High Temperature Heat Transfer Fluid Research Articles

Revealing 3D Morphological and Chemical Evolution Mechanisms of Metals in Molten Salt by Multimodal Microscopy.

Growing interest in molten salts as effective high-temperature heat-transfer fluids for sustainable energy systems drives a critical need to fundamentally understand the interactions between metals and molten salts. This work utilizes the multimodal microscopy methods of synchrotron X-ray nanotomography and electron microscopy to investigate the 3D morphological and chemical evolution of two-model systems, pure nickel metal and Ni-20Cr binary alloy, in a representative molten salt (KCl-MgCl2 50-50 mol %, 800 °C). In both systems, unexpected shell-like structures formed because of the presence of more noble tungsten, suggesting a potential route of using Ni-W alloys for enhanced molten-salt corrosion resistance. The binary alloy Ni-20Cr developed a bicontinuous porous structure, reassembling functional porous metals manufactured by dealloying. This work elucidates better mechanistic understanding of corrosion in molten salts, which can contribute to the design of more reliable alloys for molten salt applications including next-generation nuclear and solar power plants and opens the possibility of using molten salts to fabricate functional porous materials.

Entropy Generation Minimization Analysis of Solar Salt, Sodium, and Lead–Bismuth Eutectic as High Temperature Heat Transfer Fluids

Abstract The amount of entropy generation in heat transfer devices impacts their operation economy and should therefore be minimized during the design phase. Entropy generation also depends on the individual thermophysical properties of the heat transfer fluid (HTF). An entropy generation minimization analysis of three different liquid coolants, namely, solar salt (SS), sodium, and lead-bismuth eutectic (LBE) is thus performed for fully turbulent flow in a circular tube under circumferentially uniform heat flux by considering a heat rate, inlet and outlet temperatures and heat flux densities typical of a concentrated solar tower plant. The Reynolds number is determined at which the proper combination of Nusselt number and friction factor minimizing the entropy generation, and consequently the exergy loss, is obtained as well as the best thermodynamically performing coolant fluid over the operating range of Reynolds numbers. Sodium can operate at 60% lower entropy generation than solar salt while providing a smaller wall-to-bulk temperature difference. Despite its high thermal conductivity, LBE performs similarly to solar salt. However, it can be advantageous compared to solar salt if operated at higher temperatures that cannot be achieved by the latter due its thermal stability limit.

Performance study on eutectic molten salt as a high temperature working fluid in the parabolic trough collector

The growth of renewable energy technologies has made remarkable changes in the engineering field. In this paper, a progressive study has been made on the Solar Parabolic Trough Collector (SPTC) using molten salt as an effective high temperature Heat Transfer Fluid (HTF). Molten salt blend is formed with 60% of sodium nitrate (NaNO3) as and 40% of potassium nitrate (KNO3) since this ratio can tackle a temperature of about 550°C. The experiment has been conducted in two phases, one with a cylindrical circular cross-section absorber tube and another with a new model tube with elliptical cross-section. The test also records outlet temperature by altering three mass flow rates of HTF, 0.014, 0.021 and 0.028 kg/s, from August 2017 to July 2018. The efficiency with the elliptical absorber is 14.5% which is higher than that of circular absorber tube while using molten salt as an HTF.

Hot corrosion of different alloys in chloride and carbonate molten-salt mixtures under argon atmosphere

Molten eutectic salt mixtures can be used as thermal energy storage materials or heat transfer fluids at concentrated solar power plants. The targeted operating temperature for the next generation concentrated solar power plants is 700 °C. Although chloride and carbonate eutectic salt mixtures with high thermally stable temperatures were reported to have great thermophysical properties, their corrosion to alloys at high temperature has not been thoroughly studied. Corrosion of the thermal energy storage materials and heat transfer fluids to alloys is critical, and oxygen in air is confirmed to accelerate the corrosion significantly. In the present study, three eutectic mixtures of NaCl-KCl-ZnCl2, Li2CO3-Na2CO3-K2CO3 and LiF-Na2CO3-K2CO3 were considered as high temperature thermal energy storage materials or heat transfer fluids. Corrosion of stainless steel 316, Hastelloy C276, Inconel 625 and 718 immersed in the three salt mixtures up to 21 days were experimentally investigated at 700 °C under argon atmosphere. The specimens after corrosion tests were characterized by scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. The results show that SS316 is only compatible with the LiNaK carbonate-salt and the corresponding corrosion rate is lower than 95 μm/year. Both C276 and In625 have relatively good corrosion resistance for the NaKZn chloride-salt and LiF-NaK carbonate-salt, and the corrosion rates are lower than 78 μm/year. The highest corrosion rate over 1700 μm/year is detected for In625 immersed in the LiNaK carbonate-salt. In addition, In718 shows excellent corrosion resistance less than 10 μm/year immersed in all the three salt mixtures.

Microstructures and Thermal Properties of Sn–Bi–Zn–Ga Alloys as Heat Transfer and Heat Storage Materials

Low melting point alloy is a potential high-temperature heat transfer medium because of the high thermal conductivity, low solidus temperature and wide range of use temperature. Consequently, we investigated the possibility of using Sn–Bi–Zn–Ga alloys as heat storage and heat transfer material. Moreover, we investigated the microstructure and phase compositions by electron probe micro-analysis (EPMA) and X-ray diffusion (XRD). Results show that the new structures and phases are formed in the alloy matrix with Ga additions, which lead to the improvement of the thermal properties. An extensive thermophysical characterization of the Sn–Bi–Zn–Ga alloys has been performed by differential scanning calorimeter (DSC) analysis. The addition of Ga lowers the peak temperature and increases the heat capacity of the alloys. The thermal expansion of the test alloys increases with increasing temperature and the densities decreases with Ga additions. As the density, specific heat capacity and thermal diffusivity change with temperature and physical state, the thermal conductivity of the alloys first decreases and then increases. These results demonstrate the feasibility of using Sn–Bi–Zn–Ga alloys as the high-temperature heat transfer fluid.

Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials

Molten chloride salts are promising advanced high-temperature (400–800 °C) thermal energy storage (TES) and heat transfer fluid (HTF) materials in next generation concentrated solar power (CSP) plants for higher energy conversion efficiencies. However, severe corrosion of structural materials in contact with molten chloride salts is one of the most critical challenges limiting their applications at elevated temperatures. In this work, two corrosion mitigation strategies are investigated to alleviate the hot corrosion of structural materials in molten chloride salts: (1) adding corrosion inhibitor and (2) using a Fe-Cr-Al alloy with a protective alumina layer on the surface after pre-oxidation. Three commercial high temperature Fe-Cr-Ni alloys (SS 310, Incoloy® 800 H and Hastelloy® C-276) were exposed to molten MgCl2-NaCl-KCl (60–20–20 mol%) mixed salts with 1 wt% Mg as corrosion inhibitor, for 500 h at 700 °C under inert atmosphere. By addition of the Mg inhibitor, the corrosion rates of the studied alloys were found to be significantly reduced, more precisely by ~ 83% for SS 310, ~ 70% for In 800 H and ~ 94% for Ha C-276 compared with the exposure tests without Mg addition. The corrosion mitigation mechanism of Fe-Cr-Ni based alloys in molten chloride salts by adding Mg is discussed based on corrosion thermodynamics. To assess the second mitigation strategy two pre-oxidized alumina forming Fe-Cr-Al alloys were exposed to the same molten chloride salts without Mg corrosion inhibitor under the same conditions. It is observed that the adherent alumina scales can effectively inhibit the dissolution of Cr and Fe and the bulk penetration of corrosive impurities. Overall, both strategies offer enormous potential for enhancing the expected lifetime of commercial alloys in molten chloride salts.

Experimental and numerical studies on molten salt migration in porous system with phase change

Molten salt is promising high temperature heat transfer fluid, and its transport in porous media is an important problem for molten salt application. In this paper, molten salt migration and phase change in cold porous system packed with sand particles is experimentally and numerically studied. Experimental results show that high temperature molten salt continuously migrates and a transparent liquid molten salt layer appears during discharge stage with molten salt pouring into porous bed, and then it solidifies as white opaque solid block during post-discharge stage. A transient axial-symmetrical flow and heat transfer model is developed using volume of fluid model and linear approximation in mushy zone, and the simulated results very well fit with experiment. After molten salt discharges and contacts with the cold surface, a thin solid layer quickly forms for solidification, and then it gradually expands and finally becomes a solid block. Since the earlier solid layer hinders molten salt vertical flow, liquid molten salt will flow across the outer boundary of solid layer, and then molten salt layer below the surface becomes thicker. After molten salt totally solidifies, an inner region with little molten salt will probably exist inside molten salt solid block. The maximum migration region for molten salt in cold porous system is affected by structural and operating parameters. For larger porosity and particle diameter or higher molten salt temperature, molten salt flow in porous system has less flow resistance, and then the maximum migration height can be increased, while the migration diameter is reduced.

Molten carbonate salts for advanced solar thermal energy power plants: Cover gas effect on fluid thermal stability

The eutectic mixture Li2CO3-Na2CO3-K2CO3 is investigated as a high temperature heat transfer fluid and storage medium alternative for molten salt solar thermal power plants. This salt has an operating temperature range of 400–700 °C, enabling the use of higher temperature/efficiency power cycles. However, this carbonate mixture is known to thermally decompose in air. This study evaluates the thermal stability of the salt mixture under different cover gases: air, nitrogen, carbon dioxide, and an 80/20 carbon dioxide/air mixture. Initial characterization is performed through thermogravimetric analysis (TGA), followed by larger scale testing in a custom-made reactor to simulate conditions closer to its practical use. The results show improved thermal stability with a CO2 atmosphere. The decomposition kinetics under different cover gases are estimated from TGA data. However, larger-scale, longer duration experiments show much slower decomposition rates compared to the classical TGA approach. These findings indicate that the main contribution to mass loss in TGA is due to vaporization rather than thermal decomposition. Thus, a proper evaluation of the molten salt´s thermal stability can only be obtained from reactor experiments where vaporization is inhibited. Very long induction periods (of the order of days) are observed, suggesting that the kinetic decomposition mechanism is a nucleation and growth type. Other considerations for future plants incorporating these high temperature salts are discussed.

Corrosion behavior of metallic alloys in molten chloride salts for thermal energy storage in concentrated solar power plants: A review

Recently, more and more attention is paid on applications of molten chlorides in concentrated solar power (CSP) plants as high-temperature thermal energy storage (TES) and heat transfer fluid (HTF) materials due to their high thermal stability limits and low prices, compared to the commercial TES/HTF materials in CSP-nitrate salt mixtures. A higher TES/HTF operating temperature leads to higher efficiency of thermal to electrical energy conversion of the power block in CSP, however causes additional challenges, particularly increased corrosiveness of metallic alloys used as containers and structural materials. Thus, it is essential to study corrosion behaviors and mechanisms of metallic alloys in molten chlorides at operating temperatures (500–800 °C) for realizing the commercial application of molten chlorides in CSP. The results of studies on hot corrosion of metallic alloys in molten chlorides are reviewed to understand their corrosion behaviors and mechanisms under various conditions (e.g., temperature, atmosphere). Emphasis has also been given on salt purification to reduce corrosive impurities in molten chlorides and development of electrochemical techniques to in-situ monitor corrosive impurities in molten chlorides, in order to efficiently control corrosion rates of metallic alloys in molten chlorides to meet the requirements of industrial applications.

Experimental Test of Properties of KCl–MgCl2 Eutectic Molten Salt for Heat Transfer and Thermal Storage Fluid in Concentrated Solar Power Systems

The eutectic mixture of MgCl2–KCl molten salt is a high temperature heat transfer and thermal storage fluid able to be used at temperatures up to 800 °C in concentrating solar thermal power systems. The molten salt thermophysical properties are reported including vapor pressure, heat capacity, density, viscosity, thermal conductivity, and the corrosion behavior of nickel-based alloys in the molten salt corrosion at high temperatures. Correlations of the measured properties as functions of molten salt temperatures are presented for industrial applications. The test results of tensile strength of two nickel-based alloys exposed in the molten salt at a temperature of 800 °C from 1-week length to 16-week length are reported. It was found that the corrosion and strength loss is rather low when the salt is first processed to remove water and oxygen.

Photo-thermal characteristics of hybrid nanofluids based on Therminol-55 oil for concentrating solar collectors

Hybrid nanofluids are the new generation efficient heat transfer fluids allowing greater control over the properties of base fluid as compared to mono-nanofluids. In this study, attempt has been made for increasing the efficiency for photo-thermal conversion by heat transfer fluid for high temperature solar collectors. Therminol-55, a high temperature heat transfer fluid is doped with Al2O3 and TiO2 nanoparticles with an aim to improve the thermal and optical properties. Effects of concentration and type of nanoparticle on photo-thermal conversion properties and absorbance in Therminol-55 have been studied. Spectrophotometric analysis has been carried for all nanofluids, namely, Al2O3–Therminol-55, TiO2–Therminol-55 and hybrid (Al2O3–TiO2)–Therminol-55 nanofluids with varying concentrations of 0.05, 0.075, 0.1, 0.25, 0.5 wt%. It was found that TiO2 nanofluids possess the maximum absorbance with minimal effect of nanoparticle concentration above 0.1 wt% followed by hybrid (Al2O3–TiO2) nanofluid (HNF) with strong dependence of concentration. Al2O3–Therminol-55 nanofluids exhibited least absorbance. The peak values of absorbance are 0.47, 2.15 and 2.144 in the visible region for Al2O3–Therminol-55, TiO2–Therminol-55 and hybrid (Al2O3–TiO2)–Therminol-55 nanofluids, respectively. It was observed that hybrid nanofluids show both bathochromic and hyperchromic shifts. Further, performance testing has been carried out using artificial source of light and it has been observed that hybrid nanofluids provide efficient photo-thermal conversion as compared to TiO2 and Al2O3–Therminol-55 nanofluids. Maximum temperatures of 152.9, 149.6, 158.6 °C were observed for 0.5 wt% Al2O3–Therminol-55, 0.1 wt% TiO2–Therminol-55, and 0.5 wt% hybrid (Al2O3–TiO2) nanofluid, respectively, against 125.8 °C of Therminol-55. Hybrid nanofluids based on Therminol-55 could be a potential candidate for high temperature concentrating collectors based on the superior properties over mono-nanofluids and base fluid.

Basic properties of eutectic chloride salts NaCl-KCl-ZnCl2 and NaCl-KCl-MgCl2 as HTFs and thermal storage media measured using simultaneous DSC-TGA

Concentrated solar power (CSP) technologies tend to work at high temperatures for better energy efficiency, which need high temperature heat transfer fluids (HTF) and thermal storage. A good HTF should have low melting point and be thermally stable at high temperatures. A thermal storage medium needs a high heat capacity in order to store more energy in per unit of mass or volume. The objective of this work is to study the basic thermophysical properties of several eutectic molten salts for HTF and thermal storage media, which can work at upper limit temperatures of 850 °C. The eutectic salts are mixtures by NaCl-KCl-ZnCl2, and by NaCl-KCl-MgCl2, respectively. The heat capacity, melting point, and heat of fusion of the eutectic salts at different temperatures were measured using Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA) simultaneously. Details of the measurements and obtained experience of operating the equipment are provided in the work. For each of the eutectic salts multiple samples were tested to check repeatability and to minimize random errors. In theoretical analysis, mixing rules from literature was used to estimate the heat capacity of the new HTFs, which showed good agreement with experimental data.

Properties and heat transfer coefficients of four molten-salt high temperature heat transfer fluid candidates for concentrating solar power plants

Heat transfer fluid is one critical component for transferring and storing heat energy in concentrating solar power systems. Molten-salt mixtures can be used as high temperature heat transfer fluids because of their thermophysical properties. This paper studied the thermophysical properties of Li2CO3-Na2CO3-K2CO3 eutectic salt and three eutectic chloride salts NaCl-KCl-ZnCl2 with different compositions in the range of 450-600°C and 250-800°C, respectively. Properties including specific heat capacity, thermal conductivity, density and viscosity were determined based on imperial correlations and compared at different operating temperatures. The heat transfer coefficients of using different eutectic salts as heat transfer fluids were also calculated and compared in their operating temperature range. It is concluded that all the four eutectic salts can satisfy the requirements of a high-temperature heat transfer fluid.

Entropy generation and Carnot efficiency comparisons of high temperature heat transfer fluid candidates for CSP plants

Heat transfer fluid is a critical component in a concentrating solar power plant. A large quantity of heat transfer fluid is required to transfer heat between the solar collector and the power block, thus it is crucial to select the most appropriate heat transfer fluid in order to maximize the system performance. The present study compared the performances of five molten-salt eutectic mixtures in regarding with the entropy generation rate and the Carnot efficiency of using them as heat transfer fluids. All the five molten-salt eutectic mixtures have thermal stability temperatures above 600 °C. Effects of the tube lengths in the steam generation heat exchanger and the receiver heat exchanger as well as the heat transfer fluid flow rate on both the entropy generation rate and the Carnot cycle efficiency were investigated. The results indicate that the carbonate salts has the worst performances compared to the other eutectic mixtures. The three chloride salts have slightly higher entropy generation rate and 5% higher Carnot efficiency than the Solar Salt. Therefore the three chloride salts are suggested to be used in advanced concentrating solar power tower plants as potential high temperature heat transfer fluids.

An experimental study of the effect of exfoliated graphite solar coating with a sensible heat storage and Scheffler dish for desalination

The high-temperature solar coatings and heat transfer fluids play a key role in increasing the performance of concentrating solar power (CSP)/desalination plant. In this paper, the performance of a new type of exfoliated graphite (EG) solar coated on the receiver with sensible heat storage and Scheffler dish is evaluated. The results show that using of exfoliated graphite coating on absorber plate, the system enhances performance up to 40%, thermal stability of 643.9°C on a rough surface with an absorptive of more than 97% of the solar incident. The several experiment trials conducted in microwave oven show that, EG has high thermal stability even upto 750°C and observed no significant degrading of thermal properties. The outcome of this research confirms the suitability of such a system for the cavity, volumetric air and surface receivers used for the various solar thermal applications.

Minimum system entropy production as the FOM of high temperature heat transfer fluids for CSP systems

Abstract High temperature heat transfer fluids are used in concentrated solar thermal power (CSP) plants to receive heat from a solar concentrator and then transfer it to a working fluid (such as water, air, or super critical CO 2 ) in a thermal power system. For various high temperature heat transfer fluids, such as, synthetic oils, various molten salts, and liquid metals, a general criterion is proposed in this work to evaluate the merit of fluids regarding their transport properties. For the goal of transferring a desired amount of heat, a fluid that causes less entropy production is believed to have better figure of merit (FOM). This is due to the fact that entropy production is associated with the destruction of exergy or useful energy. The entropy production in a heat transfer system in a solar thermal power plant includes the part due to the processes of heat addition and removal and the other part due to pressure losses in the flow in heat exchangers and pipes. Theoretical analysis and relevant equations for total entropy production are derived. As an example, the FOM for several heat transfer fluids used in CSP industry are compared for the goal of heat transport in the range of 50 MW th to 600 MW th . The modeling provides a fundamental approach for the comparison of various heat transport systems which may have different designs and using different heat transfer fluids/media (gas, liquid, or solid particles) in CSP systems.

Survey and evaluation of equations for thermophysical properties of binary/ternary eutectic salts from NaCl, KCl, MgCl2, CaCl2, ZnCl2 for heat transfer and thermal storage fluids in CSP

Abstract Considered as promising candidates of composing eutectic chloride molten salts for high temperature heat transfer fluid (HTF) with low corrosivity to nickel-based alloys, NaCl, KCl, MgCl2, CaCl2, and ZnCl2 are formulated into proper binary and ternary mixtures to satisfy the property requirement for HTFs. Experimental and predicted thermophysical properties of 16 selected eutectic binary and ternary molten salts are provided. The molten salts can work in a temperature range from after eutectic melting to at least 800 °C with low vapor pressure and acceptable low corrosivity. Surveyed equations and correlations for thermophysical properties of salt mixtures are used to predict heat capacity, density, thermal conductivity, and viscosity. The results are compared to experimental data from the author’s tests as well as reported in literature. Evaluated equations and correlations that can better predict the transport properties of the salts are selected and recommended. Authors expect that more experimental tests will be conducted to provide data for application in CSP industry.

Thermal and Transport Properties of NaCl–KCl–ZnCl2 Eutectic Salts for New Generation High-Temperature Heat-Transfer Fluids

Three eutectic salts from a system of halide salts NaCl–KCl–ZnCl2 were chosen for detailed study of thermal and transport properties with the objective of developing a next generation high-temperature heat-transfer fluid (HTF) for concentrated solar thermal power (CSP) technology. The acceptable range of the working temperatures for the HTF is from below 250 °C to at least 800 °C. The tested properties are presented here for the three candidate eutectic salts, including melting point, heat of fusion, heat capacity, vapor pressure, viscosity, density, and thermal conductivity. Data-fitted equations are provided for all the measured properties for convenience in engineering application. It is concluded that the three eutectic salts can satisfy the needs for a high-temperature HTF and thus are recommended as a new generation high-temperature HTF.

Novel methodology to prepare homogenous ternary molten salts for concentrated solar power applications and their thermo-physical characterization

Homogeneous ternary molten salt mixtures have been synthesized using novel and robust preparation method. In a typical preparation method, the ternary molten salt compositions contain two salts in common (KNO3 and LiNO3) and one additional hydrated salt. Thermo-physical properties of various synthesized ternary molten salt mixtures were evaluated by using simultaneous TGA-DSC, Dynamic Shear Rheometer and Thermal conductivity analyzer. Thus prepared ternary molten salt mixtures exhibited improved thermo-physical properties in the form of low melting point, high thermal stability, high specific heat capacity, high thermal conductivity and low viscosity, which are essential for the high temperature heat transfer fluids in concentrated solar power (CSP) applications.

On-site comparison of flowmeters installed in a parabolic-trough solar collector test facility

This article presents a comparison of flow measurements taken by three different types of flowmeters in a parabolic-through solar collector testbed working with a high-temperature silicone heat transfer fluid. Two of them were vortex volumetric flowmeters and the third one was a Coriolis mass flowmeter. They were tested at mass flow rates from 1.5 kg/s to 5 kg/s, and at heat transfer fluid temperatures of 140–340 °C. The Coriolis mass flowmeter was used as the reference to check the performance of the two vortex flowmeters. Relative errors and En numbers calculated were compared. The maximum relative error between vortex flowmeters measurements and Coriolis mass flow measurements was 2.5%. Correction factors were calculated to correct the vortex flowmeters readings.