Molten Salt Flow Research Articles

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.

A study on heat transfer characteristics of quinary molten salt mixture

In this study, experimental analysis of the heat transfer characteristics of a quinary molten salt mixture was carried out. This mixture was developed at Korea Institute of Energy Research as a heat storage material for a solar thermal power plant. Experiments were conducted to obtain the heat transfer coefficient in a vertical tube and to generate steam in a heat exchanger. To obtain the heat transfer coefficient, the temperatures of the outer wall, inlet, and outlet were measured in upward molten salt flow. Heat transfer correlations were proposed for the molten salt based on experimental data. The accuracies of the correlations were 60% and 88% for the transition and turbulent flow regions, respectively, with an error range of ±20%. A concentric heat exchanger was used for steam production to use the molten salt for power generation. Superheated steam at 100 bar and 400 °C was generated through heat transfer between the high temperature molten salt and water, with an efficiency of 81–91%.

Enhanced heat transfer in a parabolic trough solar receiver by inserting rods and using molten salt as heat transfer fluid

With the aim to enhance the reliability and overall heat transfer performance of a parabolic trough receiver, concentric rod and eccentric rod are introduced as turbulators, and the flow and convective heat transfer characteristics of molten salt in a parabolic trough receiver are analyzed. A three-dimensional model was developed and has been validated with experimental results and empirical equations. Highly non-uniform heat flux was provided by a novel parabolic trough collector. The result shows that both concentric rod insert and eccentric rod insert can enhance the heat transfer performance effectively. For a parabolic trough receiver with a concentric rod insert, with the increasing of dimensionless diameter B, the normalized Nusselt number is about 1.10 to 7.42 times over a plain parabolic trough receiver. The performance evaluation criteria can't reasonably evaluate the effect of B growth on the comprehensive heat transfer performance. By introducing integrated performance factor, it can give a reasonable solution, and it shows that the integrated performance factor has a significance decreases with the increase of Reynolds number when B is larger than 0.8. With B increasing, the integrated performance factor of parabolic trough receiver with concentric rod insert decreasing under a certain Reynolds number. For an eccentric rod insert, the performance evaluation criteria and the integrated performance factor decrease with the increasing of Reynolds number under a certain dimensionless eccentricity H. The performance evaluation criteria decreases from about 1.84 to 1.68 times over a plain parabolic trough receiver when H is 0.8. Moreover, the temperature distribution can be uniformed and the maximum temperature on the absorber tube also can be remarkably reduced with the increasing of B and H under a certain Reynolds number, which helps to reduce the thermal deflection and increase the reliability for a parabolic trough receiver.

Study on the coupled photon-thermal-stress integration method, characteristics with time and failure criterion in the solar molten salt cavity receiver

In this paper, the studies on the characteristics with time and failure criterion in the solar molten salt cavity receiver are carried out based on the coupled photon-thermal-stress integration method. Especially, the truly non-uniformed solar heat flux in the cavity receiver with different time are simulated using the optical simulation approach coupling the Monte Carlo ray tracing and Gebhart methods proposed by our team. Besides, the complex coupled heat transfer process of conduction, convection and radiative in the receiver are also comprehensively considered. Based on the integrally model, the solar flux, temperature and thermal stress on the absorber surfaces with different time are obtained. Then, the influences of different tube materials, flow layouts of molten salt on the coupling performances of photon-thermal-stress conversion process are discussed. Finally, the stress failure criterion in the molten salt cavity receiver is proposed based on the fracture mechanics theory. The results show that the temperature and thermal stress are influenced significantly by the non-uniformed solar flux, the distribution of temperature and thermal stress are also extremely non-uniformed. The solar flux and thermal stress change remarkably with different time, the peak heat flux and stress are maximum at 12:00 noon, and the peak thermal stress could increase by 11% and 10% compared with 11:00 and 13:00. It is also found that the peak thermal stress would be reduced effectively when the receiver tube with better thermal conductivity property and the molten salt flow layout which is from low-flux area to high-flux area are adopted. For example, compared with 800H receiver tube, the peak thermal stress of 316H tube could be reduced by 37%. Meanwhile, the peak thermal stress of flow layout which is from low to high flux area would be 20% lower than that with the opposite layout. The fracture toughness of receiver tube, which is influenced by the molten salt corrosion, is one of the key parameters to assess stress failure. Based on the stress failure criterion of critical solar flux proposed by this paper, the critical heat flux of stress failure would exhibit a decreasing trend when the fracture toughness degradation is serious in the molten salt receiver.

Novel thermosiphon-powered reverse osmosis: Techno-economic model for renewable energy and fresh water recovery

Recently, thermosiphon has been considered as an economically competitive approach for the withdrawal of thermal energy from molten salt. This feature of thermosiphon is particularly attractive for concentrated solar power (CSP) technology because molten salt can be used both as the direct receiver and thermal energy storage material, and pumping requirements can be minimized. However, the thermosiphon loop must be properly designed; otherwise the heated molten salt can flow in the reverse direction or solidify before heat exchange is fully carried out. In this paper, the dependence of important molten salt flow characteristics such as temperature, velocity and friction factor on the thermosiphon loop dimensions is theoretically investigated. The model is carried out for the withdrawal of 1MW of thermal energy from the molten salt. The economic assessment of the conversion of this energy to electricity for reverse osmosis desalination is also carried out. It is observed that by using the minimum possible value of leg length and maximum possible values of diameter and pressure head, the lowest decline of temperature and highest velocity of flow can be achieved along the loop. Fresh water production is more valuable than electricity and steam production by 460% and 480%, respectively.

Integrated model development for safeguarding pyroprocessing facility: Part I-model development and validation

Pyroprocessing is a method involving electrochemistry, thermodynamics, and transport theory. Driven by the current, nuclear materials redistribute between anode, electrolyte, and cathode to realize the desired separation. In the present study, a model for predicting the mass transport for the electrochemical process was developed. The model took into account the following key parameters: diffusion and mass transfer in both the electrode and electrolyte, the anodic dissolution and cathodic precipitation kinetics, the molten salt flow condition, and the solubility of species in molten LiCl-KCl and liquid cadmium. It was validated by available transport data between different pairs of the electrodes, namely liquid cadmium anode to solid cathode, solid anode to solid cathode, and solid anode to liquid cadmium cathode. Time for Pu depositing on the solid cathode as well as Zr dissolving from U-Pu-Zr alloy anode was precisely predicted. The potential and composition changes in various experiments were also well reproduced. The plausible explanation was given to explain the minor disagreement and future work was suggested to improve its capability. The model is useful to understand the reaction mechanisms and safeguard a pyroprocessing facility by monitoring the separation process and tracking the material flow.

Thermal management of a Sabatier reactor for CO2 conversion into CH4: Simulation-based analysis

Converting CO2-rich waste streams such as raw biogas, landfill gas and power plant flue gas into synthetic fuels and chemicals will reduce greenhouse gas emissions, providing revenue at the same time. One option is to convert CO2 into CH4 by hydrogenation via Sabatier reaction. This synthetic methane will be renewable if the H2 required for the reaction is generated via water electrolysis using solar and wind energy or hydroelectricity. However, to realize the potential of this approach, a number of technological challenges related to the Sabatier reactor design have to be resolved, including thermal management. The high exothermicity of the Sabatier reaction can lead to reactor overheating. High temperatures are unfavorable to the exothermic and reversible methanation process, resulting in low CO2 conversions. A simulation-based study of a Sabatier reactor was performed in order to optimize the removal of heat, while maximizing CO2 conversion and CH4 production. The heat-exchanger type packed bed reactor with internal cooling by a molten salt was simulated using a transient, pseudo-homogeneous mathematical model. Reactor performance was evaluated in terms of CO2 conversion and CH4 yield. The simulation results show that feed temperature, feed flow rate, and molten salt flow rate are the crucial parameters affecting the reactor performance. For the optimized operating conditions, the model predicts CO2 conversions and CH4 yields above 90% at high reactor throughputs, with space velocities up to 10,000h−1. A preliminary techno-economic evaluation is provided: opportunities and challenges are outlined.

Simplified model of a dual-media molten-salt thermocline tank with a multiple layer wall

Thermal ratcheting is a critical phenomenon associated with the cyclic operation of dual-media thermocline tanks in solar energy applications. To study this phenomenon, it is necessary to develop a comprehensive model of a thermocline tank that includes both the heterogeneous filler region and the composite tank wall. Because CFD models require a high computational cost to simulate a thermocline tank considering transient state operation, a simplified dual-phase model that includes the unsteady heat transfer through a multiple layer wall has been developed. The filler region consists of a rock bed with interstitial molten salt, and the tank wall is composed of a steel shell with two layers of insulation (firebrick and ceramic). In this simplified model, the fluid flow inside the tank is considered to be one-dimensional along the tank axis direction, whereas the heat conduction in the composite wall is considered to be two-dimensional. Therefore, a convective heat transfer coefficient from the bed to the wall is necessary to couple the molten salt flow with the heat transfer in the tank shell. In this work, the effects of both convective heat transfer from the bed to the wall and molten salt flow rate on the time-dependent thermal response of both the steel shell and molten salt have been analyzed. The simplified model is able to predict the temperatures of the molten salt, filler material and layer wall as well as the mechanical stress in the tank shell.

Thermal Performances of Two Stage Molten Salt Steam Generator

Abstract Thermal performances of two stage molten salt steam generator were experimentally investigated in this article. The two stage system consisted of primary U shape heat exchanger and secondary shell and tube heat exchanger, and its overall heat transfer coefficient, steam generation rate and thermal efficiency were further analyzed under different molten salt flow rates and water flow rates. According to the experimental results, the overall heat transfer coefficients of the primary and secondary heat exchanger both increase as the molten salt flow rising. As the water flow rate increases, the overall heat transfer coefficient of the primary heat exchanger first increases and then decreases, while that of the secondary heat exchanger gradually increases. As the molten salt flow rate and water flow rate increase, the steam generation rate and the thermal efficiency of the secondary heat exchanger both increase, while the thermal efficiencies of the primary heat exchanger and the whole system first increase, approaching their maximums with optimal molten salt flow rate and water flow rate respectively, and then begin to drop.

Development of a heat storage demonstration unit on the basis of Mg2FeH6 as heat storage material and molten salt as heat transfer media

The development and preliminary tests of a 5 kg Mg2FeH6 heat storage system which is useable for short and long-term storage applications at temperatures around 500 °C are described. The heat transfer for the heat storage process (dehydrogenation of the hydride Mg2FeH6) and heat release (hydrogenation of the hydride precursor 2Mg-Fe) is done by the flow of molten salt in appropriate heat exchangers serving as heat source or heat sink. The construction of the tube bundle reactor as a heat storage tank is presented. 1.6 kWh of heat could be released and 1.5 kWh of heat could be stored during the first experimental tests. Difficulties, which occured during the preliminary tests, are described.

High temperature thermal hydraulics modeling of a molten salt: application to a molten salt fast reactor (MSFR)

An overview of the ongoing efforts in the area of the thermal hydraulics modeling of a Molten Salt Fast Reactor (MSFR) is presented. The MSFR employs a flowing liquid fuel based on a high temperature lithium fluoride salt. A molten salt flow can be considered in many situations as an incompressible flow. However, several phenomena intrinsic to a molten salt flow pose unique challenges (radiative heat transfer, volumetric heat source, strong neutronics feedbacks, etc.). To study some of these phenomena and to improve current Computational Fluid Dynamics (CFD) models an experimental facility called SWATH (Salt at WAll: Thermal ExcHanges) will be built as part of the European project SAMOFAR (2015-2019).

Transient freezing of molten salts in pipe-flow systems: Application to the direct reactor auxiliary cooling system (DRACS)

The possibility of molten-salt freezing in pipe-flow systems is a key concern for the solar-energy industry and a safety issue in the new generation of molten-salt reactors, worthy of careful consideration. This paper tackles the problem of coolant solidification in complex pipe networks by developing a transient thermohydraulic model and applying it to the ‘Direct Reactor Auxiliary Cooling System’ (DRACS), the passive-safety system proposed for the Generation-IV molten-salt reactors. The results indicate that DRACS, as currently envisioned, is prone to failure due to freezing in the air/molten-salt heat exchanger, which can occur after approximately 20minutes, leading to reactor temperatures above 900°C within 4hours. The occurrence of this scenario is related to an unstable behaviour mode of DRACS in which newly formed solid-salt deposit on the pipe walls acts to decrease the flow-rate in the secondary loop, facilitating additional solid-salt deposition. Conservative criteria are suggested to facilitate preliminary assessments of early-stage DRACS designs. The present study is, to the knowledge of the authors, the first of its kind in serving to illustrate possible safety concerns in molten-salt reactors, which are otherwise considered very safe in the literature. Furthermore, and from a broader prospective, the analytical tools developed in this study can also be applied to examine the freezing propensity of molten-salt flows in other complex piping systems where standard, finite element approaches are computationally too expensive.

Experimental investigation on turbulent heat transfer characteristics of molten salt in a shell-and-tube heat exchanger

A new experimental platform was built to measure the flow and heat transfer characteristics of molten salt in some large-scale heat exchangers. Based on this experimental platform, the turbulent heat transfer characteristics of a shell-and-tube heat exchanger designed for molten salt were investigated. The tube-side and shell-side Reynolds number vary from 10,000 to 91,000, and from 11,000 to 27,000, respectively. The heat transfer characteristics of water in the same heat exchanger were also measured to confirm the accuracy and usable range of experimental results. The results showed that the deviation of tested convective heat transfer coefficients between molten salt and water is just 14.1% which is comparable to the experimental uncertainty of 12.8%. Therefore, in view of decreasing the difficulty and cost of experiment, water is suggested to replace molten salt for measuring the heat transfer characteristics of molten salt heat exchanger. In addition, some empirical correlations for heat transfer characteristics in both sides of heat exchanger were obtained. The tube-side correlations agree well with the typical Dittus-Boelter correlation with an allowable difference of 10%. The difference between shell-side correlation and the Dittus-Boelter correlation is also less than 20% when the proper hydraulic diameter is selected as the characteristic dimension.

Thermal performances of molten salt steam generator

Molten salt steam generator is the key technology for thermal energy conversion from high temperature molten salt to steam, and it is used in solar thermal power station and molten salt reactor. A shell and tube type molten salt steam generator was set up, and its thermal performance and heat transfer mechanism were studied. As a coupling heat transfer process, molten salt steam generation is mainly affected by molten salt convective heat transfer and boiling heat transfer, while its energy efficiency is also affected by the heat loss. As molten salt temperature increased, the energy efficiency first rose with the increase of heat flow absorbed by water/steam, and then slightly decreased for large heat loss as the absorbed heat flow still rising. At very high molten salt temperature, the absorbed heat flow decreased as boiling heat transfer coefficient dropping, and then the energy efficiency quickly dropped. As the inlet water temperature increased, the boiling region in the steam generator remarkably expanded, and then the steam generation rate and energy efficiency both rose with the overall heat transfer coefficient increasing. As the molten salt flow rate increased, the wall temperature rose and the boiling heat transfer coefficient first increased and then decreased according to the boiling curve, so the overall heat transfer coefficient first increased and then decreased, and then the steam generation rate and energy efficiency of steam generator both had maxima.

Numerical simulation of mixed convection heat transfer of molten salt in horizontal square tube with single surface heating

A numerical simulation was presented on the mixed convective heat transfer of molten salts in single-side heated horizontal square tube. The flow and heat transfer characteristic of molten salt with bottom, lateral and top heated positions were analyzed, respectively. The results showed that under non-uniform heating conditions, the velocity profiles of main core area changes with the increase of flow distance due to buoyancy effects and the core area is close to heating surfaces. For different heating modes, the numbers of vortex flow formed in each cross section of channel are different. Heat transfer ability is strengthened due to the existence of vortex in different scales. Good agreement between simulation results and mixed convective correlation is reached and the deviation between 92% of simulation results and correlation is within 20%. Under the same heating modes, the heat transfer performance of mixed convection is strengthened 1.2–2.5 times than that of forced convection.

Integrated numerical study on the coupled photon-thermal conversion process in the central solar molten salt cavity receiver

In a solar power tower system, the concentrating and collecting subsystem, including the heliostat field and the receiver, contains some of the most costly and technically challenging components. Many types of solar receivers make up the solar power tower system. Among them, the cavity molten salt receiver has been widely applied due to its lower heat loss. The accurate simulation of the coupled heat transfer process in the cavity receiver and the prediction of receiver performance are of great importance for the receiver design and safe operation. The solar flux distribution on cavity interior surfaces is extremely non-uniform, which has great effect on the receiver thermal performance. Taking the non-uniform solar flux distribution into consideration, the photon-thermal conversion process is simulated integrally in this paper. A hybrid simulation approach, which couples Monte Carlo ray tracing and Gebhart methods, is applied to investigate the complete solar radiation transfer process in the solar power tower system with a cavity receiver and obtain the non-uniform solar flux distribution. On this basis, the coupled photon-thermal conversion process in the molten salt cavity receiver is simulated. The effects of the fluid flow layout on the receiver performance under the non-uniform solar flux distribution are particularly revealed. Also, the variation of the receiver performance with time is analyzed in detail. The results show that the solar flux distribution on the interior surfaces of the cavity receiver is extremely non-uniform, under which the molten salt flow layout has great effects on the receiver performance. In the case of flow layout that the cold molten salt flows into the receiver in the high-flux area, the temperatures of both the absorber wall and the molten salt increase rapidly along the flow path. The temperature of the whole absorber wall is relatively high, which leads to greater heat loss and less amount of hot molten salt, and even seriously, “heat transfer deterioration” near the outlet of the receiver. When the cold molten salt flows into the receiver in the low-flux area, the temperature of both the absorber wall and the molten salt increase slowly along the flow path. The temperatures of the whole absorber wall is relatively low, which results in less heat loss and larger amount of hot molten salt. Although the shape of the solar flux distribution on the interior surfaces varied greatly with time, the evolutions of the molten salt (absorber wall) temperature at different time display similar variation tendency along the flow path. Moreover, the reflective loss varies with time greatly, while the heat loss varies with time slightly.

Procedures for testing valves and pressure transducers with molten salt

Thermal energy storage has been identified as an effective method for improving the capacity of concentrating solar power plants. The most commonly-used storage media is molten salt (also known as solar salt) comprising 60%w NaNO3 – 40%w KNO3. However, their specific features such as material incompatibilities, high surface tension and a high solidification point (which is around 240 °C) can create technical issues and systematic failures in hydraulic components and instruments where the molten salt flows. The current standard procedures for testing hydraulic components, such as valves and pressure transmitters, do not take into account their real behavior under the working conditions and type of fluid used in these commercial thermal storage systems.This study reports the results of a specific set of tests for commercial globe valves and pressure transmitters operated at high temperature in molten salt thermal energy storage systems in CSP facilities. These tests were performed using two experimental devices, BES-I and BES-II, designed specifically for the task and constructed at the Plataforma Solar de Almeria (PSA), which belongs to the Spanish Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT). While BES-I can test components up to 3 MPa and 400 °C, BES-II can extend the working conditions up to 4 MPa and 600 °C.To support the proposed tests, some experimental results for a globe valve and a pressure transmitter are presented and discussed. The tests are conducive to improving component design and allowing manufacturers to adapt their products for operation in molten salt thermal energy storage systems.

Molten salt flow and heat transfer in paddle heat exchangers

Molten salt flow and heat transfer in paddle heat exchangers

Molten salt flow and heat transfer in paddle heat exchangers

Molten salt flow and heat transfer in paddle heat exchangers

Experimental study on the heat transfer characteristics of a low melting point salt in a parabolic trough solar collector system

An experimental system of parabolic trough solar collector and heat transfer was set up with a new molten salt employed as the heat transfer medium (with a melting point of 86 °C and a working temperature upper limit of 550 °C). The circulation of molten salts in the system took place over 1000 h. Experiments were conducted to obtain the heat loss of the Heat Collector Element (HCE), the total heat transfer coefficient of the water-to-salt heat exchanger, and the convective heat transfer coefficients for the low melting point molten salt in a circular tube. The results show that the thermal loss of the tested HCE is higher than that of the PTR70, and the thermal loss at the joints of the collector tube represents about 5% of the total loss in the entire tube. The total heat transfer coefficient of the water-to-salt heat exchanger was between 600 and 1200 W/(m2·k) in the ranges of 10,000 < Re < 21,000 and 9.5 < Pr < 12.2. The experimental data show good agreement with existing well-known correlations presented by the Sieder-Tate equation and the Gnielinski equation. This experimental study on heat loss from molten salt flow in a receiver tube will hopefully serve as a helpful reference for applications in parabolic trough systems.