Thermal Conductivity Of Molten Salts Research Articles

An improved laser flash method for thermal conductivity measurement of molten salts

Thermal conductivity measurement of high-temperature heat transfer fluids provides a crucial basis for designing utility-scale thermal systems. Molten salts are promising heat transfer and thermal storage fluids in high-temperature thermal energy storage systems, while the molten salt thermal conductivity obtained in existing studies exhibits large deviations due to the high experimental complexity and unstandardized test procedures. In this work, we improve the conventional laser flash analysis method by proposing a theoretical heat transfer model for multi-layer heat conduction and providing a near-optimal molten salt container design. With water as a test sample, the relative error of thermal conductivity measurement using the improved method is 6.3%. The thermal conductivity of Solar Salt from 250 to 400 ​°C, and of Hitec Salt from 160 to 250 ​°C are measured and compared with the previous work. Both results show that the thermal conductivity increases with the temperature rising. This work will promote the technology standardization for accurately acquiring the thermal conductivity of molten salts or other similar high-temperature heat transfer fluids.

Impact of nanoparticle agglomeration on thermal conductivity of molten salt based nanofluids: Insights from molecular dynamics and lattice Boltzmann methods

The nanoparticle aggregation has significant influence on heat transfer properties. The contradictions between different researches exist and the underlying mechanisms of heat conduction remain unclear. To address these issues, the thermal conductivity of solar salt-SiO2 nanofluids in the non-aggregated and aggregated states was investigated, based on the combination of the advantages of molecular dynamics (MD) in revealing physical insights and lattice Boltzmann method (LBM) in simulating properties for complex structures. LBM models were simplified on the basic of MD results. Different heat transfer physics in nanofluids at nanoscale and mesoscale were compared. The results show that the thermal conductivity of aggregated nanofluid is higher than the non-aggregated nanofluid. By analyzing the diffusion coefficient, number density and vibrational density of states, we found that the previous hypotheses (such as Brownian motion, interfacial layer and interfacial thermal resistance) cannot explain the enhanced thermal conductivity induced by the nanoparticle agglomeration. The heat transfer mechanisms of aggregated nanofluid were further analyzed from new perspectives, i.e., the contributions of different material components and fluctuation modes to heat flux. It is found that the nanoparticle aggregation leads to the nanoparticle contribution increase, indicating the formation of low thermal resistance channels within the aggregates. The heat conduction contributed by collision decreases in the aggregated state, while that contributed by potential energy increases. Moreover, the thermal conductivity of aggregated nanofluid is negatively correlated with temperature, aggregate size and fractal dimension of agglomerates, and positively correlated with mass fraction of nanoparticles, degree of agglomeration, and backbone length of each aggregate.

Production of low molecular oil from typical polyolefin plastics through molten salts thermal treatment

The resource recovery based on pyrolysis has become the mainstream disposal method for polyolefin plastics in recent years. However, the poor heat transfer characteristics and uneven temperature distribution of plastics lead to inferior product quality. To address these problems, molten salts thermal treatment was performed to conduct targeted regulation of the process. It revealed that the excellent heat storage and thermal conductivity of molten salts provide a strengthened heat source for plastics, promoting product homogenization and high-value conversion. It reduced the energy barrier of the reaction and prompted the conversion of macromolecular components to light oil. Meanwhile, the catalysis of alkali metal salts effectively inhibited the transition cracking of major oil components. The yield of pyrolysis oil increased while the valuable products gradually accumulated, such as α-olefins (ranging from 26.48 % to 31.06 %) and styrene (ranging from 48.89 % to 64.03 %). Additionally, the kinetics of hydrogen could be used to analyze the dynamic processes of plastic pyrolysis well and the activation energy was significantly reduced with molten salts. The contracting volume model was considered to be the most suitable reaction model, providing critical insight for the thermal treatment of plastics at larger scales.

Development of topology-optimized structural cavities macro-encapsulating chloride salt by gel-casting for high-temperature thermal energy storage

The thermal energy storage (TES) technology based on high-temperature molten salt is playing an increasingly important role in advanced solar energy utilization and waste heat recovery. However, the high corrosivity and low thermal conductivity of molten salt have limited the efficient application of TES systems. This work reports a facile and novel sacrificial paraffin template method, aiming to create Al2O3 ceramic macrocapsules with topology-optimized structural cavities by paraffin blocks with a reverse-designed structure for macro-encapsulating molten salt. The objective function of the maximum thermal conductivity of the design domain is calculated by a topology-optimized method to optimize the layout of high conductive ceramics. Among them, the optimized macrocapsules with a volume ratio of 25 % ceramic (S25) have a 62.8 % reduction in charging time and a 101.5 % increase in charging rate. Furthermore, after being filled with eutectic chloride salt, S25 could present a satisfactory heat storage density of 272.58 J∙g−1 and 748.3 J∙cm−3 at the temperature range of 550–750 °C. The macrocapsules also show good thermal cycling stability without degradation of thermophysical properties after hundred cycles. These improved results demonstrate the potential application of the macrocapsule in high-temperature TES systems.

Thermal conductivity measurement using modulated photothermal radiometry for nitrate and chloride molten salts

Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corrosive and conducting nature of these fluids also leads to time consuming sample preparation processes of many contact-based measurements. Here, we report the measurement of thermal conductivity of molten salts using a modulated photothermal radiometry (MPR) technique, which is a laser-based, non-contact, frequency-domain method adopted for molten salts for the first time. By unitizing the advantages of front side sensing of frequency-domain measurements and the vertical holder orientation, the technique can minimize the natural convection and salt creeping effects, thus yielding accurate molten salt thermal conductivity . The MPR technique is first calibrated using standard molten materials including paraffin wax and sulfur. It is then applied on measuring pure nitrate salts (NaNO3 and KNO3), solar salt (NaNO3–KNO3 mixture), and chloride salt (NaCl–KCl–MgCl2). The measurement results are compared with data from literature, especially those obtained from laser flash analysis (LFA). Our results demonstrate that the MPR is a convenient and reliable technique of measuring thermal conductivity of molten salts. Accurate thermal conductivity data of molten salts will be valuable in developing the next-generation high-temperature thermal energy storage and conversion systems.

Advances in High-Temperature Molten Salt-Based Carbon Nanofluid Research

Molten salt is an excellent medium for heat transfer and storage. The unique microstructure of carbon nanomaterials leads to good mechanical stability, low density, high thermal conductivity, and high strength, etc. The addition of carbon nanomaterials to molten salt to form molten salt nanofluid can remarkably enhance the specific heat capacity and thermal conductivity of molten salt and reduce the molten salt viscosity, which is of great importance to increase the heat storage density and reduce the heat storage cost. Nevertheless, some challenges remain in the study of such nanofluids. The main challenge is the dispersion stability of carbon nanomaterials. Therefore, to improve research on carbon nanofluids, this paper summarizes the progress of carbon-based molten salt nanofluid research worldwide including the preparation methods of molten salt nanofluids, the improvement of heat transfer performance, and the improvement of heat storage performance. The effects of carbon nanoparticle concentration, size, and type on the heat transfer and storage performance of molten salt are derived, and the effects of nanoparticle shape on the heat transfer performance of molten salt are analyzed while more promising preparation methods for carbon-based molten salt nanofluids are proposed. In addition, the future problems that need to be solved for high-temperature molten salt-based carbon nanofluids are briefly discussed.

Effect of SiO2 nanoparticles concentration on the corrosion behaviour of solar salt-based nanofluids for Concentrating Solar Power plants

Recently, corrosion of nanoparticles molten salt-based nanofluids studies have emerged as Concentrating Solar Power plants provide a low carbon alternative to produce electricity. Enhancing the heat capacity and thermal conductivity of molten salts by using inorganic nanoparticles has been targeted as a strategy to decrease the overall investment cost of CSP systems. However, there is scarce and insufficient information about their effect on the corrosion behaviour of nanofluids, whether the nanoparticle content increases it or have no significant effect. The scatter data found show no clear agreement and the measurements are done under different conditions (temperature, time, impurities, nanoparticle's chemical nature and concentration, metal and alloy composition, testing method). In this context, the authors evaluated the effect of SiO 2 nanoparticles concentration in an industrial-grade Solar Salt in contact with four different alloys; AISI 1045, 304H, 316L and Inconel 600 by isothermal tests, 500 °C up to 2160 h. The effect of nanoparticles, 0.5% and 1% wt., was evaluated in comparison with Solar Salt industrial grade. The corrosion rate of the samples decreased in the following order AISI 1045 > 304H > 316L > Inconel 600 and nanoparticles increased in general and to a different extent the corrosion rate of the alloys. The one that experiences the highest nanoparticle effect is stainless steel 304H, followed by AISI 1045 and Inconel. For 316L, no significant differences can be seen. The applicability assessment carried out has shown that molten salt nitrate-based nanofluids can be used with Inconel 600, 304H and 316L in long-term service high-temperature applications such as CSP. • SiO 2 nanoparticles affect the corrosion behaviour of molten salt nitrates. • Nanoparticles increase the corrosion rate of the studied alloys. • The increase of nanoparticles from 0.5 to 1 % wt. does not affect significantly corrosion. • 304H, 316L and Inconel 600 can be used for long-term service with 1 % wt. SiO 2 Solar Salt.

Enhanced thermal energy storage of sodium nitrate by graphene nanosheets: Experimental study and mechanisms

Molten salt is considered as one of the most promising candidate materials for solar energy storage due to its consistent performance and high energy storage density. The limited thermal conductivity of molten salt, on the other hand, has become a bottleneck for efficient energy storage and heat transfer. Here, we propose a composite phase change material (PCM) to realize ultrafast thermal energy storage based on sodium nitrate (NaNO3) doped graphene nanosheets (GNS). The thermal conductivity of the composite is improved by 245 % with GNS doping ratio of 3.0 wt%, which is due to better phonon vibration matching between GNS and NaNO3. The decrease of phase change enthalpy and melting point are explained from the perspective of mean square displacement. This paper provides a new idea for the selection criteria of the mass fraction of nano-enhancers for molten salt-based PCMs from the perspective of phonon vibration matching.

Preparation and Thermophysical Properties of Sodium Nitrate/Nanoparticle/Expanded Graphite Composite Heat Storage Material

As a medium and high temperature heat storage medium, the thermal performance of molten salt plays an important role in the thermal energy storage system. In order to improve the specific heat capacity and thermal conductivity of molten salt, a mechanical dispersion method is used to prepare a shape-stable composite phase change material for thermal energy storage. Nitrate (NaNO3) has a higher phase change latent heat, which was chosen to combined with different nanoparticles (SiO2, SiO2+TiO2) to improve its specific heat capacity, and with expanded graphite (EG) as a carrier matrix to improve thermal conductivity. Through the characterization of its chemical compatibility and thermophysical properties, the results show that the nanoparticles, EG and NaNO3 have good chemical compatibility. Compared to NaNO3, the thermophysical properties of composite (NaNO3:(SiO2+TiO2):EG = 84%:0.1% + 0.9%:15%, mass ratio) was improved obviously. The average specific heat capacity before the phase change increased from 1.45 J/(g·K) to 1.81 J/(g·K), and the average specific heat capacity after the phase change increased from 1.69 J/(g·K) to 2.47 J/(g·K); The thermal conductivity is about 13.9 times higher than that of NaNO3; in the range of 100–380°C, the heat storage density of the composites is about 679.2 kJ/kg 300 thermal stability tests showed that the latent heat of the composites is reduced by 7.4%, and the specific heat capacity before and after the phase change is reduced by 6.1 and 6.0%, respectively. The research can provide a broad application prospects in the field of medium temperature energy storage.

Effect and mechanism of doped graphene nanosheets on phase transition properties of sodium nitrate

Molten salt is regarded as one of most promising candidates for solar energy storage due to possessing stable properties and large energy storage densities. However, the intrinsically low thermal conductivity of molten salt has become a bottleneck for rapid heat storage and transport. The addition of nanoparticles is generally considered to be a most effective way to improve the thermal conductivity of molten salt phase change materials (PCMs), while the phase change enthalpies of the nanocomposite phase change materials usually show two opposite trends of enhancement or decrement. Furthermore, the reason for the abnormal change of phase change enthalpy has not been clear in the literature so far, so the mechanism of change needs to be further explored. In this work, graphene nanosheets (GNS)@NaNO<sub>3</sub> (sodium nitrate) nanocomposite phase change materials are prepared by the hydration ultrasonic method. The materials are characterized by scanning electron microscope, and the phase change characteristics are measured using differential scanning calorimeter. Molecular dynamics simulation is carried out to explain the mechanism for the formation of the NaNO<sub>3</sub> dense layer and the non-collateral decrease of the enthalpy from the microscopic level. With the increase of GNS mass fraction, the melting point of the GNS@NaNO<sub>3</sub> composite phase change material decreases slightly while the phase change enthalpy decreases significantly with a non-colligative trend. A 13.81% decrease of the theoretical phase change enthalpy is observed with a GNS doping ratio of 1.5%. The NaNO<sub>3</sub> clusters observed on the surface of GNS are considered to have not melted, thereby resulting in a reduction in the phase change enthalpy. The mechanism is further investigated by molecular dynamics simulation, showing that the strong van der Waals attraction between GNS and NaNO<sub>3</sub> leads the 2–4 Å-thick NaNO<sub>3</sub> dense layer to form in the vicinity of GNS. With the increase of GNS mass fraction, the centroid equivalent distance between the dense layer and GNS gradually increases, which leads their mutual attraction to first increase and then weaken. When GNS mass fraction is 1.5%, the centroid equivalent distance reaches the position closest to the potential well, leading to a strongest mutual attraction. In other words, the phase change enthalpy decreases most obviously at this mass fraction. Thus, some conclusions can be drawn as follows. The type of interaction between molten salt and nano-enhancers and the position of the potential well are the fundamental reasons for the thickness of molten salt dense layer and the reduction of phase change enthalpy. The calculation of the interaction energy can be used to guide the selection of the mass fraction of the nano-enhancers, so as to avoid the loss of core material cluster and phase change enthalpy caused by the introduction of the nano-enhancers to a greatest extent. The preparation cost of the composite phase change material can also be reduced to a certain extent.

Ab-initio molecular dynamics calculation on microstructures and thermophysical properties of NaCl–CaCl2–MgCl2 for concentrating solar power

NaCl–CaCl2–MgCl2 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.

Enhanced heat conduction in molten salt containing nanoparticles: Insights from molecular dynamics

The addition of nanoparticles is a promising strategy to manipulate the thermal transport property of molten salt, which is an important thermal energy storage material for concentrating solar power. The molecular dynamics simulations have been conducted to investigate the mechanisms of enhanced heat transfer in binary molten salt based nanocomposite with nonmetallic nanoparticle SiO2. The results show that the thermal conductivity of molten salt is increased by introducing nanoparticles, which is up to 54.5% increase under 10%wt. loading of SiO2 nanoparticle. Moreover, the thermal conductivity increases as the nanoparticle loading rises. To elucidate the underlying mechanisms for the enhancement of thermal conductivity, the mean square displacement and size effect of nanoparticle, the structure and density of the ordered layer were analyzed. It is found that the possible mechanisms, including the Brownian motion of nanoparticle, the micro-convection of base fluid, and the ordered layer at the solid-liquid interface, for enhanced heat transfer proposed in previous literatures cannot explain the change in thermal conductivity of molten salt. The increment in thermal conductivity can be attributed to the improved probability and frequency of ion collision, as evidenced by the change of potential energy. The present findings verify the applicability for the molten salt-nanoparticle composite, moreover, highlight the correlation between the potential energy and the enhancement of thermal conductivity, which may provide guidance on the chosen of materials and design of the molten salt-nanoparticle composite.

Natural convection heat transfer characteristics of molten salt with internal heat generation

Molten salt is a promising heat transfer fluid with its high heat capacity, high boiling point, and chemical stability. In liquid fueled molten salt reactors and spent fuel reprocessing unit, molten salt carries heat generating fuel itself in a liquid state. To investigate the natural convection heat transfer performance of molten salt, a vertical heater vertical cooler rectangular loop filled with chloride salt containing liquid fuel, NaCl-KCl-(Pu, 238U)Cl3, is simulated using an open source CFD package, OpenFOAM under various heat input conditions; external heat flux, internal heat generation (IHG), and a combined heat input type condition. Different aspects of natural convection heat transfer according to heat input condition are found for same amount of total heat. The external heat flux condition generates a high temperature gradient near the heater wall with very thin thermal boundary layer due to the low thermal conductivity of molten salt. On the other hand, the IHG condition has a relatively flat temperature profile from its uniform volumetric heating. Due to the different behaviors of temperature evolution, local upward flow is formed only at the wall region under the heat flux condition, but global natural circulation flow is promoted under IHG condition. The combined type condition is largely dependent on how much heat exists in heat flux type, which strongly influences local heat and fluid phenomena in natural convection heat transfer. Molten salt has a high dependence on the heat input condition from its thermo-physical properties. Therefore, the distinct characteristics of molten salt and heat input conditions should be considered, when designing and analyzing the molten salt systems with both external and internal heating conditions.

Unexpected effect of nanoparticles doping on the corrosivity of molten nitrate salt for thermal energy storage

Molten nitrate salts are currently the most common mature solution for thermal energy storage at the concentrated solar power (CSP) plants. Enhancing heat capacity and thermal conductivity of molten salts via doping by inorganic nanoparticles has attracted an explosively increasing interest due to the possibility of a considerable decrease of the investment costs for CSP technology. However, to the best of our knowledge there is almost no information on the effect of such doping on the corrosivity of the molten salts. In this work we demonstrate that adding small amounts of nanoparticles into the molten nitrate HitecXL salt considerably increases its corrosivity and modifies the corrosion mechanisms. A set of advanced techniques such as SEM-EDX, XPS and XRD are applied to get insights into the effect of inorganic nano-additives on the corrosion phenomenon.

Application of an improved firework algorithm for simultaneous estimation of temperature-dependent thermal and optical properties of molten salt

Accurate measurement of the thermal and optical properties of molten salt is important in harvesting of solar energy. In this study, a secondary optimization technique that can be used to simultaneously retrieve the temperature-dependent absorption coefficient and the thermal conductivity of molten salt was proposed. The forward coupled radiation–conduction problem in a participating slab exposed to a pulse laser was solved by the finite volume method. An improved firework algorithm (FWA) was introduced to solve inverse problems, which was proven to be more efficient and robust than the original FWA and particle swarm optimization. All research outcomes indicated that the thermal and optical properties of high-temperature molten salt could be retrieved accurately by the proposed secondary optimization technique from the optical and thermal measurement signals.

Molar thermal conductivity of molten salts

Analysis of the available thermal conductivity data for the molten alkali and alkaline-earth salts, performed with the standard least square method and the interval analysis, reveals that the molar thermal conductivity of the molten alkali halides and alkali nitrates at temperatures close to their melting point has a constant value, equal to 19.08 and 23.42 W · m2 · mole−1 · K−1 · 10−6, respectively. The molar thermal conductivity of molten salts having bi-charged ions is twice higher than that of univalent salts. It means that the molar thermal conductivity depends on the charge of ions comprising salt. The molar (equivalent) thermal conductivity was found to be temperature independent. It allows evaluating the thermal conductivity of molten salt mixtures. The thermal conductivity for some molten mixtures (LiF-KF, CsCl-BaCl2, NaNO3-KNO3) was calculated considering the equivalent thermal conductivity as a constant.

A Galinstan-Filled Capillary Probe for Thermal Conductivity Measurements and Its Application to Molten Eutectic $$\hbox {KNO}_3$$–$$\hbox {NaNO}_3$$–$$\hbox {NO}_2$$ (HTS) up to 700 K

The successful measurement of the thermal conductivity of molten salts is a challenging undertaking due to the electrically conducting and possibly also aggressive nature of the materials, as well as the elevated temperatures at which these data are required. For accurate and reproducible measurements, it is important to develop a suitable experimental apparatus and methodology. In this study, we explore a modified version of the transient hot-wire method, which employs a molten-metal-filled capillary in order to circumvent some of the issues encountered in previous studies. Specifically, by using a novel flexible U-shaped quartz-capillary, filled with a eutectic mixture of gallium, indium and tin, commercially known as Galinstan, we proceed to measure the thermal conductivity of molten eutectic $$\hbox {KNO}_3$$ – $$\hbox {NaNO}_3$$ – $$\hbox {NaNO}_2$$ . The new probe is demonstrated as being able to measure the thermal conductivity of this molten salt, which is found to range from $$0.48~\hbox {W}{\cdot }\hbox {m}^{-1}{\cdot }\hbox {K}^{-1}$$ at 500 K to $$0.47~\hbox {W}{\cdot }\hbox {m}^{-1}{\cdot }\hbox {K}^{-1}$$ at close to 700 K, with an overall expanded uncertainty (95 % confidence) of 3.1 %. The quartz is found to retain its electrically insulating properties and no current leakage is detected in the sample over the investigated temperature range. The thermal conductivity data reported in the present study are also used to elucidate a partial disagreement found in the literature for this material.

A theoretical model for predicting the thermal conductivity of binary molten salts

Molten salt is popular as heat transfer fluid in solar energy applications in the temperature level of hundreds of degrees or higher, but it would be a tedious work to measure the thermal conductivity at elevated temperature due to convective and radiative effects. In this paper, a theoretical model is presented for predicting the thermal conductivity of binary molten salts, in which only data of thermal conductivity, density and mass fraction of each component salt are needed. Comparisons with experimental data show that the model gives prediction of thermal conductivity with errors less than 6.4%. The linear mixing rule, however, always gives large positive errors, especially at about 0.5–0.5mol fraction, which may sometimes reach 33.8%. The present work indicates that accurate prediction of thermal conductivity of molten salt mixture, even at elevated temperature, can be achieved if the thermal conductivity of pure compounds potentially applied in molten salt mixtures is measured with enough accuracy.

Recommended values for the thermal conductivity of molten salts between the melting and boiling points

A model for the prediction of the thermal conductivity of molten alkali and alkaline earth salts is proposed. The model is a function of simple thermophysical properties of salts. The parameters required in the calculations are density, heat capacity, velocity of sound and melting temperature. Predictions are in good agreement with reliable experimental and molecular dynamics simulation data. The model displays the correct dependence of the thermal conductivity on temperature, and this is a useful method for a first evaluation of future experimental data.

A New Model for the Thermal Conductivity of Molten Salts

Anewmodelbasedonroughhard-spheretheoryisproposedforthethermal conductivity of molten salts. The model incorporates a smooth hard-sphere contribu- tion using the properties of argon, as well as characteristic parameters based on the melting point of the molten salt. It is demonstrated that it is possible to correlate the thermal conductivity of monovalent and multivalent molten salts within experimen- tal error using this approach. Furthermore, in salts with a common anion, the single adjustable parameter in the model exhibits regular behavior with the molecular weight ofthesalt.Itisalsoshownthatthethermalconductivityofseveralmolten-saltmixtures can be predicted without any mixture parameters.