Nitrate salt-based nanofluids for thermal energy storage

Abstract

For the development of more compact and efficient thermal systems, nanofluids are presented as a promising option to replace conventional heat transfer fluids. Nanofluids are a suspension of low concentration of nanoparticles (1-100 nm) in a liquid medium. Since nanofluids' discovery in 1995, a veritable science has been created around this concept with an infinity of new applications such as electronics, biomedicine, machining, and renewable energies. The scientific interest in nanofluids is due to the improvement of the base fluid's thermophysical and chemical properties. One of the phenomena that have attracted the interest of the scientific community is the anomalous variation in heat capacity, with increases of up to 40 % in different liquid media such as water, thermal oils, paraffin, or molten salt. However, despite many efforts and significant advances, still no robust theoretical framework explains the observed phenomena. Therefore, there is a lack of understanding of the behaviour of nanofluids and their properties. Among the different types of heat transfer fluids, molten salt-based nanofluids (MSBNFs) show exceptional properties with the incorporation of nanoparticles. For this reason, one of the potential applications is in thermal storage systems (TES) for concentrated solar power plants (CSP) that use nitrate salts (eutectic mixture of NaNO3 (60 %)-KNO3 (40 %)) as a TES medium and heat transfer fluid (HTF). MSBNFs with higher energy density than conventional salts would allow the design of more compact TES systems and therefore improve the overall efficiency of CSP plants and reduce their LCOE. Economically competitive CSP technology is essential for the energy transition and solving renewable energy intermittency by storing solar energy during the day and dispatching it at night. In addition, these plants are an ideal complement to PV technology, resolving the imbalance between maximum demand and renewable production. However, for the scalability of nanofluids to higher TRLs, many scientific and economical barriers still must be overcome. In this thesis, through a review of the state-of-the-art and a bibliometric study, this field of research's social and scientific impact has been shown, identifying the main limitations and obstacles that the scientific community is subjected to. Throughout the chapters, answers to each of the key points that limit the development of nanofluids and especially nanofluids based on nitrate salts, are given from a computational, statistical and experimental point of view. The main result of the thesis is the identification of the main mechanisms involved in the variation of the heat capacity that respond to the contradictory results of the literature. It has been determined that nanofluids based on nitrate salts behave as a biphasic system due to the formation of nanostructures with a high specific area around the surfaces of the nanoparticles, demonstrating the layering phenomenon observed computationally. Furthermore, the formation of the nanostructured phase with a heat capacity higher than 100 % with respect to the nitrate salt is very susceptible to the concentration and dispersion

of the nanoparticles. Therefore, these variables play a fundamental role in the value of the heat capacity of the system, where slight variations have a significant impact on the variation of Cp. Additionally, the last chapter of this thesis shows the potential of the Small Angle X-ray Scattering (SAXS) technique to characterize nanofluids as a function of temperature, offering relevant structural information to understand their thermal behaviour. Finally, without pointing out the final application, this thesis shows the potential of nitrate salt-based nanofluids as a TES medium for improving thermal efficiency and the possible reduction of the volume of storage tanks, consequently reducing storage costs.

Para el desarrollo de sistemas térmicos más compactos y eficientes los nanofluidos se presentan como una opción prometedora para la sustitución de los fluidos de transferencia térmica convencionales. Los nanofluidos son una suspensión de bajas concentraciones de nanopartículas (1-100 nm) en un medio líquido. El interés científico de los nanofluidos se debe a la mejora de las propiedades tanto termo-físicas como químicas del fluido base. Uno de los fenómenos que más han llamado la atención a la comunidad científica es la variación anómala de la capacidad calorífica, con incrementos de hasta el 40 % en diferentes medios líquidos. Una de las aplicaciones con gran potencial son los sistemas de almacenamiento térmico (TES) para plantas de concentración solar (CSP), donde el empleo de nanofluidos basados en sales fundidas permitirían diseñar sistemas TES más compactos y por tanto mejorar la eficiencia general de las plantas CSP. Pero para la escalabilidad de los nanofluidos a mayores TRLs todavía se han de solventar numerosos obstáculos científicos como barreras económicas. Una de las principales limitaciones es la falta de un marco teórico robusto que de explicación a los fenómenos observados. A lo largo de los capítulos se van dando respuesta desde un punto de vista tanto computacional, como estadístico y experimental, a cada uno de los puntos clave que limitan el desarrollo de los nanofluidos y especialmente de los nanofluidos basados en sales de nitrato. El principal resultado de la tesis es la identificación de la formación de fases de altas áreas especificadas con elevados valores de capacidad calorífica como el principal mecanismo involucrado en la variación de la capacidad calorífica, que dan respuesta a los resultados contradictorios de la literatura. Adicionalmente, se muestra el potencial de la técnica de Dispersión de Rayos-X de Ángulo Pequeño para caracterizar nanofluidos. Finalmente, sin perder de vista la aplicación final, se muestra el potencial del uso de nanofluidos basados en sales de nitrato como medio TES, para la mejora de la eficiencia térmica y consecuentemente la reducción de los costes de almacenamiento.