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Molten salt is an excellent medium for heat transfer and storage. The unique microstructure of carbon nanomaterials leads to good mechanical stability, low density, high thermal conductivity, and high strength, etc. The addition of carbon nanomaterials to molten salt to form molten salt nanofluid can remarkably enhance the specific heat capacity and thermal conductivity of molten salt and reduce the molten salt viscosity, which is of great importance to increase the heat storage density and reduce the heat storage cost. Nevertheless, some challenges remain in the study of such nanofluids. The main challenge is the dispersion stability of carbon nanomaterials. Therefore, to improve research on carbon nanofluids, this paper summarizes the progress of carbon-based molten salt nanofluid research worldwide including the preparation methods of molten salt nanofluids, the improvement of heat transfer performance, and the improvement of heat storage performance. The effects of carbon nanoparticle concentration, size, and type on the heat transfer and storage performance of molten salt are derived, and the effects of nanoparticle shape on the heat transfer performance of molten salt are analyzed while more promising preparation methods for carbon-based molten salt nanofluids are proposed. In addition, the future problems that need to be solved for high-temperature molten salt-based carbon nanofluids are briefly discussed. [ABSTRACT FROM AUTHOR]

Summary: Dispersing nanomaterials can effectively improve the thermal energy storage performance of molten salts. However, research on such improvement mechanism is still immature, as the understanding of the performance improvement levels induced by different nanoparticles has not been consistent. Using silica nanoparticles as additives, this work prepares nine molten salt nanofluids by aqueous solution method with the use of potassium nitrate, sodium nitrate, and their binary mixture as base salts. The key thermal performance of these nanofluids are evaluated through different characterization approaches, such as differential scanning calorimetry, thermal gravimetric analysis, laser flash analysis, and scanning electron microscopy for microstructural characteristics. The possible performance enhancement mechanism caused by silica nanoparticle is also explored. Results show that, for these nine molten salt nanofluids containing silica nanoparticles, the change in the melting point is negligible. The potassium nitrate salt dispersed with 20‐nm silica nanoparticles achieve the highest performance improvement not only in latent heat but in specific heat and thermal conductivity, while sodium nitrate salt nanofluids reach maximum improvement at their decomposing temperature. The improvement level can be associated with the electronegativity of alkali ions. Cloud nuclei cause microstructure differences among molten salt nanofluids and may be the main reason for the thermal performance enhancement of the molten salt nanofluids. [ABSTRACT FROM AUTHOR]

Summary: Specific heat and thermal conductivity are important thermal properties of high‐temperature heat transfer fluids and thermal storage materials for supercritical solar power plants. In the present work, nanofluids composed of ternary carbonate Li2CO3‐K2CO3‐Na2CO3 (4:4:2, mass ratio) and 1.0 wt.% carbon nanotubes (CNT) were prepared to obtain high‐temperature heat transfer and storage media with enhanced specific heat and thermal conductivity. The dispersion of CNTs in the nanofluids was tuned by changing the evaporation temperature (100, 140, 180 and 220 °C) and adding surfactants such as sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), or gum Arabic (GA). The results showed that GA and SDS facilitate good dispersion of CNT in nanofluids at the evaporation temperatures of 140 °C and 180 °C, resulting in the formation of more needle‐like nanostructures. The higher increase in the specific heat and thermal conductivity of the nanofluids with SDS at 500 °C was 78.3% and 149.2%, respectively. Additionally, the specific heat of as‐prepared ternary carbonate nanofluids exhibits a good thermal stability after 30 cycles of thermal shock experiments. [ABSTRACT FROM AUTHOR]

Molten salt is an important working fluid in the field of solar energy. However, the mixed convection heat transfer of molten salts has not been investigated. In this study, the performance of the mixed convection heat transfer of molten salt in horizontal square tubes under different side heating conditions is experimentally and numerically examined. The bottom side heating is experimentally studied, and a correlation is fitted on the basis of the experimental data. A good agreement between the correlations and the experimental data is achieved. The experimental results show that the mass flow rate of molten salt and the Reynolds number ( Re ) increase with the increase in the pump frequency. Moreover, the Nusselt number ( Nu ) increases with the increase in Re . The maximum enhancement of Nu in the mixed convection heat transfer compared with the forced convection heat transfer is approximately 50%. The experimental results and previous simulation results of the mixed convection heat transfer of molten salt with bottom surface heating are also compared. The comparison results also show a good agreement. The two-side heating condition (bottom and lateral side) is numerically examined. The simulation results of the two-side heating show that the vortexes caused by the buoyancy effect change the shape of the mainstream core area and strengthen the performance of heat transfer. [ABSTRACT FROM AUTHOR]

In order to meet the requirements of high temperature solar thermal power generation better, the melting point of mixed carbonates should be further reduced. Starting with ternary carbonates, we modified their composition ratios and added NaNO3 or NaOH as additives. Their new melting points, initial crystallization points, decomposition temperatures, specific heats and thermal stabilities were analyzed. While the melting points of the mixed nitrate/carbonate salts were maintained at 394.7°C, the melting points of the mixed hydroxide/carbonate salts were reduced to 323.2°C. Also the initial crystallization temperature of the mixed hydroxide/carbonate salts was dependent on their composition. Among the hydroxide-mixed molten salts, there are six samples with lower initial crystallization temperature. The decomposition temperature of the selected six samples was nearly 800°C. Their specific heats were fitted with the quadratic polynomial of the temperature and their thermal stability was shown by the excellent repeatability of five heating curves. Only the very first heating curves showed that the initial measured value of a melting point was higher than all the later values of the molten salts due to starting with the powder. Furthermore, possible issues that arise with the addition of NaOH are discussed. [ABSTRACT FROM AUTHOR]

Nitrate salts have been applied as heat transfer and heat storage medium in the Concentrating Solar Power (CSP) plants. Corrosion is critical to the long-time safe operation of the CSP system. This experimental study was conducted to research the dynamic corrosion behaviors of 316 L stainless steel in high-temperature quaternary nitrate nitrite mixture. The effects of corrosion on 316 L stainless steel were investigated under the varied flow rates of 0.6 m/s, 1.3 m/s and 2 m/s with 565 °C constant temperature for 1000 h. The results showed that oxidation corrosion occurred in 316 L stainless steel at all working conditions. The dynamic corrosion curves were parabolic, and the value corrosion weight loss (m mass) increased with the increase of the flow rate. Meantime, when the products flow rates were 0.6 m/s, 1.3 m/s and 2 m/s, the values of m mass were 3.6 mg cm−2, 3.77 mg cm−2 and 4.36 mg cm−2. The values of corrosion depth (R depths) were 0.039 mm/a, 0.041 mm/a and 0.048 mm/a, respectively. Besides, the corrosion products were consisted of hematite (Fe 2 O 3), magnetite (Fe 3 O 4), trace amounts of MgFe 2 O 4 and NaFeO 2 with the structure of lamellar. Furthermore, the corrosion products gradually transformed into FeCr 2 O 4 with structures of dense and massive, and the increase of the flow rate deepened the degree of corrosion, but it had little effect on the types of corrosion products. • Dynamic corrosion behavior of 316 L steel are experimentally studied in quaternary nitrate-nitrite mixed salt at 565 °C. • The corrosion rates of 316 L steel are obtained at different flow velocities. • The analysis of dynamic corrosion mechanism is comparatively discussed by SEM, XRD and EDS method. [ABSTRACT FROM AUTHOR]