Nanofluids with optimised thermal properties based on metal chalcogenides with different morphology

Over the last decades, the interest around renewable energies has increased considerably because of the growing energy demand and the environmental problems derived from fossil fuels combustion. In this scenario, concentrating solar power (CSP) is a renewable energy with a high potential to cover the global energy demand. 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. One of the current lines of research is the replacement of the heat transfer fluid used in the absorber tube of parabolic trough collectors with nano-colloidal suspensions of nanomaterials in a base fluid, typically named nanofluids. Nanofluids are considered as a new generation of heat transfer fluids since they exhibit thermophysical properties improvements compared with conventional heat transfer fluids. But there are still some barriers to overcome for the implementation of nanofluids. For example, obtaining nanofluids with high stability is a priority challenge for this kind of system. Also ensuring that nanoparticles will not clog pipes or cause pressure drops. In this Doctoral Thesis, the use of transition metal dichalcogenide-based nanofluids as a heat transfer fluid in solar power plants has been investigated for the first time. Specifically, nanofluids based on one-dimensional, two-dimensional and three-dimensional MoS2 , WS2 and WSe2 nanostructures have been researched. The base fluid used in the preparation of these nanofluids is the eutectic mixture of biphenyl and diphenyl oxide typically employed as heat transfer fluid in concentrating solar power plants. Mainly two preparation methods have been explored: the liquid phase exfoliation method, and the solvothermal synthesis of the nanomaterial and its subsequent dispersion in the thermal oil by ultrasound. Experimental parameters such as surfactant concentration, time and sonication frequency for preparation of nanofluids have also been analysed. The nanofluids have been subjected to an extensive characterisation which includes the study of colloidal stability over time, characterisation of thermal properties such as isobaric specific heat or thermal conductivity, rheological properties and optical properties. The results have revealed that nanofluids based on 1D and 2D nanostructures of transition metal dichalcogenides are colloidally stable over time and exhibit improved thermal properties compared to the typical thermal fluid used in solar power plants. The most promising nanofluids are those based on MoS 2 nanosheets and those based on WSe 2 nanosheets with heat transfer coefficient improvements of 36.2% and 34.1% respectively with respect to thermal oil. Furthermore, the dramatic role of WSe2 nanosheets in enhancing optical extinction of the thermal oil suggests the use of these nanofluids in direct absorption solar collectors. In conclusion, the present work demonstrates the feasibility of using nanofluids based on transition metal dichalcogenide nanostructures as heat transfer fluids in concentrating solar power plants based on parabolic trough collectors.En las últimas décadas, el interés en torno a las energías renovables ha aumentado considerablemente debido a la creciente demanda energética y a los problemas medioambientales derivados de la combustión de combustibles fósiles. En este escenario, la energía solar de concentración (CSP) es una energía renovable con un alto potencial para cubrir la demanda energética mundial. Sin embargo, es necesario trabajar para mejorar la eficiencia y reducir el coste de las tecnologías basadas en este tipo de energía con el objetivo de hacerla más competitiva. Una de las líneas de investigación actuales es la sustitución del fluido caloportador utilizado en el tubo absorbedor de los colectores cilindroparabólicos por suspensiones nanocoloidales de nanomateriales en un fluido base, típicamente denominados nanofluidos. Los nanofluidos se consideran una nueva generación de fluidos de transferencia de calor, ya que presentan mejoras en sus propiedades termofísicas en comparación con los fluidos de transferencia de calor convencionales. Pero aún quedan algunos obstáculos por superar para la aplicación de los nanofluidos. Por ejemplo, obtener nanofluidos con alta estabilidad es un reto prioritario en este tipo de sistemas. También garantizar que las nanopartículas no obstruyan las tuberías ni provoquen caídas de presión. En esta Tesis Doctoral se ha investigado por primera vez el uso de nanofluidos basados en dicalcogenuros de metales de transición como fluido caloportador en centrales solares. En concreto, se han investigado nanofluidos basados en nanoestructuras unidimensionales, bidimensionales y tridimensionales de MoS2, WS2 y WSe2. El fluido base utilizado en la preparación de estos nanofluidos es la mezcla eutéctica de bifenilo y óxido de difenilo empleada habitualmente como fluido de transferencia de calor en las centrales de concentración de energía solar. Se han explorado principalmente dos métodos de preparación: el método de exfoliación en fase líquida y la síntesis solvotermal del nanomaterial y su posterior dispersión en el aceite térmico mediante ultrasonidos. También se han analizado parámetros experimentales como la concentración de surfactante, el tiempo y la frecuencia de sonicación para la preparación de los nanofluidos. Los nanofluidos han sido sometidos a una extensa caracterización que incluye el estudio de la estabilidad coloidal a lo largo del tiempo, la caracterización de propiedades térmicas como el calor específico isobárico o la conductividad térmica, propiedades reológicas y propiedades ópticas. Los resultados han revelado que los nanofluidos basados en nanoestructuras 1D y 2D de dicalcogenuros de metales de transición son coloidalmente estables en el tiempo y presentan propiedades térmicas mejoradas en comparación con el fluido térmico típico utilizado en las centrales solares. Los nanofluidos más prometedores son los basados en nanoláminas de MoS2 y los basados en nanoláminas de WSe2, con mejoras del coeficiente de transferencia térmica del 36,2% y el 34,1%, respectivamente, con respecto al aceite térmico. Además, el espectacular papel de las nanoláminas de WSe2 en la mejora de la extinción óptica del aceite térmico sugiere el uso de estos nanofluidos en colectores solares de absorción directa. En conclusión, el presente trabajo demuestra la viabilidad del uso de nanofluidos basados en nanoestructuras de dicalcogenuros de metales de transición como fluidos de transferencia de calor en centrales solares de concentración basadas en colectores cilindro-parabólicos

WSe2 nanosheets synthesized by a solvothermal process as advanced nanofluids for thermal solar energy

Nanofluids are colloidal systems based on the suspension of nanoparticles in a fluid. Their thermal properties mean that they are promising heat transfer fluids with possible applications in different fields, concentrating solar energy being one of particular interest. Thus, this study presents the preparation of nanofluids based on WSe2 nanoparticles suspended in the eutectic mixture of biphenyl and diphenyl oxide, which is a heat transfer fluid widely used in concentrating solar power plants. To this end, solvothermal synthesis was used to prepare WSe2 nanosheets, which were characterized by means of scanning transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. The physical and chemical stability of the nanofluids was analyzed, observing that they became more stable when Triton X-100 was used as the surfactant. The presence of WSe2 nanosheets did not result in a significant increase in density or viscosity, but, by contrast, improvements were obtained in their isobaric specific heat and thermal conductivity, up to 4.7 and 64%, respectively. Spectral optical properties were investigated as well, showing a significant effect of the WSe2 nanosheet addition to the extinction coefficient of the base fluid in the wavelength range of the solar spectrum, promising for direct absorption solar collectors. Finally, the efficiency of the nanofluids was analyzed in a solar collector considering the Ur parameter, obtaining a remarkable increase in the efficiency of up to 34% with respect to the pure heat transfer fluid. This proves the possibility to obtain a sustainable production of energy from the sun using these WSe2-based nanofluids

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

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

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

The Role of the Interactions at the Tungsten Disulphide Surface in the Stability and Enhanced Thermal Properties of Nanofluids with Application in Solar Thermal Energy

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

Improving the efficiency of the concentrating solar power plants using heat transfer nanofluids with gold nanoplates: An analysis from laboratory to industrial scale

We report about the remarkable changes in the thermophysical properties of the heat transfer fluid used in concentrating solar power plants with parabolic-trough collectors (Dowtherm A, a mixture of diphenyl oxide and biphenyl) by addition of Au nanoplates in mass fractions around 10−2 wt%. The resulting nanofluids are stable for weeks, and their enhanced physical properties make them good candidates for the application. Particularly, with 4.8·10−2 wt% of Au nanoplates, specific heat increases by 12.0 ± 1.2 % at 523 K and thermal conductivity increases by 24.9 ± 6.1 % at 373 K, with no measurable changes in density or dynamic viscosity. This set of physical properties allows to make a realistic estimation of the performance of a prototypical concentrating solar power plant using these nanofluids for solar-to-thermal energy conversion. We determine, using computational cost-free numerical models available in literature, that the performance of a concentrating solar power plant could increase up to 35.1 %, compared to the predicted 24.7 % with the conventional heat transfer fluid, with neither rheological penalties nor economically prohibitive structural changes. The findings here reported may contribute to encourage the application of heat transfer nanofluids in order to improve the efficiency of concentrating solar power plants, and to consolidate a working scheme that positively promotes the transition from laboratory scale to industrial scale. © 2023 The Author

Novel WS2-based nanofluids for concentrating solar power: performance characterization and molecular-level insights

Nano-colloidal suspensions of nanomaterials in a fluid, nanofluids, are appealing because of their interesting properties related to heat transfer processes. Whilst nanomaterials based on transition metal chalcogenides (TMCs) have been widely studied in catalysis, sensing, and energy storage applications, there are few studies of nanofluids based on TMCs for heat transfer applications. In this study, the preparation and analysis of nanofluids based on 2D-WS2 in a typical heat transfer fluid (HTF) used in concentrating solar power (CSP) plants is reported. Nanofluids prepared using an exfoliation process exhibited well-defined nanosheets and were highly stable. The nanofluids were characterized in terms of properties related to their application in CSP. The presence of WS2 nanosheets did not modify significantly the surface tension, the viscosity, or the isobaric specific heat, but the thermal conductivity was improved by up to 30%. The Ur factor, which characterizes the thermal efficiency of the fluid in the solar collector, shows an enhancement of up to 22% in the nanofluid, demonstrating great promise for CSP applications. The Reynolds number and friction factor of the fluid were not significantly modified by the addition of the nanomaterial to the HTF, which is also positive for practical applications in CSP plants. Ab initio molecular dynamics simulations of the nanoparticle/fluid interface showed an irreversible dissociative adsorption of diphenyl oxide molecules on the WS2 edge, with very low kinetic barrier. The resulting ‘decoration’ of the WS2 edge dramatically affects the nature of the interface interactions and is therefore expected to affect significantly the rheological and transport properties of the nanofluids

Investigation of enhanced thermal properties in NiO-based nanofluids for concentrating solar power applications: A molecular dynamics and experimental analysis

Nanofluids could be a promising alternative to the typical heat transfer fluids (HTF) used in concentrating solar power. This study analyses nanofluids based on a typical HTF for concentrating solar power (CSP) applications and NiO nanoparticles. The optimum nanoparticle concentration was determined by analysing the stability of the nanofluids. Some of their properties, such as density, viscosity, isobaric specific heat and thermal con- ductivity, were characterized to evaluate their performance. Their thermal conductivity increased by up to 96% and the heat transfer coefficient by 50%. Molecular dynamics calculations were performed to explain from a molecular perspective how the presence of equal proportions of two surfactants, benzalkonium chloride (BAC) and 1-Octadecanethiol (ODT), enhanced the thermal properties of the NiO nanofluid. The isobaric specific heat and thermal conductivity values followed the same experimental tendency. The analysis of the radial distribu- tion functions (RDFs) and spatial distribution functions (SDFs) revealed an inner layer of base fluid and sur- factant molecules around the NiO cell. This first layer contained BAC molecules at all the temperatures, while ODT was only incorporated at higher temperatures. The exchange of surfactant and base fluid molecules around the NiO as the temperature increases may play an important role in the enhancement of the thermal properties

MoS2 nanosheets vs nanowires: preparation and theoretical study of highly stable and efficient nanofluids for Concentrating Solar Power

The nano-colloidal suspension of nanomaterials in a base fluid, typically named a nanofluid, is a promising system that shows interesting properties, such as those related to heat transfer processes. Obtaining nanofluids with high stability is a priority challenge for this kind of system. So, a rationalization of the preparation of nanofluids is clearly needed. Thus, this study presents a methodology based on liquid phase exfoliation that makes it possible to prepare stable nanofluids and control the morphology of the nanostructures, which is defined by the surfactant used. Two stable nanofluids were prepared based on MoS2 nanosheets and MoS2 nanowires and a typical heat transfer fluid (HTF) used in high temperature applications. Periodic-Density Functional Theory (periodic-DFT) calculations were performed to rationalize why different nanostructures were obtained according to the surfactant used. Finally, enhancements in thermal properties were found, being up to 57% for thermal conductivity and up to 7.5% for isobaric specific heat. Therefore, these nanofluids are a promising alternative to the typical HTF used, which is a eutectic mixture of biphenyl and diphenyl oxide. Also, to our knowledge, controlling the nanostructures obtained and the rationalization of the methodology for the preparation of stable nanofluids is reported for the first time. This leads to highly stable nanofluids with improved thermal properties, promising for application in concentrating solar power