Molten Salt-based Nanofluids Research Articles

Enhancements in thermal properties of binary alkali chloride salt by Al2O3 nanoparticles for thermal energy storage

The cost-effective molten chloride salts are promising heat transfer fluids for next-generation concentrating solar plants. However, their limited specific heat capacities and thermal conductivities hinder their applications. In this paper, molten salt-based nanofluids based on a binary chloride (50 mol.% NaCl, 50 mol.% KCl) with Al2O3 nanoparticles were proposed to enhance the thermal properties. Firstly, molecular dynamics simulations were employed to investigate the thermal properties of these nanofluids, focusing on various nanoparticle volume fractions within 1100∼1600 K. The findings reveal that as the nanoparticle fraction increases from 0% to 7%, the specific heat capacity and dynamic viscosity improve by≈14.9% and 34.8%–38.6%, respectively. Concurrently, the thermal conductivity increases by 10.0%–16.5%. Then, microstructure evolution was analyzed to elucidate the enhancing mechanism of the thermal conductivity. Specifically, the negatively charged nanoparticle surface selectively attracts Na+ and K+, resulting in the formation of a compressed layer around the nanoparticle where the short-range order of ions was intensified. As a result, the compressed layer serves as a thermal conduit between the Al2O3 nanoparticle and the surrounding salt, resulting in the enhancement of the thermal conductivity. Furthermore, additional analyses of thermal diffusion properties and energy density distributions provide indirect evidence for the existence of the compressed layer where ionic motion is restricted, further validating the enhancing mechanism.

Effect of Morphological Characteristics of Aggregates on Thermal Properties of Molten Salt Nanofluids

Molten salt-based nanofluid is a thermal storage and heat transfer medium for concentrated solar thermal power plants formed by adding nanoparticles to molten salt, which can enhance the thermal performance of molten salt. However, the nanoparticles tend to aggregate in nanofluids, causing changes in thermal properties. In this work, molecular dynamics simulations were used to study the effect of morphological characteristics of aggregates on the thermal conductivity and specific heat capacity of molten salt-based nanofluids. The results show that the aggregated nanoparticles cause a greater increase in thermal conductivity and specific heat capacity than dispersed nanoparticles. Additionally, the increase in fractal dimension leads to thermal conductivity reduction, while there is no clear correlation between the fractal dimension and specific heat capacity. New insights into the thermal properties of aggregated nanofluids are provided by analyzing the contribution of material components, heat flux fluctuation modes, and energy compositions. It is found that the thermal conductivity of aggregated nanofluids is mainly dominated by the base liquid and collision term. However, the specific heat is not related to the variation in the contribution of different energy compositions. Moreover, compared to the dispersed nanofluid, the increased specific heat capacity of aggregated nanofluids is attributed to the thicker semi-solid layer. This study provides guidance for the design and control of the thermal properties of molten salt-based nanofluids.

Recent Advances in Molten Salt-Based Nanofluids as Thermal Energy Storage in Concentrated Solar Power: A Comprehensive Review.

This study critically reviews the key aspects of nanoparticles and their impact on molten salts (MSs) for thermal energy storage (TES) in concentrated solar power (CSP). It then conducts a comprehensive analysis of MS nanofluids, focusing on identifying the best combinations of salts and nanoparticles to increase the specific heat capacity (SHC) efficiently. Various methods and approaches for the synthesis of these nanofluids are explained. The article presents different experimental techniques used to characterize nanofluids, including measuring the SHC and thermal conductivity and analyzing particle dispersion. It also discusses the challenges associated with characterizing these nanofluids. The study aims to investigate the underlying mechanisms behind the observed increase in SHC in MS nanofluids. Finally, it summarizes potential areas for future research, highlighting crucial domains for further investigation and advancement.

New insights into the heat capacity enhancement of nano-SiO2 doped alkali metal chloride molten salt for thermal energy storage: A molecular dynamics study

Molten salts and their based nanofluids are important media for heat transfer and thermal energy storage in concentrated solar power (CSP) system. To improve the overall efficiency of the CSP plants, it is important to increase the specific heat capacity of molten salt to the extent possible. Nanomaterials have been doped in molten salt to enhance the specific heat capacity of molten salt; however, the mechanism of enhancement in specific heat capacity caused by doping of nanoparticles in molten salt is still unclear. In this work, the Buckingham force field parameters for alkali metal chloride molten salts (KCl, NaCl, LiCl) were first derived and carefully validated. The specific heat capacities, viscosities, and self-diffusion coefficients of pure molten salts and the specific heat capacities of molten-salt-based nanofluids with SiO2 nanoparticles were investigated by molecular dynamics (MD) simulations. The calculated thermophysical properties of the chloride molten salts using the derived Buckingham potential parameters are in good agreement with the experimental data, and the overall prediction performance is better than that of the BMH potential. Results show that the specific heat capacity of molten salt first increases and then decreases with increasing SiO2 doping ratio, reaching a maximum value at 1 wt%. The specific heat capacities of KCl, NaCl and LiCl nanofluids with 1 wt% SiO2 nanoparticle are increased by 6.2 %, 8.6 % and 19.8 % compared to the base salts, respectively. Number density, radial distribution function, coordination number curves, and energy of the nanofluid system were systematically analyzed. It is found that the increase of the specific heat capacities of the molten salt-based nanofluids can be explained by the energy and coordination number change caused by the number density mismatch of cations and anions. Interestingly, the local delamination of cations around SiO2 nanoparticle surfaces in LiCl-SiO2 nanofluids is clearly observed and visualized for the first time; it is a phenomenon well explained by the analysis of number density and contributes to the further enhancement of the heat capacity. The findings of this work could provide important fundamentals in understanding and designing more efficient molten salt nanocomposites for thermal energy storage in future studies.

An Overview of the Molten Salt Nanofluids as Thermal Energy Storage Media

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.

Experimental and numerical study on heat transfer characteristic of nitrate molten salt-based nanofluids in tube with twisted tape

Experimental and numerical study on heat transfer characteristic of nitrate molten salt-based nanofluids in tube with twisted tape

Development of a New Method for Synthesizing HITEC Salt-Based Alumina Nanofluids

This study presents a new two-step method to synthesize molten salt-based nanofluids by replacing water with butanol and using an Emax high-energy mill to ensure good stability and homogeneity. Commercial HITEC molten salt was selected as the base fluid, and alumina nanoparticles (nominal size of 5,1 nm) were used as an additive in three different proportions: 0,5, 1,0, and 1,5 wt.%. The specific heat capacity was evaluated through two different methods: differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (MDSC). According to the evaluation by MDSC, an increment of up to 4,27% in the specific heat capacity was achieved with 1,0 wt.% of alumina nanoparticles in comparison with the raw salt, without affecting the melting point and thermal stability of the salt. This behavior may be related to the good distribution of the nanoparticles in the salt. However, no significant improvement in the specific heat capacity of the nanofluid was observed when the standard DSC method was applied. This behavior may be due to the different sensitivities of the two methods to small changes in the sample, with MDSC being the more sensitive technique, as it establishes the contribution of the two phases that make up the nanofluid: the molten salt as the base fluid and the solid nanoparticles. Similarly, the heating rate used in each of the techniques can influence the sensitivity with regard to determining changes in nanofluids.

Effects of interfacial layer on thermal conductivity enhancement of solar salt-based nanofluids: Insights from molecular dynamics simulations

The interfacial nano-layers around the nanoparticles (NPs) affect the effective thermal conductivity (ETC) of molten salt-based nanofluids (MSBNFs), which brings challenge in fabricating MSBNFs with enhanced ETC. This paper presents insights into the interfacial layer and its effects on ETC of solar salt-based nanofluids through molecular dynamics simulation. The SiO 2 and Al 2 O 3 NPs with diameters of 1–2 nm and mass fractions of 0.5–2.0 wt% are considered. The results show that both the size and specie of NPs affect the thickness of the outer interfacial layer. The mass fraction has less significant effect on the thickness of interfacial layer, but is closely related to the degree of order in the interfacial layer. The ETC of MSBNFs varies with mass fraction and reaches the peak at a certain mass fraction. For MSBNFs with SiO 2 NPs and Al 2 O 3 NPs, the peak ETC occurs at mass fraction of 0.5 wt%, which increase by 3.5% and 7.0% at NP diameter of 1 nm respectively, and by 8.1% and 14.8% at NP diameter of 2 nm respectively. Detailed explanations to the physics of ETC enhancement of MSBNFs are presented in the view of interfacial layer from molecular level.

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

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

Performance analysis and particle swarm optimization of molten salt-based nanofluids in parabolic trough concentrators

Molten salts are typically used as energy storage media in concentrating solar power systems for their lower costs and environmental impact. This study aims to map and optimize the performance of parabolic trough concentrators (PTCs) working with molten salt-based nanofluids (MSNFs) as heat transfer media at high temperatures. The thermal, hydraulic, energetic, and exergetic performances were analyzed and optimized using a unique framework of Monte Carlo optical simulations, computational fluid dynamics, data-drive support vector regression, particle swarm optimization, and decision-making techniques. Three molten salts (Solar Salt, Hitec, and Hitec XL) and three nanoparticle types (Al2O3, CuO, and SiO2) were investigated in a broad range of volumetric concentrations (0.0–4.0%), operating Reynolds numbers (4 × 103 to 40 × 103), and temperatures (535–805 K). The results showed a maximum energy efficiency of 69.1%, achieved when using SiO2-Hitec nanofluid (1.0%) at a Reynolds number of 40 × 104 and temperature of 535 K. The maximum achieved exergy efficiency was 70.48%, obtained using pure Hitec at a Reynolds number of 40 × 104 and temperature of 535 K. The maximum possible enhancements in energy and exergy efficiencies in the covered range are 17.0 and 42.0%, respectively. The optimal combination of energy and exergy efficiencies are ∼73.1 and 69.0%, obtained using CuO-Hitec nanofluid at temperature, Reynolds number, and concentration of 535 K, 39912.98, and 0.019%, respectively. The optimum combination of percentage enhancements in energy and exergy efficiencies are 0.465 and 7.182%, respectively, which corresponds to CuO-Hitec nanofluid operating at 805 K, 32025.4, and 0.092%, respectively.

Understanding mechanism of enhanced specific heat of single molten salt-based nanofluids: Comparison with acid-modified salt

This study aimed to identify the mechanisms for enhancing the specific heat of KNO3-SiO2 nanofluids in both solid and liquid phases. The specific heat of the nanofluids was examined by a different scanning calorimetry, by varying the concentration of the nanoparticle (0.1–10 wt%). The specific heat increased with the nanoparticle concentration but decreased beyond the optimal concentration (1 wt%). The specific heat of the nanofluids was enhanced by up to 24.1% and 28.1% in the solid and liquid phases, respectively. In order to account for the reasons for the enhanced specific heat of the molten salt nanofluids, acid-modified salts in which the aqueous hydrogen peroxide (H2O2) solution was added to KNO3 were prepared, and their specific heat was examined by varying the amount of H2O2. The acid-modified salts showed an enhanced specific heat of 21.1% and 29.5% in the solid and liquid phases, respectively, and similar concentration-dependent specific heat as the nanofluids. Fourier transform infrared spectroscopy and energy-dispersive X-ray spectroscopy as well as scanning electron microscopy were used to corroborate the nanoparticle dispersion into the salt. X-ray diffraction analysis was conducted to compare the crystallite sizes of the nanofluids and acid-modified salts to that of pure KNO3. The reduced crystallite size (increased interfacial area between crystallites) accounts for the enhanced specific heat in the solid phase. Based on the zeta potential measurements, the Stern layer and Debye length, which consist of the electric double layer (EDL), were estimated to be 0.45 and 0.97 nm, respectively, for the KNO3-SiO2 nanofluid. Similar features on the Stern layer and Debye length were obtained for the acid-modified salts. The formation of the EDL adjacent to the nanoparticles accounts for the enhanced specific heat of the nanofluids in the liquid phase.

Specific heat capacity of molten salt-based nanofluids in solar thermal applications: A paradigm of two modern ensemble machine learning methods

The quantitative determination of specific heat capacity (SHC) of molten (nitrate) salt-based nanofluids helps to control the start-up heat and prevent overheating when deployed as a working heat transfer fluid in a wide range of solar thermal applications. Thus, accurate measurement of the SHC and capturing the melting point is of paramount importance in the molten salt-based nanofluids’ characterization analyses applied in solar collectors. In this research, two modern ensemble machine learning models, Extra Tree Regression (ETR) and AdaBoost Regression (ABR), were developed based on 2,384 datasets, including solid mass fraction (w), temperature (T), SHC of base fluid (CPBase), mean diameter (Dp), and density (ρp) of nanoparticle as all independent input variables and the SHC of molten salt-based nanofluids (CPMS-nf) as the target. Herein, the stepwise forward method and mutual information were addressed to determine the best input combination and sensitivity analysis. The provided models were validated using Random Forest (RF) and Boosted Regression Tree (BRT) as two powerful other ensemble models. The results demonstrated that ETR model in terms of (R = 0.9964, RMSE = 0.1566, and U95%=3.6062) outperformed the ABR (R = 0.9949, RMSE = 0.1855, and U95%=3.6009), RF (R = 0.9922, RMSE = 0.2326, and U95%=3.5904), and BRT (R = 0.9907, RMSE = 0.2508, and U95%=3.5857). The SHC of molten salt base fluid was identified as the most significant factor in estimating the SHC of molten salt-based nanofluids.

Effects of interface layer on the thermophysical properties of solar salt‐SiO 2 nanofluids: A molecular dynamics simulation

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.

Effects of SiO2 Nanoparticle Dispersion on The Heat Storage Property of the Solar Salt for Solar Power Applications

The effects of SiO2 nanoparticles on the heat storage properties of Solar Salt (NaNO3-KNO3) are studied using experimental and molecular dynamics (MD) simulations. The experiment results show the specific heat capacity of the molten salt-based nanofluids is higher than that of the pure base salt. We focus on the inference regarding the possible mechanisms behind the enhancement of the specific heat capacity which are considered more acceptable by the majority of researchers, the energy and force in the system are analyzed by MD simulations. The results demonstrate that the higher specific heat capacity of the nanoparticle is not the reason leading to the heat storage enhancement. Additionally, the analysis of potential energy and system configuration shows that the other possible mechanisms (i.e., interfacial thermal resistance theory and compressed layer theory) are only superficial. The forces between the nanoparticle atoms and base salt ions construct the constraint of the base salt ions, further forms the interfacial thermal resistance, and the compressed layer around the nanoparticle. This constraint has a more stable state and requires more energy to deform it, leading to the improvement of the heat storage property of nanofluids. Our findings uncover the mechanisms of specific heat capacity enhancement and guide the preparation of molten salt-based nanofluids.

Viscosity and stability analysis of hitec salt-based alumina nanofluids

Nanofluids based on molten salts have been recently studied thanks to the increase in some thermophysical properties compared to the base salt. Despite the interest shown in such material, information on its stability over time and viscosity is lacking, especially when Hitec is used as the base salt. This research evaluated for the first time at high temperature, the particle/agglomeration size, colloidal stability, as well as the rheological behavior of Hitec salt-based alumina nanofluids containing 0.5–1.5 wt% of alumina nanoparticles with a 13.6 nm nominal size. The results showed intermediate particle size and stability values, and that its nanofluid character (nanoparticles < 100 nm) remained after 1 h under static conditions and mechanical redispersion. The relation between the viscosity and temperature of pure Hitec followed the Guzmán-Andrade and Girifalco models and was compared to the correlations in the literature. Hitec and molten salt-based nanofluids (MSBNFs) with 0.5 wt% of Al2O3 displayed Newtonian behavior both with a rheological behavior index n = 1, while the other MSBNFs presented shear-thinning behavior with n = 0.827 and n = 0.743 for 1.0 and 1.5 wt% of Al2O3, respectively. This behavior is in accordance with the particle sizes analyzed using the Dynamic Light Scattering, the sample with the smallest initial size presented a Newtonian behavior compared to the other samples whose size was larger and therefore its behavior was shear-thinning. Finally, the experimental influence of the nanoparticle concentration on viscosity was compared to theoretical and empirical models. The Chandrasekar model was determined to provide the best experimental data fitting.

Influence of the production method on the thermophysical properties of high temperature molten salt-based nanofluids

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.

Editorial

Nanofluids are gaining attention in the exchange, conversion, and storage of energy, owing to their enhanced thermophysical characteristics. Since the thermal conductivity of metals are far higher than that of fluids such as water and oil, adding metal nanoparticles into such fluids can result in a new medium with improved heat transfer properties. Apart from pure metals, nanoparticles made of metal-oxides, silicon carbide, composites, bio-based materials, etc. are also used for this purpose. Nanofluids are produced by engineered-dispersion of nanoparticles in a base fluid, which significantly enhances the thermal conductivity of conventional heat transfer fluids (Ganji et al. 2018; Sheikholeslami 2019). However, the thermophysical characteristics of nanofluids depend on the size, shape, and volumetric fraction of the dispersed nanoparticles. An interesting breakthrough in nanofluids is the advent of hybrid nanofluids wherein two or more nanoparticles are immersed in the base fluid, which has shown further enhancement of the thermal and rheological properties (Kumar and Arasu 2018). Nanofluids play important role in energy exchange and conversion applications (Jama et al. 2016;Nagarajan et al. 2014; Said et al. 2014; Reddy et al. 2017; Hussain et al. 2019; Muñoz-Sánchez et al. 2018; Bhalla and Tyagi 2017; Islam et al. 2015). Holkar et al. (2018) listed different nanofluids employed for energy harvesting and their advantages. Apart from these, cupric oxide nanofluid was proposed to improve the heat transfer performance of asphalt solar collectors that harvest solar energy from roads (Hashim 2014). The efficiency of direct absorption solar collectors could be improved by the use of nanofluids as the absorption medium (Nagarajan et al. 2014;Otanicar et al. 2010). A major challenge in the use of nanofluids for energy applications is the contradictory behavior of thermal conductivity and specific heat. While the enhancement of thermal conductivity of base-fluid by the dispersion of nanoparticles is well established, there are no consistent findings on the enhancement of specific heat capacity (SHC) (Lu and Huang 2013), which is a desirable property for energy storage. Several studies reported decline of SHC of aqueous nanofluids, whereas non-aqueous nanofluids showed improvement of SHC (Shin and Banerjee 2010). This issue has been tackled in many studies, and it remains an ongoing challenge. Adding nanoparticles to molten salts that are used as heat transfer fluids in concentrated solar power (CSP) system could enhance the thermophysical properties including SHC and enable the CSP plants operate at higher temperatures (Shin and Banerjee 2010; Lu and Huang 2013). Muñoz-Sánchez et al. (2018) provided a comprehensive review on the use of molten salt-based nanofluids for energy storage and transfer at higher temperatures. Owing to the growing environmental concerns on the conventional synthesis techniques of nanoparticles, the emerging trend is to adopt green synthesis approaches (Genuino et al. 2013; Ghulam 2016). The nanoparticles produced by green synthesis are termed as green nanoparticles, and the nanofluids prepared from them are known as green nanofluids (Narayanan and Rakesh 2019). Several researchers have reported development and characterization of green and eco-friendly nanofluids such as silver and gold nanofluids (Mollick et al. 2014)(John et al. 2015), grapheme-based nanofluids (Mehrali et al. 2016), and coconut fiber-based nanofluids (Adewumi et al. 2018). These green nanofluids would be promising alternatives for their conventional counterparts for various energy conversion applications.

Corrosion aspects of molten nitrate salt-based nanofluids for thermal energy storage applications

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.

Colloidal stability of molten salt –based nanofluids: Dynamic Light Scattering tests at high temperature conditions

The use of molten salt-based nanofluids as heat transfer fluids or thermal energy storage materials to increase the efficiency of Concentrated Solar Power plants has gained attention due to the use of the renewable energies against Global Warming. One of the issues of interest is the colloidal stability of the nanoparticles dispersed in ionic media like molten salts. In this work a new experimental set-up to measure the particle size distribution of molten salt-based nanofluids by means of Dynamic Light Scattering was developed. The colloidal stability of silica and Al/Cu nanoparticles dispersed in solar salt (NaNO3-KNO3) was experimentally measured for the first time. Silica nanoparticles were dispersed in water, calcium nitrate tetrahydrate and solar salt, and the formation of micrometrical agglomerates was observed when molten salts were used as base fluid due to the high ionic strength of the medium and the reduced Debye length. The influence of the nanoparticle composition was proved to be also important. For the Al/Cu metal alloy nanoparticles the agglomerates formed were smaller than for silica. Besides, even though both nanoparticles settle after 4 h in static conditions, only Al/Cu nanoparticles recover the initial particle size distribution when they are mechanically redispersed.

A soft computing approach for estimating the specific heat capacity of molten salt-based nanofluids

Despite the promising potential of nanofluids as heat transfer and energy storage media, determination of their thermal behavior and properties need significant experimentation. Considering the relatively high costs of such fluids and the time-consuming procedures for synthesizing them and measuring their characteristics, machine learning techniques can be powerful tools for simulating their behaviors in the unstudied combinations of operating conditions. In this study, a machine learning model has been developed for the first time in the literature - to simulate and predict the specific heat capacity of a molten nitrate salt mixture seeded with silica, alumina and titania nanoparticles. A multilayer perceptron neural network (ANN) was selected among 1920 ANNs with different architectural features. With a prediction R2 value of 0.9998, the suggested model was found to provide much superior predictions (and validated against experimental data) as compared to the classic analytical models. The model developed in this study can, therefore, be used for estimating the values of specific heat capacity for nanofluid samples - based on the temperature and mass fraction of the nanoparticles, as well as the average (or nominal size) of the nanoparticles. The soft-computing technique itself was evaluated under extreme training conditions and it was found that the algorithm can adapt to new data sets with maximum MAPE of 2% and can enable excellent quality of predictions (R2 > 0.95) when trained with <300 data points.