Molten Solar Salt Research Articles

Effect of chlorides and sulfates on the corrosion of SS347 and GH3539 in molten solar salt

Adding an appropriate amount of chloride or sulfate salts to molten nitrates is an effective way to improve the thermal stability temperature and energy density of solar salts. However, the corrosivity of the mixed solar salts is enhanced significantly. In this paper, the effect of chlorides and sulfates on the corrosion of SS347 and GH3539 in molten solar salt is investigated. The high oxygen pressure and basic environment in molten nitrates causes the formation of an external Fe oxides layer and inner FeCr2O4 layer for SS347. Unlike SS347, the corroded GH3539 presents an exclusive film of NiO and many core-shell structured nodules with Ni as core and NiO as shell, followed by a region of internal oxidation involving Ni, W and Cr. The additives chlorides produce an oxidation-chlorination environment, causing spallation of oxide scale from SS347, but without significant effect on GH3539 due to lower thermodynamic stabilities of Ni and W chlorides than Fe and Cr chlorides. The addition of sulfates to solar salt also harms the adhesion of the scale on SS347 to a certain extent. While the additives sulfides accelerate the corrosion of GH3539, forming a thick porous NiO layer and a serious internal sulfidation zone.

Implementation of Solar Salt Properties Into ARIANT and Simulation of Pressurized Loss of Forced Circulation in a High Temperature Gas-Cooled Small Modular Reactor

Abstract Small modular reactors (SMRs) are actively being considered for use in Canada. Some proposed SMRs can make use of solar salt as an intermediate coolant for a heat storage system. The development of thermalhydraulic simulation tools is one of the key capabilities needed to examine the performance of SMRs and license this class of reactors. This article summarizes the implementation of molten solar salt fluid properties into the algorithm for analysis of network thermalhydraulics (ARIANT) and uses this code to simulate a high temperature gas-cooled SMR with helium and solar salt as its primary and secondary coolants during a pressurized loss of forced circulation (PLOFC) event. This work demonstrates the ability of ARIANT to simulate transient events in a two-loop reactor system consisting of helium and solar salt as coolants and helps to establish ARIANT as a tool for SMR analysis.

Specific Heat Capacity of Solar Salt-Based Nanofluids: Molecular Dynamics Simulation and Experiment.

In this study, a nanofluid composed of molten solar salt (MSS) and 1.0% SiO2 nanoparticles by mass was created and analyzed using differential scanning calorimetry (DSC) to determine its specific heat capacity (SHC). The SHC of the nanofluid was found to be significantly higher than that of pure MSS. The average increase in SHC of the nanofluid with 1.0% SiO2 nanoparticles (NPs) loading was found to be 15.65% compared with pure MSS. The formation of nanostructures after doping with NPs may increase the SHC of molten salt (MS) nanofluids, according to certain published research that included experimental confirmation. Nevertheless, no thorough theoretical or computational studies have been conducted to verify the experimental findings related to MSS nanofluid. Molecular dynamics (MD) simulations were conducted in various simulation boxes for different cases to verify the experimental findings and investigate the mechanism behind the enhancement of SHC caused by the addition of SiO2 NPs in eutectic MSS. The simulations used pure MSS and mixtures containing NaNO3 nanostructures bonded with SiO2 NPs. The highest SHC increase of 25.03% was observed when the simulation box contained 13.71% NaNO3 nanostructures by weight. The incorporation of NaNO3 nanostructures increased the surface area and total surface energy, leading to a positive effect on the SHC of the MSS nanofluid. However, the decrease in the base molten salt's SHC had a slight negative impact on the overall SHC of the MS nanofluid.

Modeling Microwave Heating of Molten Salt for Thermal Storage Systems

With the aim of affordably storing energy produced by photovoltaic and wind power plants, Power-to-heat-to-power (or Carnot batteries) are proposed as an outstanding system capable of transforming this energy into heat through existing commercial thermal storage systems for thermosolar plants, and again produce electricity by connecting these storage systems to available power blocks that can come from dismantled carbon plants or any other such as gas plants, or even nuclear. On this basis, microwave heating is studied as feasible to store energy in molten solar salt: 60%w NaNO3, 40%w KNO3. This study is expected to provide some key points for the design of microwave systems for molten solar salt, analysing different simulated cases with numerical modeling: continuous flow microwave heating in an elliptical cavity with different flow rates – 1 l/min and 1.6 l/min – and different positions of the fluid carrier tube, and in a microwave oven with helical tubes.

Effect of gas management on corrosion resistance in molten solar salt up to 620 °C: Corrosion of SS316-types and SS347

This study investigates the impact of stabilizing molten salt chemistry on the corrosion behavior of four steel types at temperatures ranging from 570 °C to 620 °C. The research explores the influence of temperature and gas atmosphere on corrosion mechanisms through detailed analysis of molten salt, corrosion rates, and microstructures. Findings reveal that systematic changes in salt chemistry can modify mass transport mechanisms, enabling the development of corrosion mitigation strategies even at extreme temperatures. This discovery marks a significant breakthrough in the advancement of Solar Salt-based storage systems for high-temperature applications, facilitating the utilization of renewable energy sources.

Tribological behavior of 316H stainless steel in NaNO3-KNO3 molten salt at elevated temperature

The corrosion behaviors of stainless steels in various high-temperature molten salt have been extensively investigated. However, the tribological behavior of the alloys acting as moving parts in the molten salt is rarely reported. In this work, 316H stainless steel was selected to comparatively study its high-temperature tribo-corrosion performance in molten solar salt (60% NaNO3+40% KNO3, wt.). Under the drying sliding condition, the wear rate increases at 500 °C though the friction coefficient decreases compared to that at room temperature. In contrast, compared to the 500 °C dry sliding condition, the wear resistance of 316H at 500 °C in molten nitrate salt is remarkably improved owing to the lubrication effect of the molten salt and the enhanced oxidation tendency on the worn surface.

Ascertaining the Si-substitution on the tribo-corrosion behaviors of Ti3AlC2 in molten Solar Salt

As a promising working medium for thermal transportation, nitrate-based molten salts are widely adopted for current concentrating solar power (CSP) plants. However, their corrosive nature also brings severe safety hazards to the system, e.g., the impellers and cover in molten salt pumps. To study the potential application of MAX ceramics as a proactive coating for these parts, tribo-corrosion behaviors of Ti3AlC2 and its Si-substituted analogs against GCr15 steels are evaluated in the molten Solar Salt at 300 °C to simulate the operating condition of the cold-salt pump. The Si-substituted MAXs present higher anti-wear performance than pristine Ti3AlC2. With an optimal composition of Ti3Al0.6Si0.4C2, about a 25% reduction in mean wear rate to that of Ti3AlC2 is achieved. Through a comprehensive analysis of the morphological and chemical states of both tribo-corroded and static corroded surface, the effects of Si on the tribo-corrosion behaviors of Ti3AlC2 is illuminated. The enhanced performance after Si addition is ascribed to the facilitated formation of aluminosilicates as corrosion products on the surface, which act as a lubricant as well as impeding the dissolution of Al into the molten Solar Salt.

On the corrosion cracking of austenitic stainless steel in molten solar salt: Experiments and modeling

The mechanical behaviors of 304 and 316 L steels was investigated in air and in solar salt with different contents of chloride impurity under slow strain rate tensile (SSRT) tests using a novel experimental approach at 565 °C. Results show that the yield strength (YS) and ultimate tensile strength (UTS) are negligibly affected by the molten salt corrosion whereas the ductility decreases with increasing contents of chloride. Corrosion cracks initiate and propagate along the grain boundary due to its higher corrosion rate and incremental oxide rupture. Moreover, a damage model that captures the corrosion and chloride effects is proposed and validated. • SSRT tests were performed for stainless steels in molten solar salt. • The YS and UTS are negligibly affected by molten salt corrosion. • Ductility of stainless steels decreases with increasing chloride contents. • Cracks are prone to initiate and propagate along grain boundary. • A damage model that captures the corrosion and chloride effects are proposed.

Numerical analysis of a developing turbulent flow and conjugate heat transfer for molten salt and liquid sodium in a solar receiver

• Temperature uniformity within a solar receiver using guide vanes was investigated. • Friction coefficient and Nu number were reported for liquid sodium and molten salt. • The effect of wall heat flux on fluid flow and heat transfer was studied. A numerical analysis of the developing flow and conjugate heat transfer in a solar receiver tube with a three-blade guide vane at the tube inlet was performed to investigate the effect of guide vane and working fluid on the flow and temperature distribution in a solar receiver tube subjected to the boundary condition of the non-uniform wall heat flux. Three dimensional Navier-Stokes, energy, and Laplace equations were solved numerically using Fluent code for the molten solar salt and liquid sodium as the working fluids. The SST k-ω turbulence model was used to predict the flow behavior inside the tube. The values of the friction coefficient and Nu number predicted by the numerical analysis were compared to and validated against experimental correlations from the literature. The numerical results show that a significantly lower pressure drop, and more uniform temperature distribution in the receiver tube are achievable by using the liquid sodium, compared to the molten solar salt. Application of a three-blade guide vane improves temperature uniformity across the receiver tube cross-section, but increases pressure drop in the developing flow region and, thus the overall pressure drop in the tube. The increase in the pressure drop is attributed to the swirling flow and higher friction coefficient compared to the fully developed flow for both working fluids studied in this work.

Analysis of scale and frame interface effects on the solidification properties of solar salt in mesopores

The intermittency and instability of solar irradiation can be effectively overcome by using medium- and high-temperature composite phase change materials (PCMs) for latent heat storage. Studies using metals and organics as phase change materials show that the phase change behavior of mesoporous composite phase change materials is affected by the constraint effect, and its thermophysical parameters (such as phase change point and phase change latent heat) are significantly different from those of macro volume. However, at present, research on molten salt phase change materials, which are widely used in the field of medium- and high-temperature research, is insufficient. Molecular dynamics simulations combined with experimental methods were used to analyze the changes in the freezing point, supercooling degree and enthalpy of solidification of solar salt in mesoporous composite phase change materials, which were based on the self-diffusion coefficient and the potential energy-temperature curve. The results show that a larger mesoporous size leads to a higher freezing point of the solar salt. At the same scale, with the increase in Al2O3 content in the framework, the freezing point of the solar salt first decreases and then increases. The supercooling degree of solar salt decreases with increasing scale. When the mass content of Al2O3 in the framework is in the range of 25 wt%-40 wt%, the supercooling degree of solar salt increases gradually; in contrast, it decreases in the range of 40 wt%-50 wt%, and finally, it increases again in the range of 50 wt%-75 wt%. The latent heat of phase transition increases with increasing scale. With the increase in Al2O3 content in the framework, the latent heat of phase transformation first decreases, then increases, and finally stabilizes. Al2O3 and SiO2 in the interface of the frame can promote heterogeneous nucleation and provide a substrate for the nucleation of molten solar salt. They also reduce the degree of supercooling. It is found that the effect of Al2O3 on nucleation is more significant.

Impact of the additive manufacturing process on the high-temperature corrosion of 316L steel in the presence of NaCl–KCl–ZnCl2 molten solar salt

A comparative study of the high-temperature oxidation and corrosion resistance of wrought and additive manufacturing selective laser melting (SLM) 316L stainless steel was performed in presence of molten NaCl–KCl–ZnCl2 salt used for thermal energy storage in the new generation of concentrated solar plants. Isothermal tests at 650 and 700 °C in a dry air atmosphere were conducted to evaluate (i) the effect of the manufacturing process and (ii) the effect of molten salts on the corrosion behavior of the steel by dimensional metrology analysis, scanning electron microscopy and X-ray diffraction characterization. The manufacturing process had a significant influence on the oxidation resistance. The wrought samples had a mass gain after 48 h that was four times greater than that of SLM samples at 650 °C, and 50% greater at 700 °C. The presence of the salt primarily increased the corrosion rate of all the samples and, again, the wrought pieces suffered 13% and 6% more corrosion in terms of metal loss than the SLM ones at 650 and 700 °C, respectively. The presence of more pathways for Cr diffusion associated with a higher number of defects in the microstructure of the SLM sample, as dislocations and grain boundaries, facilitated the formation of Cr2O3 scales in the SLM sample, avoiding the intergranular attack shown in the wrought one.

Impact of temperature on corrosion behavior of austenitic stainless steels in solar salt for CSP application: An electrochemical study

The compatibility between solar salt and structure materials is still a key issue in concentrated solar power (CSP) technology. The corrosivity of solar salt is significantly dependent on temperature. In this work, the temperature effect on the corrosion of 304 and 316L stainless steels (SS) in solar salt was studied at 500, 565, and 600 °C using electrochemical methods. Results indicate that higher values of i corr and corrosion rate (CR) are found for both 304 and 316L SS at higher temperatures. EIS results demonstrate that corrosion process of 304 SS in solar salt at 500 and 565 °C is controlled by ions transportation through the oxide layer. At 600 °C, however, the Warburg impedance is observed due to the porous and Cr 2 O 3 -free oxide layers formed on the surface of the sample exposed in molten solar salt for 138 h. Comparatively, the ions transportation through the oxide layer is always the rate-controlling process for 316L SS at each temperature. Protective oxide layer containing Cr 2 O 3 is formed on 316L SS even at 600 °C. Moreover, the formation of Cr-rich nitride underneath the oxide/substrate interface depends on the temperature of melt and chemical composition of substrate. • Electrochemical corrosion behaviors of 304 and 316L SS in temperature range of 500–600 °C are studied. • Higher i corr of 304 SS than that of 316L SS is observed at each temperature. • Formation of Cr-rich nitride depends on temperature and types of steel. • Cr 2 O 3 -free oxide layer forms on 304 SS at 600 °C, inducing Warburg impedance. • Cr 2 O 3 exists in protective oxide scale of 316L SS in the studied temperature range.

Theoretical estimation of temperature-dependent radiation properties of molten solar salt using molecular dynamics and first principles

The use of solar salt is important in modern solar energy utilization technology, i.e. concentrating solar power. However, there are few high-temperature-dependent radiation property data that can be used to accurately determine the temperature field, which hinders its further application. Thus, a theoretical approach using a combination of molecular dynamics and first principle methods is adopted to predict the radiation properties of molten solar salt from 525 to 875 K, which covers its working temperature range. According to the theoretical computation results, the main absorption region is observed when the wavelength is less than 500 nm. There are two absorption peaks located at approximately 130 and 250 nm, and there are also two refractive index peaks located at approximately 160 and 400 nm, which are slightly different from the absorption peaks. Moreover, increasing the working temperature subtly decreases the refractive index and extinction coefficient simultaneously. In addition, by investigating the effects of the simulation size, we found a 195-atoms system that satisfies the requirements for computations of related thermodynamic quantities. We believe that the results of this study show the significance of solar salt in the future applications.

Investigation of Regeneration Mechanisms of Aged Solar Salt.

The scope of our study was to examine the potential of regeneration mechanisms of an aged molten Solar Salt (nitrite, oxide impurity) by utilization of reactive gas species (nitrous gases, oxygen). Initially, aging of Solar Salt (60 wt% NaNO3, 40 wt% KNO3) was mimicked by supplementing the decomposition products, sodium nitrite and sodium peroxide, to the nitrate salt mixture. The impact of different reactive purge gas compositions on the regeneration of Solar Salt was elaborated. Purging the molten salt with a synthetic air (p(O2) = 0.2 atm) gas stream containing NO (200 ppm), the oxide ion concentration was effectively reduced. Increasing the oxygen partial pressure (p(O2) = 0.8 atm, 200 ppm NO) resulted in even lower oxide ion equilibrium concentrations. To our knowledge, this investigation is the first to present evidence of the regeneration of an oxide rich molten Solar Salt, and reveals the huge impact of reactive gases on Solar Salt reaction chemistry.

Development of ZrB2-Based Single Layer Absorber Coating and Molten Salt Corrosion of Bulk ZrB2–SiC Ceramic for Concentrated Solar Power Application

This study has dual objectives, one to develop single layer ZrB2-SiC-based ceramic absorber coating on stainless substrate, and another aim is to explore the degradation of ZrB2- SiC bulk ceramic in molten solar salt. In particular, we report the molten salt corrosion behavior of earlier developed ZrB2-SiC ceramic composite at 400 °C for 100 h. The corrosion rate in solar salt was determined to be 0.73 ± 0.09 mg h-1/cm2by gravimetric analysis. Binary and ternary oxide phases were formed due to electrochemical corrosion. The hot-pressed bulk ZrB2-SiC ceramic exhibits a high solar absorptance of 0.848 and also a high thermal emissivity of 0.66. A one-layer solar selective absorber coating without antireflection layer was developed using the ceramic disk as a sputtering target. When deposited using radio frequency (RF) magnetron sputtering technique on stainless steel substrate, a solar absorptance of 0.769 and thermal emissivity of 0.15 was obtained for an amorphous coating of thickness ~90 nm, without antireflection coating. The composition, determined by electron probe microanalyzer and X-ray photoelectron spectroscopy techniques, revealed nonstoichiometric phases in the layer with a relatively higher concentration of the lighter elements. The implication of the present work for concentrated solar power application is discussed. © 2021 American Chemical Society. All rights reserved.

Corrosion and electrochemical investigations for stainless steels in molten Solar Salt: The influence of chloride impurity

The influence of chloride impurity on corrosion behavior of stainless steels (304, 316 L and 347 SS) was carried out in molten Solar Salt at 565°C. Static immersion and electrochemical polarization tests were carried out at 565°C. It was found that the corrosion rates of the studied steels obtained from both static immersion and EP tests significantly increase in Solar Salt with the presence of chloride impurity. The chloride enhances the aggressiveness of the molten Solar Salt at 565°C by producing oxidants, which aggravates the corrosion extent of the investigated steels. Moreover, the oxide scale formed in molten Solar Salt containing chloride impurity is prone to spall. The synergy effect of active oxidation and the spallation of the oxide scale dominates the accelerating corrosion process.

Improving the corrosion resistance of ferritic-martensitic steels at 600 °C in molten solar salt via diffusion coatings

Molten salt corrosion in the receiver's piping system and storage tanks is still one of the major drawbacks of concentrated solar power (CSP) plants and is currently covered by using expensive high-alloyed steels or Ni-base alloys. The employment of cheaper structural materials combined with protective coatings is an attractive alternative to increase cost-efficiency. The present study investigates the corrosion resistance of three different coatings (a pure Ni, a Cr and a combined Ni + Cr coating) deposited on ferritic-martensitic X20CrMoV12-1 steel during isothermal immersion in molten solar salt in comparison to uncoated Ni-base alloy Haynes 230. Exposure tests were conducted at 600 °C for up to 1000 h. To identify the individual role of the main elements, exposure tests of pure Fe, Ni and Cr were undertaken as well. In addition to the cross-sectional investigations via light-optical microscopy and EPMA, X-ray diffraction measurements were conducted to identify the corrosion products. These were complemented by weight change measurements and chemical analysis of the salts after the exposure tests. The study reveals differences between the corrosion behaviour of the different coating approaches, which is discussed as a function of coating composition and degradation mechanisms. Whereas pure Cr or pure Ni coatings offer hardly any improved protection of the uncoated X20CrMoV12-1 substrate, the combined Ni + Cr coating performs very well and improves the scaling behaviour of X20CrMoV12-1 significantly. The outcome highlights the correlation between the solubility of the pure metals Fe, Ni and Cr in molten solar salt and the corrosion resistance of the investigated coatings.

Effect of Chloride Impurity on Corrosion Kinetics of Stainless Steels in Molten Solar Salt for CSP Application: Experiments and Modeling

Solar salt (60 wt% NaNO3 + 40 wt% KNO3) is widely used in the storage system of concentrated solar power plant. However, the chloride within the molten solar salt can affect the corrosion kinetics of stainless steels and hence affect the overall structural integrity. In this work, the corrosion behavior of 304 and 316L stainless steel (SS) in molten solar salt mixtures with different chloride impurities were investigated by immersion tests at 565 °C. Results show that as the content of chloride in the solar salt increases (up to 1.4 wt%), both 304 and 316L SS keep parabolic corrosion kinetics, and the rate constants increase. In comparison with 304 SS, the better corrosion resistance of 316L SS is ascribed to the higher thickness ratio of FeCr2O4 spinel to oxide scale. In addition, to quantitatively describe the effect of chloride impurity on corrosion kinetics of stainless steels, a novel theoretical model based on Fick’s first law is proposed and validated by the experiments.

Molten Solar Salt Pyrolysis of Mixed Plastic Waste: Process Simulation and Technoeconomic Evaluation

A process simulation and techno-economic evaluation of using molten solar salt as the heating medium for pyrolysis of mixed plastic waste (MPW) containing poly(vinyl chloride) (PVC) was performed. ...

Natural convection heat transfer from a bundle of heated circular cylinders with staggered arrangement immersed in molten solar salt

A two-dimensional steady-state natural convection heat transfer from a bundle of closely spaced isothermally heated cylinders in a staggered arrangement immersed in molten solar salt was investigated in this study for a laminar flow regime using numerical analysis. The flow and temperature fields and heat transfer from the cylinder bundle were predicted as functions of tube spacing and flow conditions. It was determined that flow patterns and heat transfer are strongly affected by the transverse and longitudinal cylinder-to-cylinder spacing ST and SL, Rayleigh number and thermo-physical properties of the heat storage fluid. Performance maps in the form of contour diagrams for the average Nusselt number Nu and dimensionless heat transfer volumetric density are presented for a staggered bundle of circular horizontal cylinders immersed in a molten solar salt.The variation of the heat transfer enhancement for the staggered bundle of horizontal cylinders shows that there is an optimal transverse cylinder spacing resulting in the highest heat transfer enhancement. The heat exchanger geometry was optimized by determining the optimal set of the tube (cylinder) spacings SL/D and ST/D in the bundle resulting in the maximum volumetric heat transfer rate q ̃, and smallest size, and lowest cost of a heat exchanger. A set of correlations were developed for the maximum and optimum dimensionless heat transfer volumetric densities as functions of the Rayleigh number RaD. The analyzed geometries are important in the solar heating and storage technology, nuclear reactor safety, and waste heat recovery applications.