Roberto Gómez-Villarejo

Transition metal dichalcogenides (TMCs) exhibit unique properties that make them of interest for catalysis, sensing or energy storage applications. However, few studies have been performed into nanofluids based on TMCs for heat transfer applications. In this study, nanofluids based on 2D-WS₂ are prepared by liquid phase exfoliation to analyze their potential usage in concentrating solar power plants. Periodic-Density Functional Theory (DFT) calculations were performed to rationalize the success of the exfoliation process. The hydrogen bond interaction between the hydroxyl group from PEG, which acts as a surfactant, and the S atoms of the WS₂ surface stabilizes the nanosheets in the fluid. Electron localization function (ELF) analysis is indicative of the stability of the S–H interaction from WS₂ with the molecules of surfactant due to the tendency to interact through weak intermolecular forces of van der Waals solids. Moreover, improvements in thermal properties were also found. Isobaric specific heat increased by up to 10% and thermal conductivity improved by up to 37.3%. The high stability of the nanofluids and the thermal improvements were associated with the high surface area of WS₂ nanosheets. These results suggest that these nanofluids could be a promising heat transfer fluid in concentrating solar power plants.

Nanofluids are colloidal suspensions of nanomaterials in a fluid which exhibit enhanced thermophysical properties compared to conventional fluids. The addition of nanomaterials to a fluid can increase the thermal conductivity, isobaric-specific heat, diffusivity, and the convective heat transfer coefficient of the original fluid. For this reason, nanofluids have been studied over the last decades in many fields such as biomedicine, industrial cooling, nuclear reactors, and also in solar thermal applications. In this paper, we report the preparation and characterization of nanofluids based on one-dimensional MoS₂ and WS₂ nanosheets to improve the thermal properties of the heat transfer fluid currently used in concentrating solar plants (CSP). A comparative study of both types of nanofluids was performed for explaining the influence of nanostructure morphologies on nanofluid stability and thermal properties. The nanofluids prepared in this work present a high stability over time and thermal conductivity enhancements of up to 46% for MoS₂-based nanofluid and up to 35% for WS₂-based nanofluid. These results led to an increase in the efficiency of the solar collectors of 21.3% and 16.8% when the nanofluids based on MoS₂ nanowires or WS₂ nanosheets were used instead of the typical thermal oil.

This article reports the preparation and characterization of nanofluids based on the heat transfer fluid (HTF) commonly used in concentrating solar power (CSP) plants. Nanofluids were prepared with commercial Ag nanoparticles and synthesized Au nanoparticles which were dispersed into a base fluid composed by a eutectic mixture of two stable compounds: diphenyl oxide and biphenyl. Both sets of nanofluids were characterized attending to its physical and chemical stability and the possible improvement on thermal properties respect to base fluid. Moreover, a theoretical analysis was achieved by Molecular Dynamics simulations to obtain a greater knowledge about this kind of systems through the study of the different interactions between the metal and the molecules of the base fluid. The results showed that the incorporation of Ag nanoparticles into a base fluid improves significantly thermal properties such as isobaric specific heat and thermal conductivity, giving an enhancement of its heat transfer coefficient. Theoretical analysis revealed that the arrangement of molecules of base fluid around the Ag nanoparticle favors the improvement of thermal properties. On his part, addition of Au nanoparticles into the base fluid did not produce any increment of thermal properties with regard to base fluid, and the theoretical analysis showed a minor participation of diphenyl oxide molecules in a first layer of Au nanoparticle.

Nanofluids are systems with several interesting heat transfer applications, but it can be a challenge to obtain highly stable suspensions. One way to overcome this challenge is to create the appropriate conditions to disperse the nanomaterial in the fluid. However, when the heat transfer fluid used is a non-polar organic oil, there are complications due to the low polarity of this solvent. Therefore, this study introduces a method to synthesize TiO₂ nanoparticles inside a non-polar fluid typically used in heat transfer applications. Nanoparticles produced were characterized for their structural and chemical properties using techniques such as X-ray Diffraction (XRD), Raman spectroscopy, Transmission Electron Microscopy (TEM), Fourier Transform Infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The nanofluid showed a high stability, which was analyzed by means of UV-vis spectroscopy and by measuring its particle size and ζ potential. So, this nanofluid will have many possible applications. In this work, the use as heat transfer fluid was tested. In this sense, nanofluid also presented enhanced isobaric specific heat and thermal conductivity values with regard to the base fluid, which led to the heat transfer coefficient increasing by 14.4%. Thus, the nanofluid prepared could be a promising alternative to typical HTFs thanks to its improved thermal properties and high stability resulting from the synthesis procedure.

Photodegradation processes are of great interest in a range of applications, one of which is the photodecomposition of pollutants. For this reason, analysing nanoparticles that improve the efficiency of these processes under solar radiation are very necessary. Thus, in this study, TiO₂ was doped with Mo and Cu using low-temperature hydrolysis as the method of synthesis. Pure TiO₂ and x%MoS₂/Cu/TiO₂ nanoparticles were prepared, where x is the theoretical quantity of MoS₂ added (0.0%, 1.0%, 5.5%, 10.0%), setting the nominal quantity of Cu at 0.5 wt.%. The samples obtained were characterised by X-Ray Diffraction, Raman spectroscopy, x-ray electron spectroscopy (XPS) and UV-Vis spectroscopy in diffuse reflectance mode (DR-UV-Vis). The results suggest that the TiO₂ structure was doped with the Mo⁶⁺ and Cu²⁺ ions in the position of the Ti⁴⁺. The x%MoS₂/Cu/TiO₂ samples presented lower band gap energy values and greater optical absorption in the visible region than the pure TiO₂ sample. Lastly, the photocatalytic activity of the samples was assessed by means of the photodegradation of methylene blue under visible light. The results show that when the quantity of Mo in the co-doped samples increased (x%MoS₂/Cu/TiO₂) there were significant increases of up to 93% in the photocatalytic activity.