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Si us plau utilitzeu sempre aquest identificador per citar o enllaçar aquest document: https://hdl.handle.net/2445/188000
Nitrate salt-based nanofluids for thermal energy storage
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[eng] 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.
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Matèries (anglès)
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SVOBODOVA, Adela. Nitrate salt-based nanofluids for thermal energy storage. [consulta: 25 de novembre de 2025]. [Disponible a: https://hdl.handle.net/2445/188000]