in concentrated solar power or other thermal storage systems. In this work, we explore size effects on density, viscosity and thermal conductivity predictions of molten nitrate and chloride salts by molecular dynamics. Equilibrium molecular dynamics and reverse nonequilibrium molecular dynamics methods are applied to calculate thermal conductivities of NaNO 3 and NaCl regarding different simulation cell sizes at the specific temperatures to study size effects on thermal conductivity predictions of melts. The results show that the size effects are neglected on density and viscosity predictions while a linear positive relationship between 1/ λ and 1/ L z is found in the reverse nonequilibrium molecular dynamics method. Through analyzing the phenomenon from a view of microstructure, phonon-phonon-scattering rates are found to be the key factors. The influences of temperature and particle species on size effects are further discussed. Subsequently, appropriate simulation cell sizes are selected for NaNO 3 , KNO 3 and Hitec salt, and the calculated thermal properties are in good agreement with the experimental values. The mean relative errors and fitting formula for density, thermal conductivity, and viscosity for the three salts are also obtained. • Size effects on thermal property predictions of MS are studied systematically. • λ of MS of different simulation cell sizes are computed by EMD and RNEMD methods. • RNEMD results of λ show large size effects when "ballistic transport" occurs. • Phonon-phonon-scattering rates are found to be the key factors. • The influences of temperature and particle species are further discussed. [ABSTRACT FROM AUTHOR]

• Molecular simulation is used to compute structures-properties of Na 2 CO 3 -K 2 CO 3. • Temperature and component dependence are studied from microscopic view. • The temperature-thermophysical properties-composition correlation are obtained. Molten carbonate eutectic salts are promising thermal storage and heat transfer fluid materials in solar thermal power plant with the feature of large specific heat capacity, wide operating temperature range and little corrosive. The high-temperature properties of molten carbonates should be determined accurately over the entire operating temperature for energy system design. In this paper, molecular dynamic simulation is used to study temperature and component dependence of microstructures and thermophysical properties of the binary carbonate molten salt. Negative linear temperature dependence of densities and thermal conductivities of binary mixtures of different components is confirmed with respect to the distances of ion clusters. Besides, positive linear temperature dependence of self-diffusion coefficient is also obtained. When temperature is constant, densities and thermal conductivities of binary mixtures are linearly related with components. Self-diffusion coefficients of CO 3 2− firstly increase and then decrease with increasing mole fraction of Na 2 CO 3. The temperature-thermophysical properties-composition correlation formulas are obtained, and the database of thermophysical properties of molten carbonate salts over the entire operating temperature is complemented, which will provide the essential data for heat transfer and storage system design, operation, and optimization in CSP. [ABSTRACT FROM AUTHOR]

Molten carbonate salts have received particular attention for high-temperature thermal energy storage and heat transfer applications due to desirable thermal characteristics, such as wide operating temperature range, low causticity and excellent thermal stability. In this study, molecular dynamics (MD) simulations were performed on molten alkali carbonate K 2 CO 3 based on an effective pair potential model, a Born-Mayer type combined with a Coulomb term. The radial distribution functions (RDF) and coordination number curves of the molten salt were characterized to explore the temperature dependences of macroscopic properties from microscopic view. The results suggest that the distance between K 2 CO 3 particles is getting larger with temperature increasing, resulting in the increase of molar volume and the diminished ability of resistance to shear deformation and heat transfer by vibration between ions. Besides, it can be concluded that the structure of CO 3 2− is inferred reasonably to be ortho-triangular pyramid from the comprehensive analysis of local structures including the angular distribution functions (ADF). Moreover, the thermodynamic properties were simulated in detail from 1200 to 1600 K including the density, thermal expansion coefficient, specific heat capacity, sheer viscosity, thermal conductivity and ion self-diffusion coefficient, which was hard to be measured from experiments under high-temperature extreme conditions. All the simulation results are in satisfactory agreement with available experimental data with high accuracy, and the minimum simulation error is as low as 1.42%. [ABSTRACT FROM AUTHOR]

Molten binary chloride salts are considered as promising heat transfer and storage mediums in a Concentrating Solar Power (CSP) Station. The properties of them over the entire operating temperature are required to be calculated, since they are difficult to be measured under high-temperature conditions. Reliable numerical method is an alternative tool to accurately predict thermal properties of molten salts from the viewpoint of micro-structure. Based on the effective Born-Mayer-Huggins (BMH) pair potential model, the local structures and thermodynamic properties of lithium-potassium chloride were computed by using Molecular Dynamics simulations (MD) and the results are consistent with available experimental literature data. The simulation results prove that BMH force field is reasonable to be adopted to predict the properties of various molten chloride salts and their mixtures. The minimum errors of thermal properties above are all within 11.1%, which is accurate and reliable. The results show that Li+ promotes the diffusion of the system efficiently. In addition, the specific heat capacity and thermal conductivity are significantly improved with the increase of Li+. The database of thermal properties and structures of this work are significant for accurate design of material structure and regulation of thermal storage system performance. • Temperature/component dependence on properties of molten lithium-potassium chloride. • Reverse non-equilibrium molecular dynamics methods were reported. • Lithium ion promotes the diffusion of the system efficiently. • Thermal conductivity/specific heat capacity improved with increasing lithium chloride. • Accurate material structure design and thermal storage system performance regulation. [ABSTRACT FROM AUTHOR]

• A workflow to construct solid-molten salt interaction potential is designed. • Deep potential could yield accurate solid-molten salt interaction at ab initio level. • Trends in interfacial heat transfer are correctly described by deep potential model. Interfacial heat and mass transfer properties at molten salt/solid interfaces are crucial for the study of heat storage/transfer properties of molten salt nanocomposite materials as well as the microscopic mechanism of thermophysical property enhancement, but accurate force field parameters for describing molten salt-solid interactions are still lacking. This study has utilized a workflow that could meet the demand of high accuracy and efficiency in molecular dynamics (MD) simulations of molten salt/solid interface with the deep potentials (DPs) trained by deep learning (DL), using SiO 2 /LiCl-KCl interface as a model system. In terms of the short-ranged polarizability and long-ranged correlations of ions, DPs can give accurate description of potential energy surface over wide temperature range at the level of ab initio calculations. Further analysis on interfacial thermal resistance (ITR) and thermal conductivity shows that the slowing down of heat transfer is related to the disordered microstructure of molten salt ions near the SiO 2 surfaces, which also leads to larger ITR. For the mass transfer of molten salt ions on the SiO 2 surfaces, the more active ionic diffusion and permeation through the solid can be observed due to the weaker adsorption between them, which is ascribed primarily to the high temperature. The workflow, of which the feasibility has been verified for molten salt/solid interfaces in this work with rigorous modeling, will be used for building accurate interatomic interaction models and calculating thermophysical properties for various complex interfacial systems containing molten salts. The influence of interfacial microstructure on heat and mass transfer will provide theoretical support for the application of molten salts in energy industry. [ABSTRACT FROM AUTHOR]

Molten chloride salt is recognized as a promising heat transfer and storage medium in concentrating solar power (CSP) in recent years, but there is a serious lack for thermal property data of molten chloride salts under high-temperature condition. In this work, temperature and component dependence of local structure and physical properties for binary KCl–MgCl 2 mixture were computed by molecular dynamics simulations (MD) method based on Born–Mayer–Huggins (BMH) potential. As the temperature rises, the distances between ions change and the structures turn into loose state, thus the density, thermal conductivity and the shear viscosity decrease, which also agree well with the experimental results. The calculation errors of thermal properties are mainly within 8%. The results show that density and thermal conductivity have negative linear correlation with temperature. Specific heat capacity is invariant with temperature at a constant pressure. When K+ increases in the salt, thermal conductivity is significantly improved, and shear viscosity and specific heat capacity decrease. The formulas of temperature-thermal properties-component correlations are obtained, which provide the fundamental property database for the precise design of high-temperature thermal storage system. ● Structures and thermal properties of KCl–MgCl 2 were calculated by MD simulations. ● Properties of KCl–MgCl 2 were influenced by component and temperature. ● With increasing K+, coordination structures became looser and densities decreased. ● The correlations of properties and component/temperature were obtained. [ABSTRACT FROM AUTHOR]

NaCl–CaCl 2 –MgCl 2 ternary molten salt is a potential material for thermal transfer and storage applications at high temperatures. In the present work, its microstructures and thermophysical properties over a wide operating temperature range were investigated by using ab-initio molecular dynamics simulations. An approach-to-equilibrium molecular dynamics method was used to calculate the thermal conductivity of molten salts. The local coordination structures and mechanisms of their temperature dependence were investigated. It can be found that the dominant bonding interactions are between cation-chlorine ion pairs, which can be considered as bridges connecting the local structures in liquids. The interaction between ions decreases with the increase of temperature as distances of ion clusters get larger. The thermophysical properties were evaluated in detail from 703 to 1173 K, showing a good accordance with the experimental data. A detailed relationship between operating temperature, microstructure, and thermophysical properties has been studied. The results suggest that the changes in thermodynamic properties with increasing temperature are caused by the variations of microstructures, in terms of the distances and bonding characteristics of ions. • Ab-initio molecular dynamics simulations are applied for NaCl–CaCl 2 –MgCl 2. • A pproach-to-equilibrium molecular dynamics method is used for thermal conductivity. • The relationship between operating temperature and properties has been studied. • Properties of molten salt are determined by the distances and interactions of ions. [ABSTRACT FROM AUTHOR]

• MgO-based sorbents were synthesized by deposition of alkali metal salts. • The CO 2 adsorption capacity of the sorbent is enlarged as 19.06 mmol·g−1. • The adsorption enhancement mechanism is discussed in detail. Carbon capture and storage is an effective way to mitigate the accumulation of greenhouse gases in the atmosphere. In this work, a series of MgO-basedadsorbents were synthesized by deposition of a mixed alkali metal nitrates and carbonates. The CO 2 capture amount of the compound adsorbent is enlarged as 19.06 mmol·g−1 at 325 °C when loading 10% mole of [(Li 0.44 K 0.56)NO 3 ] 2 [(Na 0.5 K 0.5)CO 3 ]. The as-synthesized adsorbent also exhibits stable CO 2 capture performance after long-term adsorption/desorption cycles. The effects of the molar ratio of alkali metal salts and adsorption conditions were investigated. The adsorption enhancement mechanism is discussed regarding the changes of composite microstructure during the reaction. It is found that the CO 2 uptake curve has three adsorption stages corresponding to the interactions between CO 2 , MgO/metal nitrates and carbonates. The nitrite product plays a key role in the improvement of CO 2 uptake since it not only yields more O2− but also reacts with MgO in molten nitrites to produce an intermediate nitrato compound, which leads to the rapid nucleation of MgCO 3 by triggering lattice defects. It is found that the CO 2 uptake decreased from 19.06 to 15.7 mmol·g−1 over 30 cycles, which proves that the new adsorbents have a good long-term adsorption/desorption stability. [ABSTRACT FROM AUTHOR]