Thermal Properties Of Molten Salts Research Articles

A theoretical study of thermal properties and structural evolution in binary carbonates phase change material: Machine learning-enhanced sampling strategy.

Thermal energy storage and utilization has been widely concerned due to the intermittency, renewability, and economy of renewable energy. In this paper, the potential energy function of binary Na2CO3-K2CO3 salt was first constructed using the Deep Potential GENerator (DPGEN) enhanced sampling method. Deep potential molecular dynamics simulations were performed to calculate the thermal properties and structural evolution of binary carbonates. The results show that as the temperature increases from 1073 to 1273K, the viscosity and thermal conductivity decrease from 5.011 mPa s and 0.502 W/(m K) to 2.526 mPa s and 0.481 W/(m K), respectively. The decrease in viscosity is related to the distance and interaction between the molten salt ions. In addition, the diffusion coefficients, energy barriers, ionic radius, angular distribution function, and coordination number of molten salt were calculated and analyzed. The CO32- exhibits a stable planar triangular structure. The ionic radius of Na+ is smaller than that of K+, which makes Na+ suffer less spatial hindrance during motion and has a higher diffusion coefficient. The energy barriers that Na+ needs to overcome to escape the Coulomb force is greater than that of K+ ions, so molten salt containing Na+ may possess greater heat storage potential. We believe that the potential function constructed with DPGEN enhanced sampling strategy can provide more convincing results for predicting the thermal properties of molten salts. This paper aims to provide a technical route to develop the novel complex molten salt phase change material for thermal energy storage.

Advanced two phase flow model for transient molten salt receiver system simulation

In order to realistically predict and optimize the actual performance of a concentrating solar power (CSP) plant sophisticated simulation models and methods are required. This paper presents a detailed dynamic simulation model for a Molten Salt Solar Tower (MST) system, which is capable of simulating transient operation including detailed startup and shutdown procedures including drainage and refill. For appropriate representation of the transient behavior of the receiver as well as replication of local bulk and surface temperatures a discretized receiver model based on a novel homogeneous two-phase (2P) flow modelling approach is implemented in Modelica Dymola®. This allows for reasonable representation of the very different hydraulic and thermal properties of molten salt versus air as well as the transition between both. This dynamic 2P receiver model is embedded in a comprehensive one-dimensional model of a commercial scale MST system and coupled with a transient receiver flux density distribution from raytracing based heliostat field simulation. This enables for detailed process prediction with reasonable computational effort, while providing data such as local salt film and wall temperatures, realistic control behavior as well as net performance of the overall system. Besides a model description, this paper presents some results of a validation as well as the simulation of a complete startup procedure. Finally, a study on numerical simulation performance and grid dependencies is presented and discussed.

Comprehensive thermal properties of molten salt nanocomposite materials base on mixed nitrate salts with SiO2/TiO2 nanoparticles for thermal energy storage

The purpose of this investigation is to find and prepare novel heat storage materials to meet the requirement of Concentrating Solar Power (CSP) system. This paper focuses on the formulation, fabrication and characterization of a novel molten salt nanocomposite for high-temperature heat energy storage. The molten salt nanocomposite is based on eutectic quaternary salt, and two different concentrations of SiO 2 and TiO 2 nanoparticles are added to improve its specific heat capacity and thermal conductivity by microwave method. Concerning the material characterization, a suite of techniques was used, including simultaneous thermal analysis (STA) and laser flash analysis (LFA). The consequences demonstrate that the melting point and latent heat of molten salt nanocomposites can be reduced by adding two different concentrations of nanoparticles at the same time. The specific heat is 28.1% higher than that of the pure quaternary salt, and 6.3% and 9.8% higher than that of adding SiO 2 and TiO 2 nanoparticles separately when adding 0.1 wt% SiO 2 and 0.9 wt% TiO 2 nanoparticles to the quaternary nitrate. Compared with pure quaternary salt, the thermal conductivity of the molten salt nanocomposites increased by 53.7%, which was 16.8% and 6.3% higher than that of adding SiO 2 and TiO 2 nanoparticles separately. Upon inspection with scanning electron microscopy, a high-density nanostructure and heat transfer channel were observed, which appear to be the reason for the enhancement of specific heat and thermal conductivity of the material. Finally, the molten salt nanocomposite has a broad application prospects in high-temperature thermal storage system. • Novel molten salt nanocomposite is prepared by microwave method for heat storage system. • Two different concentrations of nanoparticles are added to the salt simultaneously. • The specific heat and thermal conductivity of quaternary salt are improved. • The concentration of nanoparticles can affect the thermal properties of molten salt. • The honeycomb structure is the reason for the increase of thermal performance.

Investigation of properties change in the reacted molten salts after molten chlorides cyclic thermal treatment of toxic MSWI fly ash

To realize the thermal detoxification of municipal solid waste incineration (MSWI) fly ash in a relatively mild environment, molten salts thermal treatment technology was proposed in our previous research, which showed good effects. To investigate the properties of molten salts (NaCl-CaCl2) during cycling reusing, the change of the main components and the physical properties of the used molten salts were estimated. Results showed that the salts in fly ash would dissolve into molten salts. During this process, the concentration of K+, SO42- kept increasing while Cl- was decreased. The changing trend of Na+ and Ca2+ was dependent on the ratio of Ca/Na in raw fly ash. Ca(OH)2 in fly ash would react with CaCl2 to form CaClOH. Moreover, the introduction of the salt components on the thermal properties of molten salts were also studied. The melting point hardly changed by NaCl, CaSO4, and SiO2. Nevertheless, it was lowered to 431 °C with 15% CaCO3 addition, while increased to 523 °C with 20% KCl. Besides, there were no significant influences on the viscosity, stability, and thermal diffusivity of molten salts. KCl had the greatest influence on the specific heat capacity of molten salt, with an increase of about 20%.

Radiation-Assisted Formation of Metal Nanoparticles in Molten Salts.

Knowledge of structural and thermal properties of molten salts is crucial for understanding and predicting their stability in many applications such as thermal energy storage and nuclear energy systems. Probing the behavior of metal contaminants in molten salts is presently limited to either foreign ionic species or metal nanocrystals added to the melt. To bridge the gap between these two end states and follow the nucleation and growth of metal species in molten salt environment in situ, we use synchrotron X-rays as both a source of solvated electrons for reducing Ni2+ ions added to ZnCl2 melt and as an atomic-level probe for detecting formation of zerovalent Ni nanoparticles. By combining extended X-ray absorption fine structure analysis with X-ray absorption near edge structure modeling, we obtained the average size and structure of the nanoparticles and proposed a radiation-induced reduction mechanism of metal ions in molten salts.

Finite-size effects on thermal property predictions of molten salts

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 NaNO3 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/Lz 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 NaNO3, KNO3 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.

INFLUENCE of ELECTRIC FIELD on STRUCTURE and PROPERTIES of TERNARY CARBONATE MOLTEN SALT

Molten salt is one of the important heat storage materials in the field of solar thermal utilization, and nanoparticles can improve partial thermal properties of molten salt. To investigate whether nanoparticles with charge can affect the structure of molten salt ions, the micro electric field of nanoparticles was replaced by applied electric field and the liquid Li2CO3- Na2CO3-K2CO3 was solidified in the electric field. DSC was used to determine the phase change temperatures and latent heat, and thermal stability was analyzed by TG curve. At last, the microstructures of molten salt were analyzed by SEM. Results show that the applied electric field can reduce the starting and ending temperatures of the carbonate by about 10°C as well as the latent heat. The mass loss rates of molten salt samples at the positive and negative plates are 1.44% and 0.81% respectively at 700°, and these samples have higher thermal stability than normal molten salt. The applied electric field can produce new microstructure of molten salt near positive and negative plates.

Heat and mass transportation properties of binary chloride salt as a high-temperature heat storage and transfer media

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.

Molecular dynamics simulation of thermophysical properties of NaCl-SiO2 based molten salt composite phase change materials

It is urgently needed to improve the thermal properties of molten salt based phase change materials used for effective storage and utilization of solar energy. In this paper, the physical model of NaCl-SiO2 composite phase change materials (CPCM) was established. An effective method based on molecular dynamics (MD) simulation was proposed and validated to predict the thermal properties of CPCM. The structural deformation during phase transition process of CPCM system was observed and the radial distribution function (RDF) was calculated to analyze the local structure. The results indicate that the thermal conductivity of NaCl is enhanced remarkably with a maximum increase of 44.2% by adding 2.4% volume fraction of SiO2 nanoparticles and the mechanism of the thermal conductivity enhancement was discussed at the atomic level. The shear viscosity increases with the increase of the volume fraction of nanoparticles, with a maximum average increase of 23.6%. The relationship between self-diffusion coefficient and temperature is approximate to predict melting point. The force field and simulation methods adopted in this paper are desired to be useful for the prediction of thermal properties and further investigation into molten salts based thermal energy storage systems.

Forced convective heat transfer characteristics of solar salt-based SiO2 nanofluids in solar energy applications

Nowadays, molten salts are widely used in concentrated solar plants as working media to transfer and store solar energy. However, the relatively poor thermal properties of molten salts have restricted their further application. Nevertheless, the addition of nanoparticles to molten salts excellently improves these thermal properties. In present work, solar salt-based nanofluids were prepared using a lyophilizer. The specific heat of solar salt-based SiO2 nanofluids was experimentally measured based on the sapphire method, and its heat transfer performance was numerically investigated using the lattice Boltzmann method. Results indicated that an optimum nanoparticle mass fraction that could enhance heat transfer performance existed. This optimum nanoparticle mass fraction, which was found to be 1.0 wt%, improved the heat transfer coefficient and Nu number by 8.58 and 7.29%, respectively. The parameter analysis indicated that the considerably large change in the specific heat dominated the change in heat transfer performance. Meanwhile, the simulation results of the average Nu numbers exhibited good agreement with the Shah-London correlation prediction to within ±3.0%, indicating the feasibility of using this correlation in designing heat exchangers that utilize solar salt-based SiO2 nanofluids.

Experimental investigation into the thermos-physical properties by dispersing nanoparticles to the nitrates

In order to improve the thermal properties of molten salts, 20 nm silica particles were dispersed into nitrates by ultrasonic stirring method to study the effects of different mass concentrations of nanoparticles on the thermal properties of nitrates. The specific heat, latent heat, and decomposition temperature of the molten salts are measured by a synchronous thermal analyzer. The results show that the thermal properties of molten salt varied with the concentration of silica. Compared with the base salts, the heat of fusion improvements of the solar salt, potassium nitrate and sodium nitrate with 20 nm SiO2 nanoparticles were found to be ~3.84%, ~9.16% and ~3.31%; and the specific heat increased by ~15.89%, ~33.52% and ~11.86 % in liquid, respectively.

The effect of nano-MgO on thermal properties of ternary chloride fluid

Abstract Molten chloride salts are promising candidates for high temperature thermal energy storage. However, the thermal properties of molten salts is of significance for their usage in real applications. In this study, a series of MgO doped chloride salt nano-fluids have been prepared and their thermal properties have been studied. The specific heat capacity (cp) and thermal diffusivity (α) of nano-fluids have been measured by differential scanning calorimeter (DSC) and laser flash analysis (LFA). The results show that the fusion enthalpy of chloride salt with 100nm MgO of 4.5 wt.% was 42.4 J·g-1 higher than pure ternary chloride. The melting point of chloride salt with 60nm MgO of 3.5 wt.% is reduced to 424.75°C. Scanning electron micrographs show that nano-MgO has good dispersibility in ternary chloride .The specific heat capacity of chloride salt with 20nm MgO of 4.5 wt % can increase by at least 28.5% over 480 oC. The MgO obtained by the in-situ method was close to 3.8wt.% of the total content, which result in the increase of thermal diffusivity by 23.5% when the temperature is close to 550°C. The enhanced cp and α of nano-fluids show that doping of nano-MgO has a positive effect on the improvement of the heat transfer performance for chloride salts.

Simulation and experimental study of the specific heat capacity of molten salt based nanofluids

This work presents both simulations and experiments showing that the use of silica nanoparticles enhances the thermal properties of molten salt based nanofluids which is a provisional candidate for solar thermal energy storage. Sodium nitrate, potassium nitrate and lithium nitrate were chosen as the base material for such nanofluids. Thermodynamic and transport properties for nitrate salts and nanofluids were computed from molecular dynamics simulations. In order to validate our simulation results, different scanning calorimetry (DSC) was employed to measure the thermal properties of salt based nanofluids at 270–400°C. Both simulation and experimental results showed that the addition of nanoparticles increases the specific heat capacity. Material characterization analysis was carried out to investigate microstructural change of nanomaterials using a scanning electron microscopy (SEM). We observed a stripe-like arrangement of nanoparticles with SEM. That may be responsible for the enhancement of specific heat capacity of the nanofluids.

熔融盐在太阳能热发电中的应用及性能研究现状

熔盐储热材料具有粘度低，流动性能好，系统压力小，比热容高，蓄热能力强，成本较低等诸多优点，已成为太阳能高温传热蓄热介质的良好选择。本文从熔融盐的种类和热物性研究方面就国内外的研究现状做了详细介绍，展望了熔融盐作为储热材料的应用前景。 The molten salt heat storage material with low viscosity, good fluidity, low system pressure, high specific heat capacity, strong heat storage capacity, low cost and many other advantages, has be-come a good choice for solar heat storage medium. This paper introduced the present situation of domestic and foreign researches about the types and thermal properties of molten salt in detail. The application prospect of molten salt for storing heat was prospected.

Molten-Salt Power Towers: Newly Commercial Concentrating Solar Storage

Molten-salt storage is already commercially available for concentrating solar power (CSP) plants, allowing solar power to be produced on demand and to “backup” variable renewable sources such as wind and photovoltaics. The first CSP plants to operate commercially with molten-salt storage utilized parabolic trough concentrators, for example, the Andasol-1 plant. A new type of storage plant has now reached commercial status, with the 19.9-MWe Torresol Gemasolar power tower, featuring 15 h of molten-salt storage, having come online in Spain in May 2011. Advantages of the power tower storage system include the elimination of heat transfer oil and associated heat exchangers, a lower salt requirement, higher steam cycle efficiency, better compatibility with air cooling, improved winter performance, and simplified piping schemes. Near-term advances in molten-salt power tower technology include planned up-scaling, with SolarReserve due to begin constructing a 110-MWe plant in Nevada by August 2011. Other advances include improvements to the thermal properties of molten salts and the development of storage solutions in a single tank. With these developments at hand, CSP will continue to provide dispatchable solar power, with the capacity to provide energy storage for 100% renewable electricity grids in sun-belt countries.