Nanofluids for low-temperature solar thermal applications

Abstract

Solar energy, as a sustainable, clean, and non-polluting source of energy, is a perfect solution for addressing the growing energy crisis and the harmful impacts of fossil fuels. To efficiently utilize solar energy, this work evaluated the ability of various types of advanced nanofluids (NFs) to absorb solar radiation and convert it volumetrically to thermal energy for use in direct absorption solar thermal collectors. Three nanomaterials were synthesized and re-dispersed in ethylene glycol (EG) as the base fluid to form single-component NFs (sf-Au NPs/EG, sf-CuOx NS/EG, and CNTs/EG), as well as a hybrid of Au NPs-CNTs/EG.

The sf-Au NPs, synthesized via plasma treatment by generating atmospheric-pressure microplasma on the surface of an aqueous gold salt solution, exhibit a wide variety of sizes and shapes, while representative surfactant-based Au NPs/EG showed uniform spherical particles. As a result, sf-Au NPs/EG had relatively lower plasmonic absorption peaks but broader across the visible spectrum. At 0.0002 vol.% (39.4 mg/L), sf-Au NPs/EG outperformed surfactant-based Au NPs/EG NFs by approximately 3%, achieving a solar thermal conversion (STC) efficiency of 22.6%.

On the other hand, sf-CuOx NS, also synthesized via plasma treatment, exhibit aggregates composed of closely packed nanorod-like shapes, while CNTs, produced by floating-catalyst chemical vapour deposition, showed an entangled network of multi-walled CNTs. Across the visible spectrum, the not-annealed (NA) and annealed (AN) sf-CuOx NS/EG had exceptional absorption coefficient close to 8 cm-1at 0.01 vo.% of the AN NF, as well as relatively low scattering. At a lower concentration of 0.0018 vol.%, CNT/EG also exhibited a high absorption coefficient of 1-3 cm-1. These two NFs, CNT/EG at 0.0018 vol.% and AN sf-CuOx/EG at 0.01 vol.%, reached STC efficiency of 41.5% and 72%, respectively,
as the former represents the highest STC achieved by NFs in this work. sf- uOx/EG also demonstrated long-term stability over a 10-months span as well as under cyclic heating.

These findings highlight the benefits of using plasma-induced non-equilibrium electrochemistry (PiNE) as a potentially advantageous alternative method for the preparation of highly efficient, stable nanofluids, particularly metal oxides, for use as volumetric solar absorbers in solar thermal applications. These nanomaterials can effectively compensate for the extremely low light absorption of common transparent fluids, particularly in the visible light spectrum, which accounts for approximately 50% of solar energy. Additionally, this research demonstrates that the polydispersity of nanoparticles as well as the absence of surfactants significantly improve the STC performance of nanofluids used as volumetric absorbers. In addition, numerical simulation of the STC process in nanofluid-based direct absorption solar thermal collectors (DASCs) is highly desirable for gaining a deep understanding of the heat and light interaction mechanisms within nanofluids, as demonstrated in this work for sf-CuOx NS/EG, which can be beneficial in optimising DASC designs.

Date of Award	Aug 2021
Original language	English
Supervisor	Paul Maguire (Supervisor) & Davide Mariotti (Supervisor)