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Nanofluids based on molten carbonate salts for high-temperature thermal energy storage: Thermophysical properties, stability, compatibility and life cycle analysis
Abstract
Molten salts-based nanofluids have been widely considered for Thermal Energy Storage (TES) applications due to their enhanced thermophysical properties. However, the application of such fluids faces many challenges, among which are the correct determination of their properties, stability, compatibility with construction materials and the overall environmental impact. In this work, we attempt to provide a comprehensive analysis of nanofluids based on nano-alumina and molten carbonate salt for the benefit of next-generation high-temperature TES applications. In particular, considerable statistics, cross-verification, novel preparation and characterization methods were applied to record ~12% increase of thermal conductivity, ~7% increase of heat capacity and ~35% increase of viscosity. It was demonstrated that such nanofluids have poor dispersion stability under static conditions; however, the enhanced thermophysical properties can be maintained by mechanical stimuli, e.g. mixing or redistribution. We show that some nanoparticles interact with typical construction materials such as stainless steel 310 by forming mixed oxides and considerably reducing the corrosion rates. An erosion study has been performed demonstrating negligible effect of nanoparticles even in the case of their strong agglomeration. Finally, life cycle analysis revealed that viscosity and preparation method of such nanofluids must be targeted to minimize the environmental impact.
... The production of iron and titanium oxides is the main reason to stabilize the corrosion zone and decline the corrosion rate. In addition, comprehensive analysis of ternary carbonate molten salt-based nanofluids (K2CO3-Na2CO3-Li2CO3 + 12 nm Al2O3 nanoparticles) [26] unfolded that the addition of alumina nanoparticles to molten carbonate salts greatly reduced the corrosiveness on 310 stainless steel. The thickness of corrosion zone of the molten salt-based nanofluid was decreased by nearly a factor of two, indicating a lower corrosion rate compared to the pure molten salt. ...
... Although a variety of research including both experimental [23,26,31] and MD simulation [29,30] studies have been conducted to reveal the corrosion inhibition mechanisms of Al2O3 and SiO2, very few works have been conducted on TiO2 nanoparticles in the field of corrosion protection. For example, the experimental results [25] of doping TiO2 nanoparticles into molten salt showcased excellent corrosion protection, but its corrosion inhibition mechanism has not been well explored yet at the microscopic level. ...
... For example, some nanoparticles may be more inclined to undergo physical diffusion, while others are more likely to undergo chemical reactions. Furthermore, the type of molten salt and the impurities mixing with the molten salt may also affect the behavior of nanoparticles [26]. Both the type and concentration of impurities may change the properties of the molten salt and thus affect the behavior of the nanoparticles. ...
... In this sense, the corrosivity of carbonate salts in static conditions has been extensively analyzed in literature, and concretely numerous works reported results for the eutectic ternary system [8][9][10][11][12][13]. However, to the best of our knowledge, just one paper addressing the corrosion of K2CO3-Na2CO3-Li2CO3 in dynamic configuration has been published [14]. ...
... On the other hand, for static conditions, the determined average corrosion rate was 466 ± 3 μm/year, with a variability ranging from 364 to 668 μm/year. This average value is within the limits of previously reported data, between 73 and 438 μm/year, but it is significantly larger than the reported average corrosion rate, 157.0 ± 41.5 μm/year [11,12]. Nevertheless, the deviation obtained in this work is significantly lower. ...
... These differences could be explained by different salt mass-tostainless steel superficial area ratio. The present study tested samples with approximately 7000 mm 2 of external surface in 685 g of carbonate salt, while in refs [11,12], 5 g of salt were employed for samples with 1000 mm 2 superficial area. Thus, 20 times higher salt to metal ratio was used in this work. ...
Eutectic ternary carbonate salt is one of the candidates for 3rd generation concentrated solar power (CSP) plants. Gen3 CSP targets higher operation temperatures, which strengthens the corrosivity issues associated to molten salts. Although there are corrosion studies for this carbonate salt in static conditions, the effect of salt flow is not fully understood. In this work, corrosion experiments under static and dynamic conditions are compared for SS310 subjected to ternary carbonate salt at 600ºC. The corrosion layer formed during static and dynamic tests were completely characterized by means of SEM-EDX and XRD (surface and cross-section). The corrosion products deposited in the salt during the experiment were analyzed by ICP-OES. The tests performed under dynamic conditions demonstrated an increase spallation of the corrosion layer. This spallation produced a thinner scale and exposed the Cr containing phase to the molten salt, fostering its dissolution. These results confirmed the significant effect of dynamic conditions on the corrosivity of eutectic ternary carbonate salt and the importance of assessing them in the design of 3rd generation CSP plants.
... Similar effects have been reported for carbonate-based nanofluids. Grosu et al. analyzed ternary eutectic Li/Na/K carbonates doped with 1 wt% of SiO 2 nanoparticles, showing a peeling-off reduction and a twofold decrease of the corrosion layer thickness compared to the base salt for SS310 [32]. Also, with 1 wt% of SiO 2 nanoparticles but in eutectic Li/K carbonate salt, Iyer found the same reduction compared to the base fluid for SS304 [33]. ...
... The solubility in ethanol or other solvents is considerably lower, which could complicate and prolong the cleaning process. The use of water for cleaning the samples is general approach used in the field of molten salts corrosion and has been previously reported with favorable results [11][12][13][14]17,32,35,[37][38][39][40]. ...
... Moreover, a non-reactive inclusion of nanoparticles into the corrosion layer has been confirmed by several methods for corrosion studies for nanoparticle-based molten salts [28] and diffusion experiments [36]. Under more harsh conditions [32,41] or for less stable nanoparticles [29], it seems that nanoparticles participate in chemical reactions during corrosion layer formation. As described below, this is in line with the results obtained for SS304. ...
High-temperature molten salt systems are employed in a wide variety of industrial applications, linked to energy production and storage, such as concentrated solar power, waste heat recovery, storage plants, fuel cells nuclear, etc. The reactivity of these salts is one of the main issues to address for the employment of affordable steels as constructive materials. In this work, the performance of a polymeric anticorrosion coating based on nanoparticles is analyzed for carbon and stainless steel subjected to solar salt, at 390 and 565 • C, respectively. The application of the protective coating produced a more homogeneous corrosion layer in both steels compared to uncoated samples. For carbon steel, the spallation of the corrosion layer was mitigated. For stainless steel, the corrosion was significantly reduced. The was confirmed by SEM-EDX confirmed the inclusion of alumina nanoparticles into the corrosion scale and their reaction with stainless steel to form mixed oxides was corroborated by XRD. Molten salts were analyzed by ICP. The obtained results pave the way for anticorrosion coatings based on nanoparticles for high temperature molten salts applications.
... Moreover, a severe corrosion was described at 700 • C, making inadvisable the use of SS316L without protective coating for applications at that temperature. Grosu et al. reported corrosion experiments in eutectic ternary carbonates for SS310 and SS347 at 600 • C [35,36]. The corrosion layer was composed of mixed oxides in a layered structure for both stainless steels, with the Cr containing phase located close to the substrate. ...
... In Refs. [35,36], SS310 pieces with approximately 1000 mm 2 of external area were immersed in 5 g of salt. However, in the present work, 685 g of ternary carbonate salt were employed for testing a sample with an overall external surface of roughly 7000 mm 2 . ...
... This means that the salt to metal ratio of this test is 20 times higher than in Refs. [35,36]. Therefore, we highlight here the importance of salt-to-stainless steel ratio, which will be systematically studied in the upcoming work. ...
Li2CO3–Na2CO3–K2CO3 salt is one of the candidates for 3rd generation concentrated solar power (CSP) plants, aiming to increase the operational temperature. This rise of temperature significantly increases the corrosion issue of molten salts. However, there is a lack of studies focused on the effect of salt flow on the corrosion kinetic for this ternary carbonate salt. In this work, corrosion experiments under static and dynamic conditions are compared for SS310 subjected to ternary carbonate salt at 600 °C.
A complete characterization of the corrosion layer, surface and cross-section, was carried out by means of SEM-EDX and XRD, while the molten salts after the corrosion tests were analysed by ICP-OES and XPS.
The dynamic experiment exhibited an enlarged spallation of the corrosion layer, leading to a thinner and less homogeneous scale. This feature exposed the chromium containing phase and considerably increased the extent of its dissolution into the salt. The obtained results remark that the detrimental effect of dynamic conditions on the corrosivity of molten carbonate salt cannot be neglected and must be taken into account in the design of 3rd generation CSP plants.
... Figure 7 displays the XRD spectra of the pristine, reference, and a-C-coated 301LN after 1000 h of exposure to molten salt. For the oxide scales of the reference sample, LiFeO2, LiCrO2, and Li(Fe,Ni)O2 were detected using SEM-EDX and XRD (Figures 6 and 7), which is in good agreement with previous works [50,74]. On the Fe-rich outer layer, mainly LiFeO2 was generated, while LiCrO2 and Li(Fe,Ni)O2 formed at the Cr/Ni-rich inner layer. ...
... Thus, the a-C film on the 301LN surface exhibited a favorable chemical compatibility with the 301LN and molten salt, reducing the corrosion rate compared with the reference sample without the a-C film. (Figures 6 and 7), which is in good agreement with previous works [50,74]. On the Fe-rich outer layer, mainly LiFeO 2 was generated, while LiCrO 2 and Li(Fe,Ni)O 2 formed at the Cr/Ni-rich inner layer. ...
Ternary carbonate salts (Li 2 CO 3-Na 2 CO 3-K 2 CO 3) are promising heat transfer fluids to increase the efficiency of the electric power in concentrated solar power (CSP) technology. However, the corrosion produced at high operating temperatures is a key challenge to tackle for employing cost-effective steels as construction materials in CSP. In this work, the use of stainless steels with amorphous carbon was investigated, for the first time, as a surface modification method to mitigate the corrosion of structural CSP materials by molten salts. In doing so, an amorphous carbon (a-C) film of 100 nm in thickness was deposited on the 301LN stainless steel's surface by the carbon thread evaporation technique. The corrosion behavior of the 301LN was assessed in carbonate salt at 600 • C for 1000 h. This film decomposed forming carbide layers, contributing to corrosion mitigation due to the generation of denser oxide layers, decreasing the Li + diffusion through the stainless steel.
... There were numerous reports on the stability of nanoparticles in nanofluids [18,19] and molten salts [75,124,125]. The homogeneity of NEMS and the long-term stability without agglomeration or sedimentation are crucial issues to be addressed. ...
... The nanoparticle's high surface area, surface charge, and Vander Waals force lead to nanoparticles' sedimentation and non-homogeneous dispersion [19]. This could negatively affect the thermophysical properties of molten salt [75]. The stability of NEMS is usually evaluated through visual inspection of NEMS that has been doped into molten phase for extended periods of time [125]. ...
The emphasis on exploiting solar power has efficaciously engaged the concentrated solar power (CSP) technology towards producing electricity from solar thermal energy. CSP technology's key element is molten salts, which function as thermal energy storage (TES) to absorb and store the sun's thermal energy and as a heat transfer medium to transfer the stored thermal energy to a steam generator for electrical energy production. The thermophysical properties enhancement of molten salt through the integration of additives such as nanoparticles permits the molten salt to store additional solar thermal energy by improving its heat transfer properties. This, in due course, leads to an overall enhancement of the CSP plant efficiency and electricity cost. Various types of inorganic nanoparticles such as Alumina, Copper Oxide, and Titania have been studied with different types of molten salts to enhance their thermophysical properties, especially those related to CSP applications. As such, the present review aims to discuss the recent advances in nano-enhanced molten salt (NEMS) and the nanoparticles used to date to enhance molten salts behavior for CSP systems. In addition, important topics related to NEMS, such as the stability of nanoparticles and nanostructures' formation, are also discussed. The article also focuses on the different types of nanoparticles used as thermophysical enhancers and their enhancement logics. Besides, the effect of nanoparticle assets such as mass percentage, size, and shape and how they may positively or negatively affect the thermophysical properties of molten salts are also presented in this review. The related publications in this field have shown an increasing focus on enhancing molten salts' thermophysical properties every year. Most of these experiments were focused on improving the heat capacity of nitrate or carbonate salts. Additionally, the dendritic nanostructures were reported to have a long-range effect that may enhance the heat capacity. Also, the studies on nanoparticle concentration and morphology reveal that these factors significantly influence the molten salt's thermophysical properties. This review is anticipated to provide an outline of the works done related to NEMS and to benefit the researchers in the field.
... In a study on binary nitrate MSs, the introduction of ultrasonically dispersed, highly affordable detonation nanodiamonds (NDs) with an average particle size of 10 nm, led to the development of a new type of nanofluid MS. This nanofluid MS demonstrated significant improvements in thermal properties: the melting point decreased by 16°C, the thermal decomposition temperature increased by 35°C, the thermal conductivity increased by 93%, the volumetric heat capacity increased by 38%, and the thermal diffusivity increased by 43% [3]. ...
Solar power, which is one of the most abundant and sustainable energy sources, has attracted a lot of attention for its clean and renewable attributes amid a growing global demand for renewable energy. Molten salt (MS) energy storage technology is an innovative and effective method of thermal energy storage. It can significantly improve CSP (concentrated solar power) systems’ stability and efficiency. This review first introduces the importance of solar energy and then delves into the development and applications of MS energy storage technology. Traditional MSs (e.g., Solar Salt and Hitec Salt) face issues of thermal stability and corrosion at high temperatures, whereas improved MSs have shown significant enhancements in thermal properties. This paper summarizes research achievements in improving MS performance through the addition of alkaline substances, optimization of MS ratios, and introduction of nanoparticles to form nanofluids. Furthermore, the paper covers future research areas and challenges for MS energy storage technology. These include the creation of new MS materials, system design optimization, and cost reduction strategies. Technology utilizing MS energy storage is a promising component of energy systems of the future, as it contributes significantly to the advancement of renewable energy sources and increases energy efficiency.
... In this regard, some researchers have recently proposed several effective solutions to mitigate the corrosion of austenitic stainless steels immersed in molten Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 salts: (1) doping molten salts with nanoparticles, (2) protective oxide coatings, and (3) surface modification of stainless steels. Several studies have been reported on the corrosion of CSP materials after doping molten carbonate salts with nanoparticles, mainly SiO 2 and Al 2 O 3 [29][30][31][32]. Fernandez et al. [30] reported the positive effect of the presence of Al 2 O 3 in two aluminaforming austenitic (AFA) alloys, corresponding to modified OC4 and HR224 grades, tested at 650 • C. Frangini and Loreti investigated other rarely-used nanoparticles, such as CaO and MgO [31]. ...
Next-generation concentrated solar power (CSP) technology needs to work at high temperatures to increase its power generation efficiency. It represents overcoming some issues for the structural materials, such as corrosion mitigation. In the present work, the laser-surface texturing (LST) treatment is investigated as a surface modification method to mitigate corrosion of a duplex stainless steel for CSP structural material. Corrosion tests of duplex steel were carried out immersed in a static molten salt mixture of Li2CO3-Na2CO3-K2CO3 at 600 °C for 1000 h. Complementary microscopy, microanalysis and optical characterization techniques were used to analyse both the steel surface after LST and the oxide scales formed after corrosion tests. Results evidenced that the laser treatment favoured the formation of a denser oxide layer, improving its effectiveness as a protective barrier against further oxidation. Compared with an untreated surface, the laser-treated one significantly decreased the corrosion rate by 48 %.
... However, it represents overcoming some issues for the structural materials, such as corrosion mitigation. In this context, carbonate salts have emerged as promising candidates for thermal energy storage (TES) applications at high temperatures, [16,17]. These salts exhibit excellent thermal stability and high heat capacity, making them ideal for storing large amounts of thermal energy. ...
Laser surface texturing (LST) has emerged as a powerful technique for creating surface patterns on various materials, including metals, polymers, and ceramics. Among LST methods, laser wobbling surface texturing (LWST) offers distinct advantages over traditional surface modification techniques. It provides a precise and non-contact approach to customize surface properties without affecting the bulk material. This study investigates the effects of amplitude and frequency of LWST as a surface modification strategy to mitigate high-temperature corrosion in AISI 301LN stainless steel, representing the first investigation of its kind. The study extensively examines the impact of LWST on surface morphology, wettability, phase transformation, and microhardness of 301LN stainless steel. The results reveal that the wobbling frequency of LWST has a greater effect on the surface roughness than the wobbling amplitude. Higher surface roughness of LWST promotes the increase of the surface area and altered surface energy, resulting in an increased wetting behavior. X-ray diffraction analysis confirms a moderate phase transformation from austenite to martensite in the laser-treated samples, which increases the surface hardness compared to the non-treated material. Finally, the corrosion tests conducted in molten carbonate salt at 600 °C evidence a reduction of the corrosion rate around 20 %. LWST promotes the formation of denser oxide layer, improving its effectiveness as a protective barrier against further oxidation.
... At 0.25 wt% MgO nanoparticles, they obtained a 17.5% augmentation in thermal conductivity, with a 30% reduction in solidification time and a 42.4% improvement in discharge time. Grosu et al. [59] investigated the thermodynamic properties of Al 2 O 3 nanoparticles based on molten salt. The results indicated that with Al 2 O 3 based NEMS for 12 mm nanoparticles size and 1 wt% concentration, the augmentation in thermal conductivity, specific heat, and viscosity was 7%, 12%, and 35%, respectively. ...
... [6] In the MSO process, molten salts provide excellent heat transfer characteristics, including large heat capacities and the relatively high effective thermal conductivity, which is capable of storing a great amount of heat over a long period of time to ensure temperature uniformity and can also allow for rapid transfer of heat to the feedstock. [7][8][9] Besides, molten salts as reaction media for pyrolysis or oxidation could provide a gas-liquid interface between two reactants, replacing original gas-solid. This gasliquid interface could allow two reactants contact each other effectively, which enhances the rate of chemical reactions. ...
The molten‐salt oxidation method (MSO) can be applied for disposal of spent cationic exchange resins (CERs) after the treatment of nuclear industry wastewater. In this work, the oxidation decomposition of resins in carbonate molten salt in N2 and air atmospheres was investigated. The SEM morphology and FTIR spectrograms indicated that the addition of air obviously prompted the oxidation decomposition of the benzene ring, S−O bond and S−C bond in residues and the decomposition efficiency of resins reached 98.69 % at 800 °C. The XPS analysis showed the conversion of sulfur species in residues. The peroxide and superoxide ions in carbonate molten salt prompted the decomposition of thiophene sulfur and resulted in the formation of sulfate. The retention rate of sulfur in spent salt was 84.36 % at 800 °C. This work provided more theoretical guidance for the treatment of resins and technical support for the sustainable development of nuclear industry.
... In addition, the content of acid gas in MSO of CERs is significantly lower than oxidation of CERs, indicating that the addition of carbonate not only promotes the destruction of the C-S structure but also promotes the adsorption of acid gas, which is an ideal treatment way for CERs (Yao et al. 2011). As an alkaline engineering fluid with the lowest melting point in carbonate systems (Nunes et al. 2019), ternary salt (Li 2 CO 3 : Na 2 CO 3 : K 2 CO 3 = 44 wt.%: 30 wt.%: 26 wt.%) is the selection of the molten oxidation medium with appropriate properties (Cui et al. 2021;Grosu et al. 2021). ...
Molten salt oxidation (MSO) is an advanced method for waste resins treatment; nevertheless, the research about gas product variations of resins under different stoichiometric air feed coefficient (α) is rare. The optimal working condition of hazardous waste disposal is obtained through thermodynamic equilibrium calculation, and the method to improve the treatment efficiency is found to guide the optimization of the actual experiment. In this paper, Fact Sage was used to calculate the oxidation products of cation exchange resins (CERs) at different temperatures and α, focusing on the similarities and differences through the contents of CO, CH4, CO2, and SO2 during the oxidation of CERs, the MSO of CERs, and the theoretical calculation. The results indicated that the gas products of the calculation and reality of the oxidation process of CERs are quite different, while the CO contents of CERs during MSO are close to the calculated values. The main reason for this consequence is that in the oxidation process of CERs, the S in the sulfonic acid group will form thermally stable C-S with the styrene–divinylbenzene skeleton. Moreover, the introduction of carbonate can promote the destruction of C-S and absorb SO2 as sulfate, weakening the influence of C-S on the oxidation products of CERs. The gas chromatograph results indicated that the SO2 content is reduced from 0.66% in the process of CERs oxidation to 0.28% in MSO of CERs. When 1.25 times stoichiometric air feed coefficient is fed, the sulfate content in the carbonate is the highest at 900 °C, which is 23.4%.
... Nanofluids are nanometer-sized particles dispersed in a base fluid such as water or oil. The added particles alter the chemical and physical properties of the base fluid, and perhaps the most interesting is the enhancement of the heat capacity and thermal conductivity [1]. This causes a significant improvement in terms of efficiency and economy when applied to energy systems. ...
The demand for efficient and sustainable energy is continuously increasing. Among the many technologies with great potential within this field are nanofluids. Nevertheless, there is still a considerable lack of information regarding their erosive effects on systems materials. In this research, the tribological behaviour of aqueous 1.33 wt% TiO2 nanofluid was investigated when jet-impinged with an average velocity of 0.8 m/s at flat targets of various materials (plastic, copper, rubber). The target surfaces were analysed using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and X-ray diffraction (XRD). It was found that impinging TiO2 nanofluid caused erosion of 282 g/( yr⋅mm2) for copper and 212 g/( yr⋅mm2) for plastic. In addition, a deposition of nanoparticles was found for rubber at rate of 2.7 kg/(yr ⋅mm2).
... For systems the utilize nanofluids, this can be achieved through enhancing their thermal efficiency, reducing their overall system size, and/or reducing the number of nanomaterials used. The reader is guided to the following sources [463][464][465][466][467][468][469][470][471] for further details on the aforementioned. ...
Nanofluids have opened the doors towards the enhancement of many of today's existing thermal applications performance. This is because these advanced working fluids exhibit exceptional thermophysical properties, and thus making them excellent candidates for replacing conventional working fluids. On the other hand, nanomaterials of carbon-based were proven throughout the literature to have the highest thermal conductivity among all other types of nanoscaled materials. Therefore, when these materials are homogeneously dispersed in a base fluid, the resulting suspension will theoretically attain orders of magnitude higher effective thermal conductivity than its counterpart. Despite this fact, there are still some challenges that are associated with these types of fluids. The main obstacle is the dispersion stability of the nanomaterials, which can lead the attractive properties of the nanofluid to degrade with time, up to the point where they lose their effectiveness. For such reason, this work has been devoted towards providing a systematic review on nanofluids of carbon-base, precisely; carbon nanotubes, graphene, and nanodiamonds, and their employment in thermal systems commonly used in the energy sectors. Firstly, this work reviews the synthesis approaches of the carbon-based feedstock. Then, it explains the different nanofluids fabrication methods. The dispersion stability is also discussed in terms of measuring techniques, enhancement methods, and its effect on the suspension thermophysical properties. The study summarizes the development in the correlations used to predict the thermophysical properties of the dispersion. Furthermore, it assesses the influence of these advanced working fluids on parabolic through solar collectors, nuclear reactor systems, and air conditioning and refrigeration systems. Lastly, the current gap in scientific knowledge is provided to set up future research directions.
... In their other work [132],a helically coiled heat exchanger was applied in the system, and a further increase of mitigated CO 2 was obtained. In the recent work by Grosu and coworkers [195], it is disclosed that the viscosity of nanofluid and nanofluid preparation methods should be targeted to reduce the negative environmental impacts of nanofluids based on molten carbonate salts exploited for high-temperature thermal energy storage application. ...
It has been more than two decades since the discovery of ‘nanofluids’ – mixtures of common liquids and solid nanoparticles with at least one dimension below 100 nm in size. While colloidal suspensions of particles (which include larger particles) have been studied for several decades, the term ‘nanofluids’ designates fluid systems that have enhanced thermal and optical properties. Although barriers to their commercial adoption remain, the field of nanofluids has continued to grow. Many studies have considered the effects of adding nanoparticles on the thermal efficiency and exergy efficiency of renewable energy systems particularly solar systems, however, few have investigated their potential for emission reductions. Critically, since renewable energy technologies aim to reduce the environmental impact of energy systems, this review focuses on whether nanofluids provide a net environmental benefit. Thus, in addition to providing a comprehensive overview of this body of literature from an environmental perspective, this review also highlights areas for future work that could help ensure that nanofluids have a net positive environmental impact in renewable energy systems going forward.
Molten carbonate salt nanofluids have become an excellent strategy to enhance the power generation efficiency of the next-generation concentrated solar power (CSP) technology due to their excellent thermophysical properties at high temperatures. The corrosion behaviour in contact with salt nanofluids is one of the main issues to address for implementing cost-effective steels as construction materials for CSP. In the present work, the effects of molten salt nanofluids with Al2O3 nanoparticles (<50 nm and 1.0 wt%) on the corrosion behaviour of AISI 301LN stainless steel have been investigated. Corrosion tests were carried out in a static molten salt mixture of Li2CO3–Na2CO3–K2CO3 at 600 °C for 1000 h. Complementary electron microscopy, microanalysis and optical characterization techniques and micromechanical tests were carried out to study the oxide scales formed after corrosion tests in salt nanofluids and base salt of reference. Results evidenced that doping nanoparticles in molten salts significantly decreased the corrosion rate by ∼50%, compared with a base salt. Incorporating nanoparticles into the oxide scale, forming a nanostructure of reticulated Al2O3 nanoparticles generated a more homogeneous and dense oxide scale, increasing its hardness about 30% and enhancing its effectiveness as a protective barrier.
Many novel molten salts and their based nanofluids have been proposed, and their thermophysical properties have been extensively studied. However, their implications to performance and economic of concentrated solar power (CSP) systems are not well understood. This study has compared the performance of CSP generation systems by applying nitrate, carbonate, chloride and their based nanofluids. Based on the selected site of the study, Delingha City, Qinghai Province of China, different heat transfer fluids (HTFs) were applied to both parabolic trough collectors (PTC) systems and solar power tower (SPT) systems upon on their maximum operating temperatures. The solar multiple (SM) and thermal energy storage full-load hours of solar salt PTC and SPT systems were determined as the benchmarks, and then the results in each CSP system were compared to the benchmarks at annual, monthly and daily time scales, respectively. The results show that by comparing against the solar salt PTC systems with other HTFs, the nitrate nanofluid PTC system has the best performance, owing to an increase of 21.68% in its annual electricity generation (AEG) and an improvement of 6.0% in capacity factor (CF); among the five SPT systems, on the other hand, the performance of carbonate nanofluid SPT system is the best, and its AEG and CF are improved by 24.47% and 13.15% respectively while comparing with the solar salt SPT system. This work lends a fresh perspective to design and evaluate new CSP systems applying advanced molten salts and their based nanofluids.
Phase change materials ( P C M s ) used for thermal energy storage ( T E S ) have shown to be particularly promising, especially in light of the growing need for a wide variety of energy-related applications. This article discusses the usage of packed beds made from phase change materials in solar energy storage devices for variable temperature environments. The primary heating/cooling technologies and associated thermal energy storage difficulties have also been examined from the perspective of engineering applications. The packed bed’s history from a thermodynamic perspective, the temperature dispersion inside the packed bed, and several design factors that influence its performance are all covered in the brief description of the packed bed. Energy and exergy efficacy are the foundations of its analysis. The article delves into numerous thermal energy storage methods, with phase change materials being the primary topic and a beneficial means of storing thermal energy. In addition, phase change materials also keep latent thermal energy during phase transitions at almost constant temperatures.
Thermal energy storage systems are extensively investigated because of their fundamental role in the storage of renewable energy and in the recovery of useful heat generated from various systems. The three mechanisms of thermal energy storage are discussed herein: sensible heat storage (QS,stor), latent heat storage (QL,stor), and sorption heat storage (QSP,stor). Various materials were evaluated in the literature for their potential as heat storage mediums in thermal storage systems. The evaluation criteria include their heat storage capacity, thermal conductivity, and cyclic stability for long-term usage. This work offers a comprehensive review of the recent advances in materials employed for thermal energy storage. It presents the various materials that have been synthesized in recent years to optimize the thermal performance of QS,stor, QL,stor, and QSP,stor systems, along with the challenges associated with thermal energy storage materials. The paper concludes that latent heat storage systems via the use of inorganic phase change materials (PCMs) would be ideal for high-temperature applications.
The world demand for clean energy has increased significantly due to global population growth and economic expansion and significant investments have been recently conducted to rise electrical power generation capacity from renewable sources. Concentrating solar power (CSP) plants offer dispatchable power by integrating thermal energy storage (TES) and their costs have been reducing significantly in the last years. There are currently four CSP technologies: Linear Fresnel reflectors, parabolic trough collectors, central tower receiver, and parabolic dish systems. Nowadays, molten salts are used both as the primary heat transfer fluid (HTF) and as TES medium, in order to increase the maximum operating temperatures and the CSP plant efficiency. The contribution of the present paper is to summarize the current status of the existing 26 operational or under development CSP plants using central towers with incorporated molten salt TES. Furthermore, the global trends regarding this currently promising CSP installation configuration are highlighted.
The purpose of this work is to characterize HITEC with various nanoparticles and assess the impact of varying nanoparticle concentrations on the thermophysical properties of the formulated nano-enhanced HITEC molten salt (NEHMS) samples. It also seeks to examine the long-term thermal cycle stability of these properties as well as to assess the impact of the improved thermophysical properties of NEHMS on electricity price and the efficiency of the CSP plant.
The research in the field of the nanofluids has experienced noticeable advances since its discovery two decades ago. These thermal fluids having minimal quantities of nano-scaled solid particles in suspension have great potential for thermal management purposes because of their superior thermophysical properties. The conventional water-based nanofluids have been extensively investigated so far with emphasis in their improved thermal conductivity. A novel class of nanofluids based on inorganic salts has been developed in the last few years with the goal of storing and transferring thermal energy under high temperatures. These molten salt-based nanofluids can in general be recognized by an enhanced specific heat due to the inclusion of the nanoparticles. However, it should be emphasized that this does not always happen since this thermophysical property depends on so many factors, including the nature of the molten salts, different preparation methods, and formation of the compressed layer and secondary nanostructures, among others, which will be thoroughly discussed in this work. This peculiar performance has caused a widespread open debate within the research community, which is currently trying to deal with the inconsistent and controversial findings, as well as attempting to overcome the lack of accurate theories and prediction models for the nanofluids in general. This review intends to present an extensive survey of the published scientific articles on the molten salt nanofluids. Other important realities concerning the development and thermal behavior of the molten salt nanofluids, such as the stability over time of the nanoparticles dispersed in the molten salts, latent heat, viscosity, and thermal conductivity, will be reviewed in the current work. Additionally, special focus will be given to concentrated solar power technology applications. Finally, the limitations and prospects of the molten salts nanofluids will be addressed and the main concluding remarks will be listed.
Nanofluids are an emerging class of heat transfer fluids that are engineered by dispersing nanoparticles in conventional fluids. They represent a promising, multidisciplinary field that has evolved over the past two decades to provide enhanced thermal features, as well as manifold applications in thermal management, energy, transportation, MEMs and biomedical fields.
Fundamentals and Transport Properties of Nanofluids addresses a broad range of fundamental and applied research on nanofluids, from their preparation, stability, and thermal and rheological properties to performance characterization and advanced applications. It covers combined theoretical, experimental and numerical research to elucidate underlying mechanisms of thermal transport in nanofluids.
Edited and contributed to by leading academics in thermofluids and allied fields, this book is a must have for those working in chemical, materials and mechanical engineering, nanoscience, soft matter physics and chemistry.
Molten salts are important heat storage and heat transfer media in solar thermal power generation systems based on concentrating solar power (CSP) technology. In this study, ternary carbonate (Li2CO3: Na2CO3: K2CO3 with 31:34:35 mass ratio) nanofluids with ZnO nanoparticles were prepared and characterized, and their thermophysical and corrosion properties were investigated. The prepared ternary carbonate salt-based ZnO nanofluids exhibited good thermal stability, enhanced heat capacity, and reduced corrosivity. The average solid-state specific heat capacity of ternary carbonate nanofluids with 1.0 mass% of 30-nm ZnO increased by up to 16.56% and the average liquid specific heat increased by up to 21.61% compared to those of the base salt. The latent heat of the ternary carbonate salt with ZnO nanoparticles was reduced compared to the base salt. Meanwhile, the maximum corrosion mass gain and corrosion rate of austenitic stainless steel in nanofluids were 1.72 mg cm−2 and 9.6 μm y−1 which decreased by 27.4% and 25.8% compared to the base salt, respectively. This work offers a good reference for design of the molten salt-based nanofluids with superior thermophysical properties and low corrosivity in CSP applications.
Solar salt is the most commonly used medium for thermal storage and transfer in concentrated solar power (CSP) plants, and the utilization efficiency of solar energy is depended on the thermal properties of solar salt. In this work, atomic-layer-thick hexagonal boron nitride nanosheet (BNNSs, so called white graphene) were obtained and dispersed into solar salt uniformly to produce composites nanofluids. Attributed to the ultra-high thermal conductivity and a large specific surface area of BNNSs, which reduce the thermal resistance, and facilitate forming of more semi-solid boundary layer and nucleation sites, the as-prepared composites nanofluids show superior thermal properties. The properties enhancement mechanism and effects of the mass fraction of BNNSs on the thermal properties of nanofluids was investigated carefully. Compared with the pure solar salt, the solid-phase thermal conductivity, solid-phase and liquid-phase specific heat capacity was elevated by 76.79%, 29.8% and 12.82%, respectively. At the same time, the supercooling degree was significantly decreased from 12.2 °C to 4.7 °C. The enhanced thermal conductivity and specific heat capacity offer significant cost savings, and the reduced supercooling degree prevent phase separation and pipe blockage. Thus, the developed composites nanofluids can be used as a superior medium for CSP plants.
Molten salt nanofluids have become a hot research topic due to its excellent thermal properties and heat transfer performance. However, there is a few research on the effect of nanofluids on the corrosion of metal materials. In order to know the influence of molten salt nanofluids on the corrosion of stainless steel, a low melting point quaternary nitrate was used as the base salt to analyze the corrosion law of nanofluids. By means of weight loss method, two different stainless steels (304, 316L) were put into the molten salt nanofluids with SiO2 nanoparticles content of 1 wt% to test the corrosion behavior of stainless steel. The results of 500 h corrosion test revealed that the corrosion rate of mixed molten salt on 304 and 316L stainless steel is reduced by the addition of SiO2 nanoparticles. The corrosion rates of molten salt nanofluids on 304 and 316L stainless steel were 0.0433 mm/y and 0.0233 mm/y, respectively. The surface morphology analysis by SEM, EDS, XRD and XPS test showed that the reticular silica nanostructures corrosion layer was formed on the surface of stainless steel, such structure can inhibit the corrosion of stainless steel. Therefore, molten salt nanofluids have the good compatibility with metal materials, and adding nanoparticles in molten salt can reduce the corrosion on stainless steel.
Nanoparticle-enhanced molten salt phase change materials have been used extensively for thermal energy storage. However, the role of nanoparticles in enhancing the specific heat capacity remains elusive with contradicting reports. In this work, to clarify the mechanism behind the specific heat enhancement in nanoparticle (SiO2) embedded nitrate (NaNO3–KNO3) and carbonate (Li2CO3–K2CO3) molten salts. Due to the surface charges on nanoparticles, an orderly layered distribution of molten salt ions near the surface is observed in molecular dynamics simulations, which lead to higher electrostatic potential energies near the nanoparticle surface. This makes the ionic layers with the same charge type easier to expand when temperature rises, leading to a larger local thermal expansion coefficient. Since the thermal expansion coefficient is positively correlated with the specific heat based on the phonon interpolation, the increased local thermal expansion coefficient is the main reason for the enhancement in the specific heat. The enhancement in the specific heat and the enrichment of charged functional groups on the nanoparticle surface are also confirmed experimentally. The specific heat can be significantly enhanced, by up to 19% here with 1 wt% nanoparticle, after we tailor the surface chemistry of nanoparticles, in particular, by coating active functional groups such as hydroxyl groups on the surface. This study clarifies the fundamental role of surface changes on nanoparticles in enhancing the specific heat of molten salts, and suggests that nanoparticles can have higher surface charge densities, if protonated in an acidic environment, can lead to larger specific heat enhancement in molten salts.
Amongst various alternative energy storage and energy-producing technologies that have been developed and introduced in the past years, advanced heat transfer technologies are constantly growing popular. The efficiency of these systems is exclusively determined by the heat transfer fluid and its chemical and thermophysical properties. The application frequency of various mixtures of inorganic salts, which offer stability in a greater temperature range than organic compounds, is increasing over time. The most important properties such as the specific heat capacity, along with the thermal conductivity, viscosity, or the melting point can be significantly influenced by a well-designed addition of nanomaterials to the base fluid, leading to a formation of a multi-phase composite system often called nanofluid. Apart from the various energy-storage technologies, preparation techniques, and theoretical fundamentals, this review is aimed at a clear summarization of the up to date described molten salt-based composites with enhanced thermophysical properties, including the most important and often overlooked influencing factors such as the input materials, preparation techniques, and measurement conditions.
Molten salts play a key role in the heat transfer and thermal energy storage processes of concentrated solar power plants. A novel composite material was prepared in this work by adding micron-sized magnesium particles into Li2CO3-Na2CO3-K2CO3 molten salt, the heat transfer and thermal energy storage properties of the composites were studied experimentally. A stable composite nanofluid can be obtained, and a thermal conductivity of 0.728 W/(m·K) at 973 K with an enhancement of 31% is achieved for the Mg/molten carbonate nanofluid. And the strengthening mechanism of thermal conductivity was revealed by using ab-initio molecular dynamics method. It is found that the main bonding interactions exist between Mg and O atoms at the surface of Mg particles. A compressed ion layer with a more compact and ordered ionic structure is formed around Mg particles, and the Brownian motions of Mg particles lead to the micro-convections of carbonate ions around them. These factors are helpful to the enhancement of thermal conduction with the improved probability and frequency of ion collisions. This work can provide a guidance for further studies and applications on metal/molten salt composites with enhanced heat transfer and thermal energy storage capacity.
Nanofluids is a new type of efficient cooling mass. In the present experimental work, a new SiO2-ethylene-glycol/deionized water nanofluids with excellent static storage and dynamic stability was prepared. The static storage and high temperature dynamic stability of the nanofluids was investigated experimentally (particle size measurements, TEM photographs, Zeta Potential experiments). The corrosion resistance of nanofluids was investigated by means of aluminium pump cavitation experiments. The effects of concentration, temperature and Reynolds number on the thermophysical properties and heat transfer efficiency of nanofluids were studied. Based on the thermophysical parameters (thermal conductivity and viscosity) of nanofluids at different temperatures, the experimental correlations were fitted. The results show that the stability period of the nanofluids is more than 36 months. The convective heat transfer coefficient can be increased by more than 36.1% with the addition of 6 wt% nanofluids at 25 °C. The nanofluids provides improved heat transfer and good corrosion protection and impact resistance.
The research and development of new thermal energy storage materials with high working temperatures are key topics to increase the efficiency of thermal energy to electricity conversion. The use of molten salt combinations with a wide range of operating temperatures is one of the ways to fulfil this purpose, and among them, molten carbonates present several advantages, such as high thermal stability, moderate cost, and less corrosiveness, compared to other molten salt mixtures. The present work contains a state-of-the-art review of the most important thermophysical properties for the thermal energy storage capacity of binary mixtures of potassium and lithium carbonates (K2CO3–Li2CO3). The available literature on the properties that play a key role in the heat transfer rate (viscosity and thermal conductivity) and volumetric storage capacity (melting point, density, latent heat of fusion and specific heat) is reviewed and presented. This includes the works that deal with nanofluids based on these binary mixtures of molten carbonates by analysing the influence of nanoparticles on thermophysical properties. Special attention is paid to specific heat as abnormal increases are registered in molten salts when introducing nanoparticles. Although future research is necessary about the thermophysical properties enhancement of these materials, the advanced capacities they offer for high-temperature thermal energy storage are promising, and this work aims to compile the available data on them until the present day.
As high-temperature heat transfer medium and thermal energy storage material, molten salts are widely applied in concentrating solar power (CSP) plants and molten salt reactors (MSR). The molten salt corrosion is a crucial influence factor for the system safety operation. Different influence factors such as temperature, impurity, alloy composition, gas atmosphere, nanofluids and flow state on molten salt corrosion are comparatively summarized in this work. Meanwhile, the corrosion mechanisms of nitrate, carbonate, chloride and fluoride salts are emphatically reviewed. Furthermore, the anti-corrosion strategies of metallic materials to molten salt are presented. Finally, the future research directions of molten salt corrosion characteristics are proposed. This work could provide practical guidance for the application selection of metal materials in the CSP and MSR system.
With renewables as the main candidate for future energy sources, efficient energy storage is considered a key to unlock its full potential. In this context, thermal energy storage (TES), particularly molten salt-based TES, is considered as an efficient, simple and a low-cost option. One of the main drawbacks for this technology is the corrosion of vessels storing the salt at elevated temperatures. Recently, the addition of nanoparticles has been reported to be able to mitigate the corrosivity of molten salts by several independent groups. However, no convincing explanation has been established so far as to why such behaviour takes place. In this work, we attempt to give a new insight into anticorrosion performance of molten salts dopped with nanoparticles (molten salts nanofluids) both experimentally and through Molecular Dynamic simulations. XPS-depth profiling, SEM-EDX and XRD experiments together with macroscopic and atomistic modelling were used in this work and the results suggest that the diffusion of nanoparticles into construction materials at high temperatures is one of the corrosion mitigation mechanisms of molten salts nanofluids. This new finding allows the explanation of some of the reported corrosion results and hence paves the way for a new anticorrosion strategy for molten salts and other high-temperature applications.
The eutectic salt-based nanofluids have recently been explored as a Thermal Energy Storage (TES) material for high-temperature applications due to their improved thermal properties. However, the investigation of such fluids in corrosion, rheology, and thermal ageing is minimal. This study focused on the effect of newly developed hybrid nanofluids based on GO/TiO2 nanoparticles and the eutectic salt mixture on thermophysical properties for high-temperature applications. The results showed that the SS316 immersed in hybrid nanofluid formed a protective layer to prevent corrosion compared to the eutectic salt mixture after 7 days. The surface morphologies and cross-sections were studied using SEM (Scanning Electron Microscope) and EDX (Energy-dispersive X-ray spectroscopy). The addition of hybrid nanoparticles into the eutectic salt mixture exhibits minimal effect on the viscosity that varies about -0.51-11.05% from 250-400 °C range at a shear rate of 250/s. Moreover, the rheological study displayed a strong correlation (R² of 0.99) with the experimental data. Comparative studies showed that the viscosity values of both eutectic salt mixture and hybrid nanofluid showed good agreement with the literature. Finally, the thermal ageing behaviour of eutectic salt mixture and hybrid nanofluids was investigated for 15 days, and the samples were collected on the 1st, 7th, and 15th day. The average specific heat capacity of hybrid nanofluids and eutectic salt was reduced by 8.76% and 28.94%, respectively, after 15 days, indicating poor thermal stability of eutectic salt compare to hybrid nanofluids.
The microstructure and behaviors of the interface layer around the nanoparticle are strongly related to the overall thermophysical properties of molten salt‐based nanofluids (MSBNFs). Understanding this link may enable the advanced heat transfer fluid (HTF) for concentrated solar power and other applications. In this study, a molecular dynamics (MD) model for solar salt (60 wt% NaNO3/40 wt% KNO3)‐SiO2 nanofluid system is developed to estimate the characteristics of the interface layer and their effects on the overall specific heat capacity (SHC) and effective thermal conductivity (ETC) of MSBNFs. The results show that the SHC and thermal conductivity (TC) of the interface layer in MSBNFs are, respectively, 1.44‐1.85 and 1.05‐1.97 times higher than those of pure solar salt. The SHC of the interface layer has a significant effect on the overall SHC of the MSBNFs, but the interface layer is not the dominant influencing factor for the ETC enhancement of MSBNFs and thus has less effect on the overall ETC. Molecular dynamic simulation was conducted on the structures and thermophysical properties of interface layer in solar salt‐SiO2 nanofluids. The specific heat capacity and thermal conductivity of interface layer are respectively, 1.44‐1.85 and 1.05‐1.97 times higher than those of base liquid. The influencing mechanisms of the interface layer in the over specific heat capacity and effective thermal conductivity of solar salt nanofluids were identified.
In order to enhance the hydraulic efficiency of a liquid molten salt pump, the improvement on the pump was carried out through numerical and experimental methods. The internal flow field obtained by the numerical simulation was analysed. The results show that there are low-velocity area in the scroll region and large curvature of the streamline at the outlet. Geometric modification was made by trimming the back-blades of the impeller and filleting the sharp corner of the outlet pipe. The modified pump performance was verified by the experiments. The hydraulic efficiency, the pressure fluctuation, vibration characteristics between the original and modified pump were compared. The results showed that the hydraulic efficiency of the modified pump increased 7.4%. In addition, the pressure fluctuation and vibration intensity were also reduced compared with the original pump. This result shows that the geometric modification improves not only the hydraulic performance but also the structural properties.
Proper recording of thermophysical properties for molten salts (MSs) and molten salts based nanofluids (MSBNs) is of paramount importance for the thermal energy storage (TES) technology at concentrated solar power (CSP) plants. However, it is recognized by scientific and industrial communities to be non-trivial, because of molten salts creeping (scaling) inside a measuring crucible or a sample container. Here two strategies are proposed to solve the creeping problem of MSs and MSBNs for the benefit of such techniques as differential scanning calorimetry (DSC) and laser flash apparatus (LFA). The first strategy is the use of crucibles with rough inner surface. It was found that only nanoscale roughness solves the creeping problem, while micron-scale roughness does not affect the wetting phenomena considerably. The second strategy is the use of crucible made of or coated with a low-surface energy material. Both strategies resulted in contact angle of molten salt higher than 90° and as a result, repeatable measurements in correspondence to the literature data. The proposed methods can be used for other characterization techniques where the creeping of molten salts brings the uncertainty or/and unrepeatability of the measurements.
Nanoparticles have been used in thermal applications to increase the specific heat of the molten salts used in Concentrated Solar Power plants for thermal energy storage. Although several mechanisms for abnormal enhancement have been proposed, they are still being investigated and more research is necessary. However, this nanoparticle-salt interaction can also be found in chemical applications in which nanoparticles have proved suitable to be used as an adsorbent for nitrate removal given their high specific surface, reactivity and ionic exchange capacity. In this work, the ionic exchange capacity mechanism for the nanoparticles functionalization phenomenon was evaluated. The ionic exchange capacity of silica and alumina nanoparticles dispersed in lithium, sodium and potassium nitrates was measured. Fourier-transform infrared spectroscopy tests confirmed the adsorption of nitrate ions on the nanoparticle surface. A relationship between the ionic exchange capacity of nanoparticles and the specific heat enhancement of doped molten salts was proposed for the first time.
A novel ternary eutectic salt mixture for high-temperature sensible heat storage, composed of sodium chloride, potassium chloride and magnesium chloride (NaKMg–Cl) was developed based on a phase diagram generated with FactSage®. The differential scanning calorimetry (DSC) technique was used to experimentally validate the predicted melting point of the ternary eutectic composition, which was measured as 387 °C, in good agreement with the prediction. The ternary eutectic was compared to two binary salts formulated based on prediction of the eutectic composition by FactSage, but unfortunately DSC measurements showed that neither binary salt composition was eutectic. Nonetheless, the measured thermo-physical properties of the ternary and the two binary mixtures are compared. Liquid heat capacities of both the ternary and binary salts were determined by using DSC with sapphire as the standard reference. The average heat capacity of the ternary mixture was recorded as 1.18 J g−1 K−1. The mass loss of the molten eutectic salts was studied up to 1000 °C using a thermogravimetric analyser in nitrogen, argon and air. The results showed a significant mass loss due to vaporisation in an open system, particularly above 700 °C. However, simulation of mass loss in a closed system with an inert cover gas indicates storage temperatures above 700 °C may be feasible, and highlights the importance of the design of the storage tank system. In terms of storage material cost, the NaKMg-Cl mixture is approximately 4.5 USD/kWh, which is 60% cheaper than current state-of-the-art nitrate salt mixtures.
Molten salts are used as heat transfer fluids and for short-term heat energy storage in solar power plants. Experiments show that the specific heat capacity of the base salt may be significantly enhanced by adding small amounts of certain nanoparticles. This effect, which is technically interesting and economically important, is not yet understood. This paper presents a critical discussion of the existing attendant experimental literature and the phenomenological models put forward thus far. A common assumption, the existence of nanolayers surrounding the nanoparticles, which are thought to be the source of, in some cases, the large increase of a nanofluid’s specific heat capacity is criticized and a different model is proposed. The model assumes that the influence of the nanoparticles in the surrounding liquid is of long range. The attendant long-range interfacial layers may interact with each other upon increase of nanoparticle concentration. This can explain the specific heat maximum observed by different groups, for which no other theoretical explanation appears to exist.
In this study different nanofluids with phase change behavior were developed by mixing a molten salt base fluid (KNO3 selected as phase change material) with nanoparticles using the direct synthesis method. The thermal properties of the nanofluids obtained were investigated. Following the improvement in the specific heat achieved, these nanofluids can be used in concentrating solar plants with a reduction of storage material. The nanoparticles used (1.0 wt.%) were silica (SiO2), alumina (Al2O3), and a mix of silica-alumina (SiO2-Al2O3) with an average diameter of 7, 13, and 2-200 nm respectively. Each nanofluid was prepared in water solution, sonicated, and evaporated. Measurements of the thermophysical properties were performed by DSC analysis, and the dispersion of the nanoparticles was analyzed by SEM microscopy. The results obtained show that the addition of 1.0 wt.% of nanoparticles to the base salt increases the specific heat of about 5-10 % in solid phase and of 6 % in liquid phase. In particular, this research shows that the addition of silica nanoparticles has significant potential for enhancing the thermal storage characteristics of KNO3. The phase-change temperature of potassium nitrate was lowered up to 3 °C, and the latent heat was increased to 12 % with the addition of silica nanoparticles. These results deviated from the predictions of theoretical simple mixing model used. The stored heat as a function of temperature was evaluated for the base salt, and the nanofluids and the maximum values obtained were 229, 234, 242, and 266 J/g respectively. The maximum total gain (16 %) due to the introduction of the nanoparticles (calculated as the ratio between the total stored heat of the nanofluids and the base salt in the range of temperatures 260-390 °C) was also recorded with the introduction of silica. SEM and EDX analysis showed the presence of aggregates in all nanofluids: with silica nanoparticles they were homogenously present while with alumina and silica-alumina also zones with pure salt could be detected.
The viscosities of unary, binary, and ternary eutectic alkali metal carbonate molten salts were measured for the first time by a rotation method using a high-temperature rheometer system and the reliability of the viscosity values was evaluated. The viscosity values obtained in this study are similar to that reported by Sato’s research group, which is considered to have the highest reliability among the studies reported till date. The standard errors in the viscosity values at each temperature are less than ±5%.
GEMASOLAR is Torresol Energy first project to use central tower technology and molten salt system. The plant incorporates significant technological innovation, including the 120 MW th solar receiver, and also a molten salt thermal storage system, able to reach temperature up to 565ºC (1050ºF). GEMASOLAR is the first commercial plant to apply this type of technology in the world and is therefore of considerable importance in the field of renewable energies as it opens the path to a new thermosolar power generation technology which could be the best alternative to the parabolic trough commercial thermosolar power plants currently being built. The molten salt thermal storage system deployed at this project helps avoid fluctuations in power supply through a system that is capable of 15 hours of electricity production without sunlight. After commissioning in May 2011, the plant was finally ready to operate at full-blast and achieve its 24 hours of uninterrupted electricity production in late June.
In this paper the authors propose a framework for combining life cycle assessment (LCA) and Risk Assessment
(RA) to support the sustainability assessment of emerging technologies. This proposal includes four steps of
analysis: technological system definition; data collection; risk evaluation and impacts quantification; results
interpretation. This scheme has been applied to a case study of nanofluid alumina production in two different
pilot lines, “single-stage” and “two-stage”. The study has been developed in the NanoHex project (enhanced
nano-fluid heat exchange). Goals of the study were analyzing the hotspots and highlighting possible trade-off
between the results of LCA, which identifies the processes having the best environmental performance, and
the results of RA, which identifies the scenarios having the highest risk for workers. Indeed, due to lack of data
about exposure limits, exposure–dose relationships and toxicity of alumina nanopowders (NPs) and nanofluids
(NF), the workplace exposure has been evaluated by means of qualitative risk assessment, using Stoffenmanager
Nano. Though having different aims, LCA and RA have a complementary role in the description of impacts of
products/substances/technologies. Their combined use can overcome limits of each of them and allows a
wider vision of the problems to better support the decision making process.
The investigation experimentally studies the optimal concentration of alumina nanoparticles in doped molten Hitec that maximizes its specific heat capacity. A simplified model of the interfacial area is developed to explain the optimal concentration. The specific heat capacities of pure Hitec and nano-Hitec fluid are measured using a differential scanning calorimeter (DSC), and the microstructures following solidification are observed using a scanning electron microscope (SEM). A novel sampling apparatus and process for preparing molten Hitec nanofluids were developed to prevent the precipitation of nanoparticles. An optimal concentration of 0.063 wt.% is identified as yielding the greatest enhancement of specific heat capacity of 19.9%. At a concentration of 2 wt.%, the detrimental effect of the dopant nanoparticles on the specific heat capacity is evident at all temperatures. The negative effect is more significant than that predicted by the thermal equilibrium model. The SEM images following the solidification of samples and the developed model reveal the uniform dispersion of nanoparticles with negligible agglomeration at concentrations of under 0.016 wt.%. The agglomeration becomes significant and the particle clusters seem to be inter-connected at high concentrations. Moreover, the optimal concentration is approximately the concentration at which the contributions of isolated particles and clusters of sizes from 0.2 to 0.6 μm in the interfacial area to the specific heat capacity are equal.
In this study, different nanofluids with phase change behavior were developed by mixing a molten salt base fluid (selected as phase change material) with nanoparticles using the direct-synthesis method. The thermal properties of the nanofluids obtained were investigated. These nanofluids can be used in concentrating solar plants with a reduction of storage material if an improvement in the specific heat is achieved. The base salt mixture was a NaNO3-KNO3 (60:40 ratio) binary salt. The nanoparticles used were silica (SiO2), alumina (Al2O3), titania (TiO2), and a mix of silica-alumina (SiO2-Al2O3). Three weight fractions were evaluated: 0.5, 1.0, and 1.5 wt.%. Each nanofluid was prepared in water solution, sonicated, and evaporated. Measurements on thermophysical properties were performed by differential scanning calorimetry analysis and the dispersion of the nanoparticles was analyzed by scanning electron microscopy (SEM). The results obtained show that the addition of 1.0 wt.% of nanoparticles to the base salt increases the specific heat of 15% to 57% in the solid phase and of 1% to 22% in the liquid phase. In particular, this research shows that the addition of silica-alumina nanoparticles has a significant potential for enhancing the thermal storage characteristics of the NaNO3-KNO3 binary salt. These results deviated from the predictions of the theoretical model used. SEM suggests a greater interaction between these nanoparticles and the salt.
In this letter, we have presented an experimental investigation of the specific heat cp of water-based Al2O3 nanofluid with a differential scanning calorimeter. The result indicates that the specific heat cp of nanofluid decreases gradually as the nanoparticle volume fraction phi increases from 0.0% to 21.7%. The relationship between them exhibits good agreement with the prediction from the thermal equilibrium model [Eq. (2)]. The other simple mixing model [Eq. (1)] fails to predict the specific heat cp of nanofluid.
Molecular dynamics simulations were carried out to investigate the diffusion-rebonding of nanoscale α-Al2O3 on the Bcc-Fe surface at high-temperature. The results indicated that the mutual diffusion occurred between Al2O3 and Fe in the temperature range of 498-1098 K. The diffusion modes of Fe and Al2O3 are confirmed as interstitial diffusion and surface diffusion, respectively. An amorphous layer was gradually formed at the interface due to diffusion and rebonding at the interface. The thickness of the amorphous layer extended with the temperature increasing. More importantly, because the Fe ions occupied the space of the Al2O3 by the strong diffusion effect induced by high temperature, the different Fe ions rebonded with Al and O to form the new coordination complexes such as AlnFem (AlFe, AlFe3, and Al6Fe) and FeO. The formation of these coordination complexes induced the phonons transporting in the scattering mode at the interface and resulted in the reduction of the mean free path of the phonon. Consequently, the thermal resistance of the interface remarkable increased, which deteriorated the thermal transport performance of the material.
The potential use of molten salt-based nanofluids as thermal energy storage material in Concentrated Solar Power plants has gained attention over the last years due to their enhanced storage capacity. The possible effects of the salt-based nanofluid production at industrial scale have not been yet investigated, as this could influence the nanoparticles agglomeration and therefore their thermal and flow properties. Four methods were evaluated for the production of solar salt-based nanofluids containing 1 wt% of silica nanoparticles. The particle size distribution, the stability, the rheological behaviour and the specific heat of the samples were measured. Nanofluids prepared by means of a dry mixing method presented the lowest viscosity, trimodal particle size distribution and lack of stability. The commonly used dissolution method coupled with oven drying in a petri dish as well as the ball milling method presented non-Newtonian behaviour and intermediate values of particle size and stability. The new spray drying method proposed provided a monomodal particle size distribution with high stability but the highest viscosity and shear thickening behaviour. Results suggest that the four methods evaluated are appropriate for specific heat enhancement (up to 21.1%) but a commitment between stability and viscosity has to be achieved.
High temperature corrosion of molten salt containment materials is of great interest for thermal energy storage systems used with concentrating solar power. Mitigating this corrosion is critical for the design, life cycle and economics of these systems and requires understanding the mechanisms which drive corrosion. In molten salts these mechanisms are complex, and heavily influenced by factors such as impurities, atmosphere, temperature and metal composition. This review aims to illustrate the mechanisms of molten salt corrosion in thermal energy storage systems and the primary factors which affect them. As these factors are so important for corrosion mechanisms, much of the published corrosion rate data will be not be applicable to many thermal energy storage systems. This means that controlling these conditions and corrosion testing will be an indispensable part of developing cost-effective thermal energy storage systems.
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.
Efficient energy storage is a bottleneck for nearly every renewable energy technology. Thermal energy storage (TES) is widely considered as a relatively simple and reliable method, particularly for concentrated solar power (CSP) plants. Currently, considerable scientific effort is focused on the development of new molten salt-based nanofluids as storage materials with enhanced thermophysical properties and lower cost for TES purpose. However, an understanding of the effect of nanoparticles on the corrosivity of such nanofluids is practically absent. In the present work, using nanofluids based on eutectic mixture of NaNO3-KNO3 we demonstrate that nanoparticles doping has complex effects on the corrosion rates of carbon steel. In particular, if the negative effect of microbubbles of air trapped between the nanoparticles is not predominant, one can obtain reduced corrosion rates due to the incorporation of the nanoparticles into the oxidation layer. The obtained results are important both for expanding the very limited knowledge on the corrosion aspects of molten salts-based nanofluids, as well as for comprehensive evaluation of the feasibility of such nanofluids for TES applications.
Thermal energy storage (TES) is an efficient solution for improving the dispatchability of Concentrated Solar Power (CSP) plants. A system, consisting of two tanks with Solar Salt (NaNO3 60% wt. and KNO3 40% wt.) is commonly used. However, the investment cost of this technology is very high, due to the huge amount of salts required (thousands of tons). A pronounced interest is evident for improving the thermophysical properties of molten salts by adding small amounts of nanoparticles in order to reduce the mass of molten salts at CSP. At the moment, the effect of nanoparticle addition on corrosion of container materials is poorly explored. In particular, there are no works regarding the dynamic effect of nanoparticles on the corrosivity of molten salts. In this work we present first ever dynamic corrosion tests for Solar salt doped with alumina nanoparticles (1% wt.). Carbon Steel A516 and SS347, used in double-tank system, were tested. Corrosion rates were 94.8 μm yr⁻¹ and negligible respectively (1000 h, 385 °C). Detailed examination of construction materials revealed incorporation of nanoparticles into the corrosion layer and considerably lower corrosion rate as compared to the previously reported work on the nanoparticles-free Solar salt.
Molten salt mixtures with alkali and alkaline earth metal chlorides were developed for high-temperature sensible thermal energy storage in support of concurrent efforts to develop high-temperature advanced power cycles for concentrating solar power applications. Four ternary chloride mixtures with different cation combinations (Na, K, Li, Mg) were designed using the FactSage® software, and for three of these, the eutectic point was experimentally validated by differential scanning calorimetry. Specific heat capacity measurements were conducted following the ASTM E1269 standard, and were measured between 1.18 J/g/K and 1.31 J/g/K. The mass loss of the molten chloride salts was studied under three different gas blankets of nitrogen, argon and air by thermogravimetric analysis. All the selected salt mixtures were stable up to 700 °C, although weight loss due to vaporisation becomes significant around this temperature due to the high vapour pressure of the chloride salt mixtures. However, it is expected that operation at a temperature up to around 750 °C will be feasible in a closed system with an inert environment. Additionally, removal of chemically-bonded water and salt purification may need to be considered for extending the operating temperature. In terms of economic performance, although the inclusion of LiCl in the ternary eutectic mixtures is advantageous for reducing melting point and increasing specific heat capacity, at current costs, these benefits are unlikely to be justified unless LiCl cost reduces by a factor of three. The NaCl-KCl-MgCl2 mixture has the lowest cost per unit energy stored, at 4.5 USD/kWh.
Recently, a number of theoretical and experimental studies have been performed to understand the effect of nanoparticles on thermal properties and heat transfer performance but there is a lack regarding their corrosion properties. In this work, an extended corrosion characterization (at central tower plant storage temperature (565 °C)) has been carried out in two different grades of solar salt (industrial and refined purity) doped with the addition of 1 wt% Al2O3 nanoparticles or 1 wt% SiO2 nanoparticles. Corrosion rates were determined in commercial stainless steel commonly used in CSP technology (347SS) by gravimetric tests, measuring the weight gain during 1000 h, identifying the corrosion products by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). The lowest corrosion rate (0.007 mm/year) was obtained in the refined solar salt with the addition of 1 wt% Al2O3 nanoparticles. A protective layer was formed in the steel-salt interphase, identified through XRD as Al2O3. Additionally, hematite (Fe2O3) and magnetite (Fe3O4) were obtained as unprotective corrosion products throughout the test carried out with or without nanoparticles. In addition, the presence of impurities on the salts generated some stable compounds, as magnesium ferrite (MgFe2O4).
Concentrating solar power coupled to thermal energy storage (TES) is a vastly growing industrial process allowing for the generation of dispatchable and green electricity. This paper focuses on direct molten salt line-focusing technology using linear Fresnel and parabolic trough collector systems. Direct molten salt technology utilizes molten salt as heat transfer fluid in solar field and TES medium. Nitrate salts can be applied since they cover a wide temperature range. As storage medium Solar Salt, a binary NaNO 3 À KNO 3 (60-40 wt%) mixture, is most commonly used but variations of this system have promising thermal properties in terms of a lower melting temperature to minimize the risk of undesired salt freezing events. These modified salts are typically ternary, ternary reciprocal or higher order systems formed by adding additional cations, anions or both. In this study five molten salt systems Solar Salt, HitecXL (CaKNa//NO3), LiNaK-Nitrate, Hitec (NaK//NO23) and CaLiNaK//NO23 are both investigated and critically reviewed. Their thermo-physical properties including phase diagrams, composition, melting ranges, melting temperature, minimum operation temperature, thermal stability, maximum operation temperature, density, heat capacity, thermal conductivity, viscosity and handling are evaluated and the most recommended values are discussed and highlighted. This review contributes to a better understanding of how the listed properties can be determined in terms of measurement conditions and provides temperature dependent data useful for future simulations of direct molten salt LF CSP plants.
In this work, the compatibility of low-Cr carbon steel A516.Gr70 and stainless steels AISI 304 and AISI 316 with NaNO3-KNO3-Ca(NO3)2 (15–43–42 wt%) molten salt known as HitecXL was evaluated with the aim of determination the most suitable constructive material for a pilot thermocline thermal energy storage (TES) system of 20 MWh, which is under development for a commercial pilot concentrated solar power (CSP) plant of 1 MWe constructed at Green Energy Park in Ben Guerir in Morocco. It has been shown that the imperfection of initial state (roughness, exposure to atmosphere, surface defects) of carbon steel and humidity of the salt, inherent conditions of real applications, play a crucial role in the ability of the material to withstand corrosion attack of the molten HitecXL salt. The corrosion rates determined for studied materials allowed to select an appropriate low cost material with the highest corrosion resistance to be used for construction of the TES tank. New insights into the corrosion process of stainless and carbon steels by the nitrate salt have been made using a combination of different techniques, such as: SEM, EDX, XPS and XRD.
Considered as promising candidates of composing eutectic chloride molten salts for high temperature heat transfer fluid (HTF) with low corrosivity to nickel-based alloys, NaCl, KCl, MgCl2, CaCl2, and ZnCl2 are formulated into proper binary and ternary mixtures to satisfy the property requirement for HTFs. Experimental and predicted thermophysical properties of 16 selected eutectic binary and ternary molten salts are provided. The molten salts can work in a temperature range from after eutectic melting to at least 800 °C with low vapor pressure and acceptable low corrosivity. Surveyed equations and correlations for thermophysical properties of salt mixtures are used to predict heat capacity, density, thermal conductivity, and viscosity. The results are compared to experimental data from the author’s tests as well as reported in literature. Evaluated equations and correlations that can better predict the transport properties of the salts are selected and recommended. Authors expect that more experimental tests will be conducted to provide data for application in CSP industry.
The thermal physical properties of Li2CO3-Na2CO3-K2CO3 eutectic molten salt were comprehensively investigated. It was found that the liquid salt can remain stable up to 658 °C (the onset temperature of decomposition) by thermal analysis, and so the investigations on its thermal physical parameters were undertaken from room temperature to 658 °C. The density was determined using a self-developed device, with an uncertainty of ±0.00712 g cm(-3). A cooling curve was obtained from the instrument, giving the liquidus temperature. For the first time, we report the obtainment of the thermal diffusivity using a laser flash method based on a special crucible design and establishment of a specific sample preparation method. Furthermore, the specific heat capacity was also obtained by use of DSC, and combined with thermal diffusivity and density, was used to calculate the thermal conductivity. We additionally built a rotating viscometer with high precision in order to determine the molten salt viscosity. All of these parameters play an important part in the energy storage and transfer calculation and safety evaluation for a system.
Enhancing the specific heat capacity of molten salts by nanoparticle dispersion has emerged as a means to reduce the capital costs of thermal storage for concentrating solar thermal power plants, enabling cheaper solar electricity. Although several studies have shown near 30% enhancement in the specific heat of nanosalts other studies have reported a decrease of similar magnitude. In order to explain discrepancies reported in the literature, this study investigates the influence of various nanoparticle morphologies and preparation methods on the specific heat of nanosalts, which has not been systematically explored. To date, the extent of initial dispersion and the dispersion stability have only been reported on an ad-hoc basis in the literature. In the present study surface chemistry and sonication energy are controlled independently during preparation. By controlling both of these factors, the change of specific heat in nanosalts, results of this study present achieving up to 18% enhancement in specific heat of nanosalts by preparing an optimal nanosalt with distinctive nanoparticles.
Considered to be the next generation of heat transfer fluids, nanofluids have been receiving a growing amount of attention in the past decade despite the controversy and inconsistencies that have been reported. Nanofluids have great potential in a wide range of fields, particularly for solar thermal applications. This paper presents a comprehensive review of the literature on the enhancements in thermophysical and rheological properties resulting from experimental works conducted on molten salt nanofluids that are used in solar thermal energy systems. It was found that an increase in specific heat of 10-30% was achieved for most nanofluids and appeared independent of particle size and to an extent mass concentration. The specific heat increase was attributed to the formation of nanostructures at the solid-liquid interface and it was also noted that the aggregation of nanoparticles has detrimental effects on the specific heat increase. Thermal conductivity was also found to increase, though less consistently, ranging from 3% to 35%. Viscosity was seen to increase with the addition of nanoparticles and is dependent on the amount of aggregation of the particles. An in-depth micro level analysis of the mechanisms behind the thermophysical property changes is presented in this paper. In addition, possible trends are discussed relating to current theorised mechanisms in an attempt to explain the behaviour of molten salt nanofluids.
A concentrating solar power (CSP) system converts sunlight into a heat source which can be used to drive a conventional power plant. Thermal energy storage (TES) improves the dispatchability of a CSP plant. Heat can be stored in either sensible, latent or thermochemical storage. Commercial deployment of CSP systems have been achieved in recent years with the two-tank sensible storage system using molten salt as the storage medium. Considerable research effort has been conducted to improve the efficiency of the CSP system and make the cost of electricity comparable to that of the conventional fossil-fuel power plant. This paper provides a comprehensive summary of CSP plants both in operation and under construction. It covers the available technologies for the receiver, thermal storage, power block and heat transfer fluid. This paper also reviews developments in high temperature TES over the past decade with a focus on sensible and latent heat storage. High temperature corrosion and economic aspects of these systems are also discussed.
Experiments were performed using a flow-loop apparatus to explore the performance of nanofluids in cooling applications. The experiments were performed using exfoliated graphite nanoparticle fibers suspended in polyalphaolefin at mass concentrations of 0.6 and 0.3%. The experimental setup consisted of a test section containing a plain offset fin cooler apparatus (gap or nongap fin), which was connected to a flow loop consisting of a gear pump, a shelf and tube heat exchanger (that was cooled or heated by a constant temperature bath chiller/heater), and a reservoir. Experiments were conducted using nanofluid and polyalphaolefin for two different fin strip layouts. Heat transfer data were obtained by parametrically varying the operating conditions (heat flux and flow rates). The heat transfer data for nanofluids were compared with the heat transfer data for neat polyalphaolefin fluid under similar conditions. The change in surface morphology of the fins was investigated using scanning electron micrography. The nanofluid properties were measured using rheometry for the viscosity, differential scanning calorimetry for the specific heat, and laser Hash apparatus for the thermal diffusivity. It was observed that the viscosity was similar to 10 times higher for nanofluids compared with polyalphaolefin and increased with temperature (in contrast, the viscosity of polyalphaolefin decreased with temperature). The specific heat of nanofluids was found to be 50% higher for nanofluids compared with polyalphaolefin and increased with temperature. The thermal diffusivity was found to be 4 times higher for nanofluids compared with polyalphaolefin and increased with temperature. It was found that, in general, the convective heat transfer was enhanced by similar to 10% using nanofluids compared with using polyalphaolefin. Scanning electron micrography measurements show that the nanofluids deposit nanoparticles on the surface, which act as enhanced heat transfer surfaces (nanofins).
How much the thermal conductivity of a liquid can be altered by dispersing a small amount of ultra-fine particles into it has been studied. Fine powders of Al2O3, SiO2 and TiO2 were used as the ultra-fine particles, and water was selected as the base liquid. Three dispersed systems were made by applying the technique of electrostatic repulsion. For the systems of water-Al2O3 and water-TiO2, effective thermal conductivities were seen to increase much more as the particle concentration was increased, but that of water-SiO2 system almost never increased. Viscosities of their dispersed systems were also measured, and the characteristics were made clear.
This study aims to investigate the specific heat capacity of a carbonate salt eutectic-based multiwalled carbon nanomaterial (or high temperature nanofluids). The specific heat capacity of the nanomaterials was measured both in solid and liquid phase using a differential scanning calorimetry (DSC). The effect of the carbon nanotube (CNT) concentrations on the specific heat capacity was examined in this study. The carbonate molten salt eutectic with a high melting point around 490°C, which consists of lithium carbonate of 62% and potassium carbonate of 38% by the molar ratio, was used as a base material. Multiwalled CNTs were dispersed in the carbonate salt eutectic. A surfactant, sodium dodecyl sulfate (SDS) was utilized to obtain homogeneous dispersion of CNT into the eutectic. Four different concentrations (0.1, 0.5, 1, and 5 wt.%) of CNT were employed to explore the specific heat capacity enhancement of the nanomaterials as the concentrations of the nanotubes varies. In result, it was observed that the specific heat capacity was enhanced by doping with the nanotubes in both solid and liquid phase. Additionally, the enhancements in the specific heat capacity were increased with increase of the CNT concentration. In order to check the uniformity of dispersion of the nanotubes in the salt, scanning electron microscopy (SEM) images were obtained for pre-DSC and post-DSC samples. Finally, the specific heat capacity results measured in present study were compared with the theoretical prediction.
