Molten Salt Nanofluids Research Articles

Experimental study on thermophysical properties of molten salt nanofluids prepared by high-temperature melting

Heat storage is a key technology in solar thermal power, helps solar thermal power eliminate the instability in energy supply for the grid, and provides continuous and stable high-quality electrical energy to improve power system efficiency and extend system life. Molten salt is an important material for heat storage and heat transfer in solar thermal power. In recent years, nanofluid technology has been applied to investigate heat storage and transfer materials, such as molten salt, to improve heat capacity. However, the nanofluid technology is hampered by the easy agglomeration of nanoparticles in molten salts, and this phenomenon degrades the performance of the molten salt nanofluids. To reduce or prevent agglomeration, a two-step method with high-temperature melting in preparing molten salt nanofluids is developed recently. Molten salt nanofluids obtained by high-temperature melting show good stability and long time to agglomerate. This paper presented the study on the thermophysical properties of molten salt nanofluids prepared by high-temperature melting method. The molten salt nanofluids possessed low-melting point salt as base liquid and SiO2 with a diameter of 20 nm as nanoparticles. The specific heat of this type of molten salt nanofluid was tested, and the optimum concentration of nanoparticles was determined. Results showed that the specific heat capacities of the molten salt nanofluid samples with different mass fractions of nanoparticles were higher than those of pure molten salt. The average specific heat of molten salt nanofluid with a mass fraction of 0.5% was the highest at 1.950 J/(g K), which was 24.5% more than that of pure molten salt. The thermophysical properties of molten salt nanofluids with a mass fraction of 0.5% and prepared by high-temperature melting were experimentally tested and compared with those of pure molten salt. These properties included melting point, primary crystallization point, thermal conductivity, viscosity, latent heat, and density. The molten salt nanofluids showed good performance for heat storage and transfer.

Natural convection heat transfer of molten salt nanofluids around vertical array of heated horizontal cylinders

To analyze the natural convection heat transfer of molten salt nanofluids around vertical array of horizontal cylinders, a numerical simulation is performed with cylinder numbers in the range of N = 2–8 and pitches in the range of S/D = 5–10. The results show that the heat transfer of nanofluids around each cylinder is affected by the position in the tube row and its distance from the adjacent cylinder. The average Nusselt number (Nua) of the natural convection heat transfer over the whole array of cylinders is determined by cylinder spacing S/D, cylinder number N, and Ra. When S/D = 5, Nua decreases as the cylinder number increases. When S/D = 10, Nua increases as the cylinder number increases. Compared with the molten salts, the natural convection heat transfer of nanofluids is enhanced. This study provides a theoretical basis for the design of a single-tank energy storage system.

Anomalous Increase in Specific Heat of Binary Molten Salt-Based Graphite Nanofluids for Thermal Energy Storage

An anomalous increase of the specific heat was experimentally observed in molten salt nanofluids using a differential scanning calorimeter. Binary carbonate molten salt mixtures were used as a base fluid, and the base salts were doped with graphite nanoparticles. Specific heat measurements of the nanofluids were performed to examine the effects of the composition of two salts consisting of the base fluid. In addition, the effect of the nanoparticle concentration was investigated as the concentration of the graphite nanoparticles was varied from 0.025 to 1.0 wt %. Moreover, the dispersion homogeneity of the nanoparticles was explored by increasing amount of surfactant in the synthesis process of the molten salt nanofluids. The results showed that the specific heat of the nanofluid was enhanced by more than 30% in the liquid phase and by more than 36% in the solid phase at a nanoparticle concentration of 1 wt %. It was also observed that the concentration and the dispersion homogeneity of nanoparticles favorably affected the specific heat enhancement of the molten salt nanofluids. The dispersion status of graphite nanoparticles into the salt mixtures was visualized via scanning electron microscopy. The experimental results were explained according to the nanoparticle-induced compressed liquid layer structure of the molten salts.

Effect of SiO2 nanoparticles on specific heat capacity of low-melting-point eutectic quaternary nitrate salt

In order to evaluate the effect of nanoparticles on the specific heat capacity of a low-melting-point eutectic quaternary nitrate salt, 1.0 wt% SiO2 nanoparticles with an average size of 20 nm was doped into the mixed salt Ca(NO3)2·4H2O-KNO3-NaNO3-LiNO3. A mechanical dispersion mixer was developed to prepare molten salt nanofluids. The effects of the mixing time (15, 45, 90, 120, and 150 min) and stirring rate (600, 750, and 1000 rpm) on the specific heat capacity of the nanofluids were analyzed using a differential scanning calorimeter. The results showed that the specific heat capacity of the nanofluids varied with the mixing time and stirring rate. It increased by ~ 17.0% at a mixing time of 15 min and stirring rate of 750 rpm. Microstructural characterization of the molten salt nanocomposites was performed using scanning electron microscopy, which showed that special nanostructures, which resemble chain-like nanostructures, were formed on the surface of the solid nanofluids. The special nanostructures with a large specific surface area and high surface energy could increase the specific heat capacity of the molten salt nanofluids.

Experimental study on the specific heat and stability of molten salt nanofluids prepared by high-temperature melting

Molten salt is an important heat storage and heat transfer medium in solar thermal power generation technology due to its high heat capacity, wide working temperature range, and low cost. Adding nanoparticles (usually by the two-step method with ultrasonic dispersion) can increase the specific heat of molten salt. Thus, the molten salt nanofluids can increase the heat capacity and decrease the heat storage cost of a solar thermal power generation system. However, nanoparticles are easy to agglomerate in the molten salt; moreover, after agglomeration, the performance of molten salt nanofluids is degraded. A two-step method with high-temperature melting in preparing molten salt nanofluids is proposed in this paper. Molten salt nanofluids were prepared by high-temperature melting. The base solution was a low-melting point molten salt and the nanoparticles were SiO2 with a diameter of 20nm. The specific heat was measured and the nanoparticle dispersity was analyzed. The stability of the molten salt nanofluids was studied, and the results were compared with those prepared by ultrasonic dispersion. The average specific heat of molten salt nanofluids prepared by high-temperature melting was 1.789J/(g·K), which was close to that of molten salt nanofluids prepared by ultrasonic dispersion and 16.4% higher than that of pure molten salt. The molten salt nanofluids prepared by ultrasonic dispersion showed poor thermal stability under high-temperature conditions, and the average specific heat decreased by 8.5% after only 200h. The thermal stability of molten salt nanofluids prepared by high-temperature melting showed a highly stable performance in long-time experiments. The variation of specific heat was less than 5% after 2000h under the same high-temperature experimental condition. The molten salt nanofluids obtained by high-temperature melting showed better stability and long performance than those obtained by ultrasonic dispersion. Therefore, the two-step method with high-temperature melting is stable and reliable for preparing molten salt nanofluids.

Experimental measurements of thermal conductivity of alumina nanofluid synthesized in salt melt

Nanoparticles were synthesized in-situ using a simple one-step synthesis protocol from a cheap additive, mixed apriori in a high temperature salt melt (solar salt, NaNO3-KNO3). The thermal conductivity of the nanofluid was measured using a standardized concentric cylinder (annulus) test apparatus under steady-state conditions. The thermal conductivity of the salt melt was enhanced by 20∼ 25% due to generation of nanoparticles in-situ from the additive. The level of enhancement was found to be insensitive to temperature but significantly exceeded the predictions from models in the literature. Materials characterization (using electron microscopy) showed the formation of percolation networks by secondary nanostructures in the molten salt nanofluid samples (that were induced by the nanoparticles generated in-situ). The enhancement in the thermos-physical properties of the salt-melt nanofluids can be attributed to the formation of these secondary nanostructures (which form a third phase).

A critical investigation of the anomalous behavior of molten salt-based nanofluids

A critical investigation is presented in this work to study the effect of nanoparticle addition, temperature, and nanoparticle size-dependence on the specific heat capacity of both conventional and molten salt-based nanofluids. The effects of temperature and nanoparticle volume fraction on the specific heat capacity of conventional nanofluids are in agreement in all studies cited in this review. Different correlations based on the available data were developed as a function of temperature and volume concentration only. However, the effect of nanoparticle size-dependence was ignored in these correlations. A general correlation for Al2O3–water nanofluids, one of the most commonly studied nanofluids, that takes into account the effect of temperature, volume fraction, and nanoparticle size-dependence was developed and verified in this review. Disagreement was reported for the results of the specific heat capacity of molten salt-based nanofluids. A number of studies showed an enhancement in the specific heat capacity of nanofluids using 1% concentration of nanoparticles by weight only. However, other studies have shown deterioration in the specific heat capacity of nanofluids compared with the base mixture using various volume concentrations of nanoparticles. Moreover, very few studies have demonstrated the effect of nanoparticle size-dependence on the specific heat capacity of molten salt nanofluids and disagreement in the results was reported in these studies. Few models based on the conventional specific heat model were developed to determine the specific heat capacity of molten salt nanofluids. These models suffer from the lack of knowledge of many terms in these equations which make them impractical. Different mechanisms were assumed in the literature to explain the abnormal behavior of molten salt nanofluids. Additional theoretical and experimental research studies are required to clarify the mechanisms responsible for specific heat capacity enhancement or deterioration in nanofluids.

Effect of formation of “long range” secondary dendritic nanostructures in molten salt nanofluids on the values of specific heat capacity

Several studies in recent literature have demonstrated the enhancement of specific heat capacity (Cp) of molten-salts on doping with minute concentration of nanoparticles, especially when the synthesis conditions enabled the formation of stable colloidal suspensions (which are also known as “molten-salt nanofluids”). In this study we present additional evidence in support of theory proposed earlier in the literature that stable colloidal suspensions of nanoparticles in a molten-salt medium induces the preferential surface adsorption of the constitutive chemical species in the salt mixture which in turn leads to the nucleation of solid phase of the molten salt (with perhaps a different chemical composition than in the bulk phase) on the nanoparticle surface. The surface adsorbed salt species leads to the nucleation and growth of a semi-solid layer of dendritic shaped phase (dendritic shaped secondary “long range” nanostructures). Incidentally, such nanostructures were not observed in electron microscopy images for samples of pure molten-salt mixtures subjected to control experiments (i.e., without nanoparticles). Hence, this study conclusively demonstrates that the existence of these nanostructures is primarily responsible for the enhancement of specific heat capacity. In this study, three different types of nanoparticles are dispersed in the same molten-salt mixture (“base fluid”) and the experimentally measured values of specific heat capacity enhancements obtained in this study are correlated to the formation of dendritic nanostructures that are observed in the images obtained from the electron microscopy of the molten-salt nanomaterial samples.

E232 固液界面に付着したナノ粒子層が熱抵抗に及ぼす影響に関する実験的研究(一般講演(10))

Solid-liquid interfacial thermal resistance is thermal resistance defined for temperature jump at the interface between solid and liquid which have different temperatures. Solid-liquid interfacial thermal resistance is a microscopic phenomenon that of elucidation of the heat transfer mechanism is needed. Therefore, in the precedent study, in the case of a nanoparticle layer accumulated on to a heat transfer interface, effects of thermal conductivity of the nanoparticle layer, its thickness, and wettability on solid-liquid interfacial thermal resistance has been investigated by the molecular dynamics analysis. However, about this study, the quantitative evaluation by experiments is not performed yet. Therefore, our goal is to experimentally evaluate thermal resistance using a nanoparticle layer and compare the result with the simulation. In this study, we experimentally and quantitatively investigated the thermal resistance of an interface between a molten salt nanofluid and stainless-steel heat transfer surface, where zirconia nanoparticles accumulated having an enough temperature gradient.

Comment on “viscosity measurements of multi-walled carbon nanotubes-based high temperature nanofluids”

In this short communication, we comment the recent results reported by Jo and Banerjee on the viscosity enhancement of multi-walled carbon nanotubes based molten salt nanofluids with volume fraction considering the effect of nanoparticle aggregation [Materials Letters 122 (2014) 212–215]. We explain that the model used to predict this enhancement is not reliable with the parameters considered and related to spherical particles. We suggest also the use of an appropriate model taking into account the aspect ratio of the particles.

Specific heat mechanism of molten salt nanofluids

Controversial results have been reported for specific heat of conventional nanofluids and molten salt nanofluids. Some water-based and organic-based nanofluids showed decreases in specific heat, while molten salt-based nanofluids showed highly enhanced specific heat. In this study, we propose a distinct heat storage mechanism to explain enhanced specific heat of molten salt nanofluids and compare with the specific heat mechanism of conventional nanofluids.