Desarrollo de nanomateriales 2d metálicos para nanofluidos con aplicación en energía solar de concentración

The cost-effective optimisation of renewable power generation systems is a must in the present context of a global climate crisis and a growing demand for reliable, affordable and clean power supply. Reliability is a challenging issue for renewable technologies like photovoltaics, which produces electricity under ceaseless solar irradiation and therefore suffers of daily intermittency. The integration of concentrated solar power plants in hybrid power blocks has been proposed as a solution for this intermittency by taking advantage of thermal energy storage to maintain production and compensate the supply. A limitation for this solution is the low solar-to-thermal energy conversion due to the poor thermophysical properties of the typical heat transfer fluid flowing through solar collectors. The use of nanofluids as has been proposed to improve this conversion. This thesis aims to provide knowledge and understanding of the Physical Chemistry of nanofluids, with particular attention to features and processes that determine their stability and heat transfer and storage capabilities. Here Pd nanoplate-containing aromatic oil-based nanofluids have been prepared and characterised through different techniques to assess their applicability as volumetric absorbers and heat transfer fluids in parabolic trough collectors of concentrating solar power plants. A significant contribution from this thesis refers to the influence of the structure and dynamics of solid-liquid interfaces on the physical properties of nanofluids on the basis of simulations at the density functional theory and molecular dynamics levels-of-theory. The findings presented are expected to have both practical and fundamental implications on future research.La optimización del rendimiento de los sistemas de producción eléctrica renovable es una necesidad en el contexto actual, caracterizado por una crisis climática global y una creciente demanda por un suministro eléctrico fiable, rentable y limpio. La fiabilidad es un tema desafiante para algunas tecnologías renovables como la fotovoltaica, que producen electricidad únicamente bajo irradiación solar incesante y sufre, por tanto, de intermitencia diaria. La inclusión de plantas de energía solar de concentración en bloques híbridos de potencia constituye una prometedora solución para esta intermitencia, por aprovechamiento del almacenamiento térmico de energía para mantener la producción y compensar el suministro. Una limitación de esta solución es la baja eficiencia de la conversión solar-térmica por las deficientes propiedades termofísicas del fluido de transferencia de calor que circula a través de los colectores solares. Se propone el uso de nanofluidos para mejorar la eficiencia de conversión. Esta tesis pretende aportar conocimiento y comprensión sobre la Química Física de los nanofluidos, con particular atención a todos aquellos aspectos y procesos que determinan su estabilidad y aptitud para la transferencia y almacenamiento de calor. Se han preparado nanofluidos con nanoplates de Pd en un aceite aromático y se han caracterizado mediante diferentes técnicas para evaluar su aplicabilidad como absorbedores volumétricos y fluidos transferentes de calor en colectores cilindro-parabólicos de plantas de energía solar de concentración. Una contribución significativa de esta tesis se refiere a la influencia de la estructura y dinámica de la interfase sólido-líquido en las propiedades físicas de los nanofluidos, en base a simulaciones desarrolladas en el contexto de la Teoría del Funcional de la Densidad y de la Dinámica Molecular. Se espera que los hallazgos que aquí se presentan tengan implicaciones prácticas y fundamentales en investigaciones futuras

Probing the thermal resistance of solid-liquid interfaces in nanofluids with molecular dynamics

The significance of interfacial thermal resistance in the thermal conductivity of nanofluids is not well understood, in part because of the absence of measurements of this quantity. Here, we study the interfacial thermal resistance for metal-oil nanofluids of interest as heat transfer fluids for concentrating solar power, using density functional theory and molecular dynamics simulations. Insights on the role of chemical interactions in determining the interfacial thermal resistance are revealed. The results presented here showcase a general picture in which the stronger the chemical interactions between species at the interface, the lower the associated interfacial thermal resistance. The implications toward nanofluid design are discussed. We show that, for this important family of metal-oil nanofluids, the interfacial thermal resistance values are low enough so that it is possible to afford a reduction in particle size, minimizing stability and rheological issues while still offering enhancement in the effective thermal conductivity with respect to the base fluid.40 página

Estudio teórico de la estructura y la dinámica de un nanofluido con partículas de CuO

Resumen del proyecto de líneas prioritarias titulado "Estudio teórico de la estructura y la dinámica de un nanofluido con partículas de CuO" del IMEYMAT

Thermal performance of nanofluids based on tungsten disulphide nanosheets as heat transfer fluids in parabolic trough solar collectors

Nanofluids are considered as a new generation of heat transfer fluids since they exhibit thermophysical properties improvements compared with conventional heat transfer fluids. The high thermal conductivity of nano -fluids and even the isobaric specific heat enhancements over conventional liquids make these colloidal suspensions very attractive in many research areas, including solar energy. In this work, nanofluids based on tungsten disulphide (WS2) nanosheets have been prepared from the thermal oil currently used as heat transfer fluid in Concentrating Solar Power (CSP) plants. The high aspect ratio of WS2 bidimensional nanostructures provides high long-term colloidal stability to the nanofluids and facilitates heat transport. Cetyl-trimethylammonium bromide and polyethylene glycol have been used as surfactants to improve the exfoliation process and enhance the colloidal stability of the nanomaterial dispersions. Some properties such as density and viscosity of the base fluid have not been significantly altered by the presence of WS2 nanosheets in the base fluid. However, studies on the thermal properties of nanofluids have shown promising results with increases in thermal conductivity of up to 33% and heat transfer coefficient by 21% over the base fluid. Furthermore, it has been estimated that the overall efficiency of the CSP system could be improved by 31% by replacing the conventional thermal fluid with 2D-WS2-based nanofluids

Synthesis, Characterization and Photocatalytic Performance of Calcined ZnCr-Layered Double Hydroxides

The development of new materials for performing photocatalytic processes to remove contaminants is an interesting and important research line due to the ever-increasing number of contaminants on our planet. In this sense, we developed a layered double hydroxide material based on Zn and Cr, which was transformed into the corresponding oxide by heat treatment at 500 & DEG;C. Both materials were widely characterized for their elemental composition, and structural, morphological, optical and textural properties using several experimental techniques such as x-ray diffraction, x-ray photoelectron spectroscopy, scanning and transmission electron microscopy, Fourier transform infrared spectroscopy, UV-vis spectroscopy and physisorption techniques. In addition, the photocatalytic activity of both materials was analysed. The calcined one showed interesting photocatalytic activity in photodegradation tests using crystal violet dye. The operational parameters for the photocatalytic process using the calcined material were optimised, considering the pH, the initial concentration of the dye, the catalyst load, and the regeneration of the catalyst. The catalyst showed good photocatalytic activity, reaching a degradation of 100% in the optimised conditions and showing good performance after five photodegradation cycles.This research was funded by 2014-2020 ERDF Operational Programme and by the Department of Economy, Knowledge, Business and University of the Regional Government of Andalusia, grant number FEDER-UCA18-107510

Interface chemistry effects in nanofluids: Experimental and computational study of oil-based nanofluids with gold nanoplates

The chemistry of interfaces and its relation with energy storage at and transport through solid-liquid interfaces in heat transfer nanofluids is a very unexplored terrain. Here we discuss how the magnitude of the changes in specific heat and thermal conductivity of the base fluid, upon dispersion of a nanoma-terial, depends on the surface chemistry of that nanomaterial. We focus on nanofluids with Au nanoplates from an integrated experimental and theoretical perspective, and compared our findings with those pre-viously reported for nanofluids with Pd nanoplates in the same base fluid. Pd and Au are known to have different surface chemistry, and so are the structures of the solid-liquid interfaces and the thermal prop-erties of these nanofluids. It was experimentally found that for mass fractions in the order of 0.01 wt%, Pd and Au nanoplates provide enhancements of 5.9% and 1.6% in specific heat, and enhancements of 12.5% and 17.9% in thermal conductivity, at 373 K. It was verified using density functional theory and classical molecular dynamics simulations that base fluid molecules can chemisorb on Pd surfaces, but not on Au surfaces. This work suggests that the stronger interactions between species at solid-liquid interfaces, the higher the specific heat enhancements and the lower the thermal conductivity enhancements at high temperature.(c) 2022 The Author(s). Published by Elsevier B.V

Synthesis and characterization of metal oxide-based microcapsules including phase change materials for energy storage applications

In this study, microcapsules based on Cu2O containing different phase change materials (PCM) were prepared and characterized. The elemental, structural and electronic properties of the Cu2O-based microcapsules were characterized using several techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy and Fourier-transform infrared spectroscopy. In addition, the thermal properties of the microcapsules prepared were characterized in order to analyse their possible application as a thermal energy storage medium. Heating/cooling cycles using a differential scanning calorimetry technique were performed, and the phase change temperature and enthalpy were estimated. We observed good stability after the cycles. Furthermore, the encapsulation efficiency was estimated from melting and crystallization enthalpy values, reaching a value of 14.8% for the paraffin wax-based microcapsules. Finally, isobaric specific heat was measured to evaluate the storage capability of the encapsulated PCMs with regard to pure Cu2O to evaluate their possible application as a thermal storage system. An increase of around 140% was found in the isobaric specific heat for the microcapsules based on paraffin wax with regard to pure Cu2O. © 2023, The Author(s)

MoS2-based nanofluids as heat transfer fluid in parabolic trough collector technology

Concentrating solar power is becoming one of options for producing energy to replace conventional polluting energy sources. However, improving the efficiency and reducing the cost of technologies based on this type of energy to make it more competitive is still a work in progress. This study proposes replacing the thermal oil used as the heat transfer fluid in the absorber tubes of parabolic trough solar collectors (PTCs) with nanofluids based on spherical molybdenum disulphide nanoparticles with the aim of improving the thermal efficiency of concentrating solar power plants. The colloidal stability of the nanofluids was verified by UltravioleteVisible spectroscopy, Zeta potential and Dynamic Light Scattering monitoring. The presence of spherical MoS2 nanoparticles resulted in an increase of up to 13% in specific isobaric heat and 6% in thermal conductivity compared to thermal oil. Finally, the efficiency of parabolic trough solar collectors was estimated to increase by 5%, which also favours the decrease of pumping power and the elimination of selective coatings on the absorber tube. To our knowledge, this is the first time that MoS2-based nanofluids are tested as heat transfer fluids in PTCs analysing its implementation in the solar energy application. © 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).We acknowledge Ministerio de Ciencia, Innovaci?n y Universidades del Gobierno de Espa?a for funding under Grant No. RTI2018-096393-B-I00 and for financial support related to measurements of thermal properties, which were performed using devices acquired under Grant No. UNCA15-CE-2945. Also, this research was funded by 2014?2020 ERDF Operational Programme and by the Department of Economy, Knowledge, Business and University of the Regional Government of Andalusia, grant number FEDER-UCA18-107510

Multifunctional microcapsules based on ZnO and n-octadecane: From thermal energy storage to photocatalytic activity

Energy management is one of the most important issues to be addressed in the near future in many fields, one of which is buildings. In this sense, new phase change materials (PCM) are being widely studied for storing energy. Encapsulating PCM is a good way to incorporate these materials into different applications in which energy storage is useful. In this study, microcapsules based on ZnO containing n-octadecane as a phase change material were synthesized and characterized with regard to their structural, morphological and optical properties according to several synthesis parameters, such as the proportion of precursors, stirring rate and ageing time. The microcapsules were characterized using x-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and UV–Vis spectroscopy in diffuse reflectance mode. The presence of n-octadecane inside the capsules was confirmed. Their thermal behaviour was analysed by means of differential scanning calorimetry. Heating/cooling cycles were performed, after which the microcapsules presented good stability. Furthermore, the encapsulation efficiency was estimated from melting and crystallization enthalpy values, reaching a value of 23.1%. Moreover, the isobaric specific heat of the microcapsules is higher than that of ZnO, which means that substituting ZnO with microcapsules in buildings leads to an important increase in the amount of sensible heat stored. Finally, the photocatalytic activity of the microcapsules was analysed by studying the photodegradation of Crystal Violet dye. The degradation rate increased when the microcapsules were present, so the photocatalytic activity of the microcapsules was confirmed under UV and visible irradiation, which is of interest because they can be used to remove organic pollutants from buildings.10 página

Stability and Thermal Properties Study of Metal Chalcogenide-Based Nanofluids for Concentrating Solar Power

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