Binary Nitrate Salt Research Articles

Enhancing thermal conductivity of novel ternary nitrate salt mixtures for thermal energy storage (TES) fluid

Efficient thermal energy storage (TES) is crucial for concentrated solar power (CSP) plants, necessitating the exploration of advanced heat transfer fluids with enhanced thermal conductivity. Conventional binary nitrate salt mixtures have limitations in thermal performance, prompting research into ternary mixtures and nanoparticle additives. This study investigates novel ternary nitrate salt mixtures comprising potassium nitrate (KNO₃), lithium nitrate (LiNO₃), and magnesium nitrate hexahydrate (Mg(NO₃)₂·6 H₂O) as high-performance TES fluids during the during the heat absorption phase. Seven different salt compositions were synthesized and characterized using the laser flash technique to evaluate their thermal conductivity over 100–400°C. Results revealed a significant influence of composition on thermal conductivity, with maximum values during melting ranging from 0.0777 W/m·K to 0.7373 W/m·K. Melting points varied from 335 K to 340.39 K, demonstrating tailorability through compositional adjustments. Furthermore, incorporation of 0.5 wt% aluminum oxide (Al₂O₃) nanoparticles resulted in substantial thermal conductivity enhancements, with the most significant increase observed in TSF10 (from 0.0777 W/m·K to 0.491 W/m·K). These improvements are attributed to enhanced phonon transport, increased surface area, and Brownian motion facilitated by Al₂O₃ nanoparticles. The study provides a comprehensive cost analysis, including raw material costs, and discusses the potential efficiency gains for CSP applications. The findings contribute to the development of high-performance and cost-effective TES fluids, advancing the efficiency and viability of sustainable energy generation.

Optically manipulated nitrate-salt-based direct absorption solar collectors for a photothermal energy harvesting system

This study introduces a nitrate-salt-based direct absorption solar collector (DASC) for a photothermal energy harvesting system with efficient solar energy harvesting and the reduction of thermal radiation loss. Investigations of the optical properties of snow-like solid-state binary nitrate salt in solar and thermal infrared (IR) spectral regions were first conducted in this study. Research revealed that the solid-state binary nitrate salt showcases high solar reflectivity (>90 %) and infrared emissivity (∼0.85), leading to a cooling power of 36.4 W m−2. This implies pure solid salt cannot gain energy from solar illumination. To tackle this, the proposed system incorporated nano graphite and a hot mirror. Consequently, the absorbed solar energy increased from 67.6 W m−2 to 898.7 W m−2, resulting in a heating power of 794.0 W m−2 under AM 1.5G solar irradiance. With the proposed approaches, melting the salt with lowest solar concentration ratio of 15 (∼12.5 kW m−2) was achieved.

Quaternary nitrate and chloride molten salts for the next concentrating solar power plants: Corrosion considerations for the use of AISI 304L steel

The study addresses the advancement in concentrating solar power (CSP) plants by transitioning from binary nitrate salts to nitrate and chloride quaternary salts, focusing on the corrosivity of four salts as evaluated by immersion testing of AISI 304L stainless steel. Four compositions of salts were evaluated, from solar salt to equimolar NaNO3‐KNO3‐NaCl‐KCl, up to 21 days (500 h) at 500 °C in an open atmosphere, determining the corrosion kinetics using gravimetry. The morphology, chemical composition, and microstructure of the corrosion products were characterized using XRD, FESEM-EDS, and GD-OES. Exposure to the molten salt with 0 mol% Cl‐ (Solar Salt) resulted in negligible corrosion kinetics, consistent with previous studies. The salt with 14 mol% Cl‐ caused a stable corrosion product with a corrosion rate 30 times higher than without chloride. All quaternary salts exhibited a multilayer structure of the top surface with selective chromium (Cr) removal. Specimens exposed to salts with more than 29 mol% Cl‐ displayed a similar structure with Cr and iron (Fe) removal, resulting in more brittle layers and corrosion rates 90 to 250 times higher than the salt without chloride. GD-OES analysis confirmed Cl diffusion into the oxide layer, highlighting the role of pCl2(g) and pO2(g) in driving corrosivity. Based on the results, using in salts with 29 mol% Cl‐ at 500 °C is discouraged, while using 304L with 14 mol% Cl‐ in a cold tank may be considered.

Experimental and numerical investigation of the melting process of solar salt in a spherical capsule

Solar salt is commonly employed as phase change material in various industrial applications, particularly in latent heat-based thermal storage systems such as packed beds in solar thermal power technologies. Here, a series of experimental and numerical studies are conducted of the melting process of a binary nitrate salt inside a spherical capsule under a wide range of large Stefan numbers, specifically ranging from 0.56 to 1.12. For a deep understanding of the melting behavior inside the sphere and its heat transfer characteristics under different constant wall temperatures and sphere sizes, the visualized high-temperature phase change experiment along with a 2D axisymmetric numerical model are presented. To confirm modelling reliability, the numerical model well-realized the sinking phenomena visualized in the experiment with a suitable mushy-zone parameter value of 6*108 kg/(m3·s). Subsequently, the temperature field, flow field, melting front, total transfer rate and melting time are explored. Further, a detailed parametric investigation is applied, including the effects of dimensionless parameters such as Gr, Ste on the melting process. And finally, the dimensionless correlation is succeeded to be proposed for unconstrained melting process with 0.56 ≤ Ste ≤ 1.12 and 1.80E+04 ≤ Gr ≤ 2.02E+05.

Impact of NaCl impurity on corrosion of 316 L steel in molten nitrate salts for thermal energy storage: An electrochemical study

AbstractThe impact of NaCl impurity on corrosion of stainless steel (SS) 316 L in binary nitrate salts (60 wt% NaNO3 + 40 wt% KNO3) was systematically investigated at 565°C via electrochemical measurements. SS 316 L exhibits active dissolution behavior in salts containing 0–1.4 wt% NaCl. The influence of NaCl impurity shows a strong time dependence according to the negligibly affected corrosion current density obtained from a short‐time potentiodynamic polarization sweep. The corrosion process of 316L in salts with NaCl‐free is controlled by ions transport through the oxide layer. With the presence of NaCl impurity, it is controlled by higher rate of ions transport and the diffusion of metal chlorides. The alternation in basicity in the corrosion front caused by chloride ions increases the solubility of chromium oxide and hence the dissolution of chromium oxide, which can lead to the formation of a less protective oxide layer.

Effect of silica nanoparticle size on the stability and thermophysical properties of molten salts based nanofluids for thermal energy storage applications at concentrated solar power plants

Molten salts-based nanofluids are attractive candidates for thermal energy storage applications due to their enhanced thermophysical properties. However, their stability remains an open issue. In the present work, the size effect of SiO2 nanoparticles on the stability and thermophysical properties of molten binary nitrate salt was studied. For that purpose, the effect of SiO2 based nanofluids was systematically studied by using in-situ high-temperature observations and zeta potential experiments. From the analysis, the nanofluids having nanoparticles larger than 450 nm demonstrate superior stability compared to the ones with nanoparticles of 27 nm. Moreover, in contrast to the case of 27 nm particles increase of viscosity was shown to be negligible for particles larger than 450 nm. The absence of specific heat capacity (Cp) or thermal conductivity enhancement for the cases of larger nanoparticles suggests that the development of molten salts-based nanofluids is bounded by the compromise between the stability and improvement of thermophysical properties, depending on the particle size. These results open a pathway for the development of stable molten salt-based nanofluids with enhanced thermophysical properties where the size of the nanoparticles must be optimized.

Hybridization of CSP Plants: Characterization of a Molten Salt Heater for Binary and Ternary Nitrate Salt Mixtures Fueled with Gas/Biogas Heaters

Hybridization of CSP plants with alternative energy sources (fuels) represents a means to improve flexibility of operation, power dispatchability and utilization factor of the plant. New generation CSP plants make use of molten salts as Heat Transfer Fluid (HTF) besides Thermal Energy Storage (TES) medium. Therefore, proper interfaces should be developed to effectively transfer the heat from the back-up source to the molten salt. This paper presents the results obtained in the experimental validation of an innovative gas-fueled Molten Salt Heater (MSH) prototype. The objective of this research is to validate the MSH design, where the specific properties of molten salts (compared to other HTFs, e.g., thermal oils) have to be taken into account. The developed reduced-scale MSH (90 kW thermal) consists of a heat exchanger with the molten salt flowing inside finned tubes cross-flowed with the hot flue gas generated in an upstream combustion chamber. LPG or a biogas-like mixture has been used as gas fuel. Experimental results have been obtained with two different molten salt mixtures: the “solar salt” binary mixture (NaNO3/KNO3, 60/40%w) typically used in CSP applications (up to 565 °C) and the ternary mixture known as Hitec XL® containing sodium/potassium/calcium nitrates (NaNO3/KNO3/Ca(NO3)2, 15/43/42%w) characterized by lower freezing temperatures. Experimental tests have been carried out changing some operative parameters like the flow rate of the molten salt (0.45–0.94 kg/s), the inlet temperatures of the molten salt (303–445 °C) and of the hot gas (596–632 °C). For both molten salt mixtures, it was demonstrated that heat transfer correlations based on the Dittus-Boelter equation allow to predict experimental results with <10% deviation between experimental and theoretical values of the heat transfer coefficient.

Comparative study of thermally stratified heat storage tank using different heat transfer fluids based on fluid-solid coupling method

The heat transfer fluid material can affect both the behaviors of a heat storage tank. Based on the Fluid-Solid Coupling method, the influences of five different heat transfer fluids on the thermal and mechanical behaviors of the thermally stratified tank are estimated in this paper, including the binary nitrate (KNO3+NaNO3), binary chloride (KCl + MgCl2), ternary nitrate (KNO3+Ca(NO3)2+LiNO3), liquid lead-bismuth eutectic (LBE) and liquid sodium. The simulation results demonstrate that for the charging process, the tank using the ternary nitrate has the best operation behavior, while for the discharging one, that utilizing the binary nitrate has relatively better operation behavior. The standby behavior comparison results indicate that the thermocline stability of the ternary nitrate is the best, but the tank employing the binary nitrate has the smallest cumulative total heat loss. The mechanical performance comparison results show that the binary nitrate salt and binary chloride salt can lead to the smallest and largest maximum mechanical stress of the tank, which are 96.6 MPa and 168.8 MPa. Therefore, the tank utilizing the binary nitrate has the best mechanical behavior.

Nanoencapsulation of binary nitrate molten salts for thermal energy storage: Synthesis, thermal performance, and thermal reliability

Molten salts have favorable features to serve as thermal energy storage (TES) media, and their nano-sized capsules are advantageous for efficient TES. However, their high water solubility hinders encapsulation by conventional methods, such as the sol–gel process. In this study, a binary nitrate molten salt mixture (NaNO3 and KNO3 in 60:40 mass ratio) was nanoencapsulated by a silica (SiO2) shell using emulsification and sol–gel techniques. The diameters of the molten salt capsules were controlled from a few micrometers to several hundred nanometers by regulating the stirring intensity and surfactant dosage during the synthesis process. Based on differential scanning calorimetry measurements, the encapsulation ratio and encapsulation efficiency were calculated 50.6% and 50.0%, respectively. Thermogravimetric analysis confirmed that the thermally stable SiO2 shell enhances the decomposition temperature of the binary nitrate salt mixture. Finally, the proposed nanoencapsulation protocol was also utilized to encapsulate a different nitrate salt mixture (NaNO3 and KNO3 in 80:20 mass ratio) and two single nitrate salts (NaNO3 and KNO3). This was done to validate both its applicability to all nitrate salt mixtures and the consistency of the chemical composition in each nanocapsule. The achievement for these nitrates suggests the possibility of comprehensive nanoencapsulation of water-soluble materials.

Comparative study of thermally stratified tank using different heat transfer materials for concentrated solar power plant

This paper presents a comparative analysis of thermal and mechanical behaviors of the thermally stratified heat storage system using three different heat transfer materials, which are the binary nitrate salt, ternary nitrate salt and liquid lead–bismuth eutectic (LBE) alloy. The discharging behaviors of the tank using three different heat transfer materials are investigated by using both the algebraic and numerical models. For all the three materials, the algebraic results are consistent with the simulation ones. During the discharging process, in comparison, the thermal stratification thickness of the tank using the binary nitrate salt is relatively smaller, which can lead to a better discharging performance. The comparison results of expanding behaviors of the standby tank reveal that the thermal stratification thicknesses of tanks using the three different heat transfer materials all increase with the standby time increased. Compared with the other two materials, the thermal stratification thickness of the tank using the binary nitrate salt is the smallest at the same standby time. The comparison results of mechanical behaviors of the tank demonstrate that the maximum mechanical stress of the tank using the liquid LBE is the greatest, while the tank using the molten salts can have better mechanical properties.

Influence of $$NO_2^ - $$ on the Microscopic Structure and Physical Properties of the Binary Nitrate Salts: a Molecular Dynamics Simulation Study

Molecular dynamics simulation method was used to study the influence of $$NO_2^ - $$ on both the structure and properties of the binary nitrate salts (60 wt.% NaNO3 + 40 wt.% KNO3). The density and viscosity of the mixtures were experimentally measured and the simulation results met well with the experimental ones. The simulation results showed that, with the addition of $$NO_2^ - $$, the ionic clusters tended to loose and the mobilities of all the ions tended to increase. The density, viscosity, and heat capacity decreased while the thermal conductivity increased with the increase of +++ concentration. The correlation between the microscopic structure and physical properties of the mixtures were discussed and revealed.

Effects of constant load on corrosion cracking of 304 ASS at 600°C in molten salts

AbstractThe effects of constant load (CL) on corrosion cracking of 304 austenitic stainless steel (ASS) in molten salts (binary nitrate salts, 60 wt% NaNO3, and 40 wt% KNO3) were investigated. A pair of novel clamping fixture was designed to fix the tensile sample, making the gauge section immersed in molten salts. The CL tensile tests were applied with the initial stress of 212 MPa and the temperature of 600°C. Optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy were employed to characterize the deformation behavior and corrosion features. The results show that corrosion of 304 ASS in molten salts was aggravated by the continuous deformation due to the cracking of protective oxide scale. Grain boundaries were oxidized and weakened by the molten salt, leading to a crack propagating. Compared with the creep test result, a reduced rupture life of 304 ASS under constant loading in molten salts was observed.

Graphitization as efficient inhibitor of the carbon steel corrosion by molten binary nitrate salt for thermal energy storage at concentrated solar power

Hot corrosion by molten salts has detrimental effect on various industrial and infrastructural systems particularly on the Concentrated Solar Power (CSP) plants. To enhance durability of the CSP plants′ components, while decreasing the production cost of electricity, the graphitization anticorrosion method for carbon steel in contact with molten binary nitrate salt is proposed in this work. To shed some light on the corrosion mechanisms and quantify corrosion rates a combination of advanced material characterization techniques like Focused Ion Beam, SEM, EDX, XPS depth profiling, XRD was applied. It has been demonstrated that the surface spray-coating by graphite reduces the corrosion rate at least by factor of 6 and stabilizes the corrosion scale by iron carbide crystals at 390 °C.

Nanoparticles as a high-temperature anticorrosion additive to molten nitrate salts for concentrated solar power

Hot corrosion is profoundly detrimental to construction elements in Concentrated Solar Power (CSP) plants affecting their lifetime, running costs and safety. In this work we have studied the anticorrosion effect of TiO2 nanoparticles additives on carbon steel using XRD, XPS with depth profiling, EDX and FIB/SEM techniques. The results revealed that the addition of 1 wt% of TiO2 nanoparticles to molten binary nitrate salt reduces the corrosion rate of carbon steel more than twice and stabilizes the corrosion scale at 390 °C. The anticorrosion effect of TiO2 nanoadditive was attributed to the formation of iron-titanium mixed oxide on the carbon steel surface. It was confirmed by XRD and TGA techniques that addition of TiO2 nanoparticles does not alter the stability of the salt. In view of presented results, the feasibility of molten salts based nanofluids in the CSPs can be reconsidered in terms of improved compatibility with construction materials.

The evolution of the mushy zone during the melting process of a binary nitrate salt

In this study, an experimental setup was built to investigate the evolution of the mushy zone during the melting process of a binary nitrate salt (40% mol. LiNO3-60% mol. KNO3). The thermophysical properties of the salt, including the solidus and liquidus temperatures and latent heat, were firstly determined. The melting process of the salt was then conducted in a thin container with one-sided heating. The temperature variations of the salt were measured and the change of the melting front were recorded by a digital camera for the melting processes under different heating temperatures. Based on the recorded transient locations, the moving rate of the melting front was calculated. The resulting moving rate, together with the measured temperatures, was further used to calculate the mushy zone width. Moreover, the effect of the heating temperature on the mushy zone was studied. The results show that the moving rate of the melting front decreases during the melting process while the mushy zone width increases. It is also found that the moving rate of the melting front increases while the mushy zone width decreases with the increasing heating temperature. Finally, both the dimensionless location of the melting front and the dimensionless mushy zone width during the melting process were developed as functions of Stefan and Fourier numbers.

Microkinetics of the reaction [formula omitted] in molten sodium nitrate and potassium nitrate salt

Latest development in the concentrating solar power (CSP) tower technology for energy production and storage intends to find a heat transfer and storage material that provides stability at elevated temperatures. The binary nitrate salt mixture Solar Salt (sodium nitrate, 60 wt.% and potassium nitrate, 40 wt.%) is utilized up to its state-of-the-art temperature limit of 560 °C. Knowledge of the ongoing reactions is demanded to assess the potential of Solar Salt for higher temperature applications. The reversible reaction NO3-⇌NO2-+0.5 O2 is fundamental in a sequence of decomposition reactions. It is investigated by thermogravimetric experiments and ion chromatography post-analysis with regard to the chemical equilibrium, and to reaction kinetics. The reaction enthalpy and entropy of 97 +/- 9 kJ mol−1 and 85 +/- 12 J·mol−1 K−1, respectively, coincide with literature values. A detailed reaction mechanism based on literature findings is drafted and discussed. The corresponding kinetic rate law is simplified by means of rate law approximations and by focusing on the reduction reaction path. The kinetic rate constants of this study exceed previous findings, probably due to successful exclusion of mass transport limitations in the <1 mm thin salt samples. The activation energies and pre-exponential factors amount to 212 +/- 11 kJ·mol−1 and 694546727 +/- 6 s−1, and 115 +/- 11 kJ·mol−1 and 25770 +/- 6 s−1 for the reduction and oxidation reaction, respectively.

Experimental investigation of specific heat capacity improvement of a binary nitrate salt by addition of nanoparticles/microparticles

Abstract Several recent studies reported anomalous enhancement of specific heat capacity by doping minute concentration (e.g. 1 wt.%) of nanoparticles into the eutectic salt. In this work, silica particles with three different sizes and nano-sized silicon nitride and silicon carbide particles were dispersed in solar salt with particle concentrations of 0.5–7 wt.%. To prepare the composite material, a conventional two-step aqueous solution method was used and the drying approach was modified to suit large-scale production. The specific heat capacities of the composites, pure solar salt and particles were measured using differential scanning calorimetry in a temperature range of 300–400 ᵒC and the results were compared. Silicon nitride and silicon carbide nanoparticles have marginal effect on the specific heat capacity of solar salt over the examined concentration of 0.5–3 wt.%. The specific heat capacity of silica composites with 3 wt.% of silica were enhanced by 4.3–9.7% over that of the pure solar salt. However, when 5 wt.% and 7 wt.% of silica (30 μm) were dispersed, the specific heat capacity was reduced by 10.6% and 9%, respectively. Material characterization of the composite material proved the formation of sodium silicate due to reaction of silica and sodium nitrate, which is probably correlated to the reduction of specific heat capacity.

Natural convection heat transfer for eutectic binary nitrate salt based Al2O3 nanocomposites in solar power systems

Molten salts are widely used as thermal energy storage materials in a concentrated solar plant. In these thermal storage systems, natural convection heat transfer for molten salts is important for both energy storage and transfer, however the research on nature convection heat transfer is quite limited. In present work, the natural convection heat transfer for eutectic binary nitrate salt based Al2O3 nanocomposites was numerically simulated using a two-phase lattice Boltzmann model. Various interactions affecting the distribution of nanoparticles were considered in the simulation. The effects of the nanoparticle mass fraction (0–2.0%) and Rayleigh number (104–108) on natural convection heat transfer were also analyzed. Results indicated that natural convection heat transfer increased with increasing the Ra. With the addition of nanoparticles, natural convection heat transfer decreased at low Ra numbers but increased at high Ra numbers. Among all interaction forces considered in this study, the temperature difference driving force FS was much larger than other forces, which means the nanoparticle distribution was mainly influenced by FS.

Effect of Al2O3 nanoparticle dispersion on the specific heat capacity of a eutectic binary nitrate salt for solar power applications

Molten salts can be used as heat transfer fluids or thermal storage materials in a concentrated solar power plant. Improving the thermal properties can influence the utilization efficiency of solar energy. In this study, the effect of doping eutectic binary salt solvent with Al2O3 nanoparticles on its specific heat capacity (cp) was investigated. The effects of the mass fraction of nanoparticles on the cp of the composite nanofluid were analyzed, using both differential scanning calorimetry measurements and molecular dynamics simulations. The specific heat capacity of the nanocomposites was enhanced by increasing the nanoparticle concentration. The maximum enhancement was found to be 8.3%, at a nanoparticle concentration of 2.0%. A scanning electron microscope was used to analyze the material morphology. It was observed that special nanostructures were formed and the specific heat capacity of the nanocomposites was enhanced by increasing the quantity of nanostructures. Simulation results of cp agreed well with the experimental data, and the potential energy and interaction energy in the system were analyzed. The change in Coulombic energy contributed to most of the large change in cp, which explains the discrepancy in values between conventional nanofluids and molten salt-based nanofluids.

Mechanical Dispersion of Nanoparticles and Its Effect on the Specific Heat Capacity of Impure Binary Nitrate Salt Mixtures.

In this study, the effect of nanoparticle concentration was tested for both CuO and TiO2 in eutectic mixture of sodium and potassium nitrate. Results showed an enhancement in specific heat capacity (Cp) for both types of nanoparticles (+10.48% at 440 °C for 0.1 wt % CuO and +4.95% at 440 °C for 0.5 wt % TiO2) but the behavior toward a rise in concentration was different with CuO displaying its highest enhancement at the lowest concentration whilst TiO2 showed no concentration dependence for three of the four different concentrations tested. The production of cluster of nanoparticles was visible in CuO but not in TiO2. This formation of nanostructure in molten salt might promote the enhancement in Cp. However, the size and shape of these structures will most likely impact the energy density of the molten salt.