High Temperature Heat Transfer Fluid Research Articles

Single-phase thermosyphon heat transport device, a future prospect for heat removal applications: An experimental comparison with heat pipe and two-phase closed thermosyphon

Over the years, the passive mode of heat transportation has gained immense popularity, which includes two-phase devices viz. Heat pipe (HP) and Two-phase closed thermosyphon (TPCT). But recently, the thermosyphon heat transport device (THTD) entered the pool, wherein the heat transfer fluid (HTF) in liquid phase transports heat by natural circulation (thermosyphon). Hence, to gauge their efficacy, in the present experimental investigation, the three devices (with the same geometry and HTF) are compared by imposing isothermal (40°C–90 °C) and isoflux (50–900 W) heating conditions.In isothermal mode, HP transports heat with a minimal temperature gradient compared to TPCT and THTD. The thermal resistance (TR) of TPCT and THTD is 5.4 to 10.1 times and 6.6 to 10.1 times respectively greater than HP. For isothermal temperatures below 80 °C, the TR of THTD is less than TPCT (viscous limit). Later the trend reverses as the TPCT outperforms, while THTD experiences HTF boiling. Further, the heat extracted by HP is 2.1 to 3.9 times and 3 to 3.5 times greater than TPCT and THTD respectively. Hence, its application is justified if cost is not a concern, while THTD (with high-temperature HTF) is suggested over TPCT on an economic basis. Even the isoflux mode showed a similar trend of TR. The system response (based on time constant) depicted both HP and TPCT have similar and nearly constant values over the heat load range, while in THTD, it decreases with an increase in heat load and approaches the other two. Hence, a high-temperature HTF would lead to competitive results. Further, concerning the heat transport distance, as the TR of THTD remained the same compared to the increment in TPCT, the application of the former is justified in long-distance heat transportation. Ultimately, HP is the front-runner, while TPCT and THTD perform on par depending on operating conditions and geometry.

Predicting thermophysical properties of molten salts in the MgCl2-NaCl-KCl-LiCl system with a shell-model potential

Ternary eutectic salts composed of MgCl2, NaCl, and KCl, referred to as MNK salts, have recently emerged as promising candidates as high-temperature heat transfer fluids and thermal energy storage media. In this study, we performed classical molecular dynamics (MD) simulations to predict the densities, specific heat capacities, viscosities, and ionic self-diffusivities for MNK salts over a wide temperature range. The impact of LiCl additive on their thermophysical properties was also investigated. To capture the electronic polarization of Cl anions by neighboring cations, we developed a novel shell-model potential using the force-matching method and a dataset of ab initio calculated interatomic forces. Our extensive MD simulations predict structure and properties for pure salts and binary/ternary salt mixtures in the MgCl2-NaCl-KCl-LiCl system in overall good agreement with available experimental and theoretical data, which corroborates the accuracy and reliability of our developed potential.

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.

Geothermal Power Generation: Harnessing Electrical Energy Through The Organic Rankine Cycle (ORC) System

The research employs a thermodynamic simulation method using an Engineering Equation Solver (EES), relying on theoretical calculations. This method is integrated into a geothermal power plant, precisely focusing on geothermal source temperatures of approximately 95ºC. The investigation centers on the heat transfer process within a high-temperature heat transfer fluid from geothermal sources, conveying stored heat to the Organic Rankine Cycle (ORC) evaporator. Three specific working fluids, R134a, R11, and R22, examine working fluid selection for ORC at 95ºC. The results highlight the R11 organic fluid as an optimal compromise, excelling in two crucial criteria. Firstly, R11 exhibits the highest net mechanical power, = 34.81 kW compared to alternative fluids. Secondly, it boasts the best energetic efficiency of the cycle, registering = 16.01%, outperforming both R134a ( = 13.17%) and R22 ( = 12.64%). In summary, this study conducts a focused analysis of the energy aspects of an Organic Rankine Cycle (ORC) for electricity production using geothermal sources and organic fluids. Operating at a geothermal source temperature of 95ºC with a water flow rate of 80 lt/s and environmental conditions at 20ºC, the parametric study emphasizes the superiority of the R11 organic fluid. R11 emerges as the optimal choice, demonstrating the highest net mechanical power and superior energetic efficiency compared to alternative fluids, thereby contributing valuable insights to advancing sustainable and efficient energy technologies.

Exploring Cr and molten salt interfacial interactions for molten salt applications.

Molten salts play an important role in various energy-related applications such as high-temperature heat transfer fluids and reaction media. However, the extreme molten salt environment causes the degradation of materials, raising safety and sustainability challenges. A fundamental understanding of material-molten salt interfacial evolution is needed. This work studies the transformation of metallic Cr in molten 50/50 mol% KCl-MgCl2via multi-modal in situ synchrotron X-ray nano-tomography, diffraction and spectroscopy combined with density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Notably, in addition to the dissolution of Cr in the molten salt to form porous structures, a δ-A15 Cr phase was found to gradually form as a result of the metal-salt interaction. This phase change of Cr is associated with a change in the coordination environment of Cr at the interface. DFT and AIMD simulations provide a basis for understanding the enhanced stability of δ-A15 Cr vs. bcc Cr, by revealing their competitive phase thermodynamics at elevated temperatures and probing the interfacial behavior of the molten salt at relevant facets. This study provides critical insights into the morphological and chemical evolution of metal-molten salt interfaces. The combination of multimodal synchrotron analysis and atomic simulation also offers an opportunity to explore a broader range of systems critical to energy applications.

In-situ thermophysical measurement of flowing molten chloride salt using modulated photothermal radiometry

Molten salts are leading candidates for high-temperature heat transfer fluids (HTFs) for thermal energy storage and conversion systems in concentrated solar power (CSP) and nuclear energy power plants. The ability to probe molten salt thermal transport properties in both stationary and flowing status is important for the evaluation of their heat transfer performance under realistic operational conditions, including the temperature range and potential degradation due to corrosion and contamination. However, accurate thermal transport properties are usually challenging to obtain even for stagnant molten salts due to different sources of errors from convection, radiation, and corrosion, let alone at flowing status. To the best of authors’ knowledge, there is no available in-situ technique for measuring flowing molten salt thermal conductivity. Here, we report the first in-situ flowing molten salt thermal conductivity measurement using modulated photothermal radiometry (MPR). We could successfully perform the first in-situ thermal conductivity measurement of flowing molten NaCl-KCl-MgCl2 in the typical operating temperature (520 and 580 °C) with flow velocities ranging from around 0.3 to 1.0 m s−1. The relative change of the molten salt thermal conductivity was measured. Gnielinski’s correlation was also used to estimate the heat transfer coefficient h of the flowing NaCl-KCl-MgCl2 in the given experimental condition. The work showed the potential of the MPR technique serving as an in-situ diagnostics tool to evaluate the heat transfer performance of flowing molten salts and other high-temperature HTFs.

Control strategy effect on storage performance for packed-bed thermal energy storage

Energy storage will play an essential role by complementing renewable energy sources, which are central to the decarbonization of the power sector. Sensible thermal energy storage (TES) system through heating materials with high-temperature heat transfer fluid (HTF) is the simplest and most inexpensive TES technology. However, the inherent disadvantage of thermocline degradation makes the packed-bed TES system has several negative consequences. In the present study, a passive thermocline control (TCC) method with a high ratio of height-diameter and an active TCC method by setting an ending charging temperature were proposed. The results showed that the heat transfer between air and solid thermal storage materials was enhanced because of the fast airflow and the long airflow passage of the TES system with the passive TCC method. The utilization factors were about 75%, and thermal energy efficiencies were about 78%–80% under different interval times between charging and discharging. Combined with the active TCC method, the stable discharging duration was 6%–19% longer than just using the passive TCC method.

Electrochemical Behavior of MgOH+ in Molten MgCl2-KCl-NaCl Salt

Research on molten chloride salts as a high-temperature (500-800 ℃) heat transfer fluid has been reinvigorated in recent years. Its abundance, low cost, and good thermohydraulic properties have drawn attention from the nuclear and concentrated solar power industry. Specifically, a ternary MgKNa chloride salt (45.98/38.91/15.11 wt.%) has been considered a prime candidate for commercial application. However, economic risk remains due to high corrosion of containment alloys.The rate of corrosion in ternary MgKNa chloride salt has been primarily attributed to the impurity MgOH+, stemming from the reaction of MgCl2 with moisture. Extensive research has been conducted to purify chloride salts, but little attention has been given to on-line purification strategies once the salt is molten and circulating in the plant. One proposed strategy is continuous electrochemical purification via electrolysis using a Mg anode and W cathode. This strategy has been successfully demonstrated at the laboratory scale. Work done in our group indicate significant opportunity for the electrochemical process to be scaled up commercially as an electrochemical purification unit.Here, we present a study on the electrochemical behavior of MgOH+ reduction on a W cathode in molten MgCl2-KCl-NaCl across the temperature range of 475-525 ℃. The kinetic parameters from this study aid in modeling and scaling up an electrochemical purification cell at industrially relevant scales.

Experimental Study of Eutectic Molten Salts NaCl/KCl/ZnCl2 Heat Transfer Inside a Smooth Tube for High-Temperature Application

AbstractEutectic salts NaCl-KCl-ZnCl2 and NaCl-KCl-MgCl2 are two of the chloride salt systems that are promising for being used as high-temperature heat transfer fluid (HTF) and thermal energy storage (TES) materials in a wide range of temperatures from 230 °C to 750 °C in concentrated solar thermal power systems. To conduct measurement of the heat transfer coefficient of the molten salt at high temperatures, a circulation system and instrumentation of flow and heat transfer was constructed. Experimental measurement of the convective heat transfer coefficients of NaCl-KCl-ZnCl2 (molar fraction: 13.8–41.9–44.3%) inside tubes has been accomplished to find the most suitable heat transfer correlations. Experience of salt processing and operation of the high-temperature heat transfer test system was obtained. Two correlations, Dittus-Boelter equation and Gnielinski’s correlation for Nusselt number against Reynolds number and Prandtl number, are evaluated using the test results, and the latter correlation is recommended due to its better agreement of prediction against tested data.

Solar-aided power generation in biomass power plant using direct steam generating parabolic trough collectors

Parabolic trough collectors may be used to increase the temperature of a heat transfer fluid flowing inside absorber tubes. A method of solar-aided power generation in thermal power plants is using heat transfer from the high-temperature heat transfer fluid for feed water heating. In doing so, less steam is extracted from steam turbine, and power output is increased. This paper investigates an alternative method of solar-aided power generation that uses direct steam generating parabolic trough collectors for feed water heating in a small biomass power plant. Such a power plant consists of an open feed water heater, which is more commonly known as deaerator due to its capability to remove dissolved gases from feed water. Feed water heating by solar energy in this power plant is more easily implemented by using parabolic trough collectors with direct steam generation than parabolic trough collectors with a heat transfer fluid. The proposed integration of parabolic trough collectors into a biomass power plant requires the supply of saturated steam from parabolic trough collectors to the deaerator. The steam quality reaches the maximum value when the direct normal irradiance is maximum. The performance of this system depends on the average value of effective direct normal irradiance. Simulation results show that there exists the optimum extracted steam pressure, which results in the maximum electrical energy generated by the power plant.

Characteristic analysis of thermal energy storage system using synthetic oil as a heat transfer fluid: techno-economic modeling and LAB scale demonstration

ABSTRACT A dual-media thermal energy storage system consisting of ceramic pebbles as a storage material and high-temperature heat transfer fluid (HTF) is analyzed for 1 MWe National Solar Thermal Power Plant. A numerical model has been formulated to study the thermocline behavior of tanks at different operating conditions. Based on the numerical model, a Lab-scale test facility is developed to validate the numerical model. The main objective of this study is to analyze the formation of the thermocline thickness for various operating parameters such as mass flow rate, void fraction, pebbles diameter, and thermal diffusivity of HTF and storage materials, along with a parametric optimization. The numerical result shows that, as the mass flow rate increases, the discharge time can cause the varying temperature at the outlet that is unenviable. The percentage movement of the thermocline layer was observed to be 70.37% with an increase in the mass flow rate of 18.75%. Similarly, it was found that the percentage increase in the volumetric heat transfer coefficient increased by 9.56%, with an increase in mass flow of 18.75%. The lower pebble diameter gives the superlative results for thermocline thickness, but it causes a great deal of pressure drop, which causes exergy loss. However, the percentage improvement in volumetric heat transfer coefficient was found to be 80%, with a 33.33% reduction in pebble diameter. It is concluded that a smaller size pebble diameter reduces the thermocline thickness, but it needs to be optimized to counter the problem of pressure drop. Parametric optimization has been done by considering various parameters such as D/L ratio, void fraction, mass flow rate, pebble diameter, thermal diffusivity of solid and HTF. Signal to noise ratio (SNR) has been taken into account to get the optimized value of the parameters. An empirical correlation has been developed to calculate thermocline thickness which is valid for a D/L 1. Further, a Techno-economic cost analysis of the TES (DMT) system for a 1 MWe CSP plant is done to study the economic facet of the system.

A two-step procedure for the selection of innovative high temperature heat transfer fluids in solar tower power plants

This work compares with a two-step procedure the performance of different Heat Transfer Fluids (HTF) for high temperature receiver applications (up to 715 °C) in advanced Solar Tower (ST) plants. The most promising molten salts and liquid metals are initially selected and ranked according to their performance, estimated with different Figures of Merit (FoM) available in literature or newly defined. For the best performing fluids, different hydraulic configurations and tube diameters at fixed receiver size are tested. The optimized external tubular receiver configuration for each HTF is then implemented in a ST plant and its performance is assessed through a detailed techno-economic analysis, considering the use of a direct thermal energy storage system and of a sCO2 based power cycle. As a second step, the yearly electricity yield and the LCOE are evaluated for two different sites. Results show NaCl–MgCl2 as the best option for the considered type of plant with a LCOE of 151 $/MWh, which is anyhow 10% higher than the reference Solar Salts case.

A novel parabolic trough receiver by inserting an inner tube with a wing-like fringe for solar cascade heat collection

In this work, a novel parabolic trough receiver (NPTR) with an inner tube and wing-like fringe was proposed to improve heat-collecting efficiency as well as provide different grades of thermal energy. Thermal oil and water, which flow respectively in absorber and the inner tube, are selected as high and low temperature heat transfer fluids. A three-dimensional computational fluid dynamics model was developed to investigate the performance of the NPTR. Effects of geometrical parameter and thermal conductivity of inner tube on the performance of NPTR were studied in details. Based on the results, the NPTR with β = 180° is recommended as the suggested design. Moreover, performance of the suggested design under different direct normal irradiances (300–1000 W/m2) and inlet temperature of oil (400–650 K) were evaluated. Compared to the conventional parabolic trough receiver, the heat loss of NPTR is effectively reduced by 33.1–50.1%, and the overall efficiency can be improved by 0.61%–7.67%. Moreover, the proportions of oil and water heat gains in the total input solar energy are ranged in −18.8–63.5% and 8.39–77.6%, and the temperature gains of oil and water are ranged in −1.4–19.5 K and 5.4–18.8 K, respectively.

Thermophysical Properties Experimentally Tested for NaCl-KCl-MgCl2 Eutectic Molten Salt as a Next-Generation High-Temperature Heat Transfer Fluids in Concentrated Solar Power Systems

Abstract A new eutectic chloride molten salt, MgCl2-KCl-NaCl (wt% 45.98–38.91–15.11), has been recognized as one of the most promising high-temperature heat transfer fluids (HTF) for both heat transfer and thermal storage for the third-generation concentrated solar power (CSP) systems. For the first time, some essential thermophysical properties of this eutectic chloride molten salt needed for basic heat transfer and energy storage analysis in the application of concentrating solar power systems have been experimentally tested and provided as functions of temperature in the range from 450 °C to 700 °C. The studied properties include heat capacity, melting point, heat of fusion, viscosity, vapor pressure, density, and thermal conductivity. The property equations provide essential database for engineers to use to calculate convective heat transfer in concentrated solar receivers, heat exchangers, and thermal storage for concentrated solar power plants.

Analysis of a Fluidized-Bed Particle/Supercritical-CO2 Heat Exchanger in a Concentrating Solar Power System

Abstract Concentrating solar power (CSP) development has focused on increasing the energy conversion efficiency and lowering the capital cost. To improve performance, CSP research is moving to high-temperature and high-efficiency designs. One technology approach is to use inexpensive, high-temperature heat transfer fluids and storage, integrated with a high-efficiency power cycle such as the supercritical carbon dioxide (sCO2) Brayton power cycle. The sCO2 Brayton power cycle has strong potential to achieve performance targets of 50% thermal-to-electric efficiency and dry cooling at an ambient temperature of up to 40 °C and to reduce the cost of power generation. Solid particles have been proposed as a possible high-temperature heat transfer or storage medium that is inexpensive and stable at high temperatures above 1000 °C. The particle/sCO2 heat exchanger (HX) provides a connection between the particles and sCO2 fluid in emerging sCO2 power cycles. This article presents heat transfer modeling to analyze the particle/sCO2 HX design and assess design tradeoffs including the HX cost. The heat transfer process was modeled based on a particle/sCO2 counterflow configuration, and empirical heat transfer correlations for the fluidized bed and sCO2 were used to calculate heat transfer area and estimate the HX cost. A computational fluid dynamics simulation was applied to characterize particle distribution and fluidization. This article shows a path to achieve the cost and performance objectives for a particle/sCO2 HX design by using fluidized-bed technology.

Ionanofluids with [C2mim][CH3SO3] ionic liquid and alumina nanoparticles: An experimental study on viscosity, specific heat and electrical conductivity

Ionic liquids, as well as their suspensions with nanoparticles, can be considered as the future of solar applications, due to their unique properties. This work consists of an experimental study of 1-ethyl-3-methylimidazolium methanesulfonate ([C2mim][CH3SO3]) ionic liquid and its ionanofluids with Al2O3 nanoparticles of different mass concentrations (i.e. 0.05–10 %wt.). Viscosity, specific heat and electrical conductivity are measured in the temperature range 283.15–333 K and the experimental data were discussed in terms of both nanoparticle concentration and temperature variation. Results showed that the viscosity is increasing when nanoparticles mass concentration increase, going up to 250% for the ionanofluid with 10 %wt. of alumina suspended in the ionic liquid. Ionanofluids rheological tests and viscosity variation were debated in the article and the results were correlated using the Vogel-Fulcher-Tammann approach. Specific heat was found to increase with both mass fraction and temperature. Electrical conductivity decrease with nanoparticle addition and increase linearly with temperature growth. Concluding, these new fluids can be seen as an alternative for high temperature heat transfer fluids used in solar applications.

Ab-initio molecular dynamics study on thermal property of NaCl–CaCl2 molten salt for high-temperature heat transfer and storage

NaCl–CaCl2 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–CaCl2 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.

Potential scalability of a cost-effective purification method for MgCl2-Containing salts for next-generation concentrating solar power technologies

Next-generation concentrating solar power (CSP) technology requires a high-temperature heat-transfer fluid and thermal energy storage media. Molten MgCl2-KCl-NaCl is considered a potential candidate salt due to its thermophysical properties. However, MgCl2 presents various challenges because of its hygroscopic nature. To keep corrosion under control, this molten chloride needs to remain free of hydrates and other impurities. Here, we have developed an effective purification method for MgCl2-containing salts by 1) combining known laboratory-scale, batch-style thermal and chemical purification processes to enable scalable, continuous-style processes for commercial next-generation CSP operation; 2) improving overall efficiency and minimizing the major corrosive impurity MgOHCl by optimizing key engineering parameters such as heating temperature/time and amount of elemental Mg addition; and 3) investigating the addition of halite (NaCl) to carnallite (KMgCl3) to reduce the liquidus temperature. Laboratory-scale results suggest that 1) adding 6.5 wt% of halite and less than 0.1 wt% of elemental Mg to commercial carnallite and 2) following a heating schedule to at least 650 °C with ~3 h of holding time at that temperature can produce a ternary MgCl2-KCl-NaCl salt composition with a low liquidus temperature of about 400 °C. It also reduces the presence of the corrosive impurity, MgOHCl, from ~1 to 2 wt.% to ~0.1 wt%. Findings of these key engineering parameters should provide a pathway toward a scalable, continuous-style salt-purification process at a scale of metric tons per hour that can produce a corrosion-controlled chloride molten salt for CSP applications.

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.