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Abstract
Solar thermal energy has a vital role to play in meeting tomorrow’s energy demand, particularly where heat is desired with minimum environmental impact (e.g. building HVAC, industrial process heat, refining minerals, etc.). Due to its high efficiency, solar thermal energy production significantly exceeds photovoltaic energy production today, although both technologies still have room for improvement. By bringing recent advancements of nanofabrication and nanotechnology to bear on solar thermal technology, it is possible to absorb sunlight directly inside the working fluid. This represents a simple, direct path from sunlight to useful heat. Unfortunately, the design of these beguilingly simple systems is rather complicated. Optics, heat transfer, fluid mechanics, and materials science issues are highly coupled, yielding an intricate design space. While some preliminary work on this topic has been reported in literature, a systematic modelling and design of such direct absorption collectors (DACs), using accurate tools, has not yet been performed. As such, this research work investigated the performance limits of nanofluid-based DACs and optimised them to develop a novel solar absorber which is competitive with conventional surface-based solar thermal absorbers.
First a set of theoretical tools and experimental methods, essential for studying nanofluid based DACs were developed. These tools include important improvements from much of the published literature on DACs, enabling sound, detailed design and accurate determination of DAC performance.
This is followed by a parametric study investigating the fundamental limits of selective solar absorption (maximising solar absorption and minimising emissive losses) by a DAC. For this, merit functions are derived that measure the selective performance of DAC components. It is clearly shown that (ideally) nanoparticles should strongly absorb solar radiation with minimum scattering while the basefluid should not cause long wavelength emissive losses. The cover material should let most of the solar radiation through while being able trap long wavelength radiation inside the collector.
Using these fundamental limits as guidelines, the current study optimises and compares real materials for nanoparticles, basefluids, and covers. Optimisation processes are introduced to determine nanoparticle and cover parameters that maximises DAC performance. Multi-walled carbon nanotubes were found to be the best nanoparticle among current options, resulting in a strongly absorbing and cost effective nanofluid. Several other particles commonly considered in previous literature were deemed unsuitable when scattering is taken into account. All common heat transfer liquids were found to inherently, strongly emit long wavelength radiation, necessitating an approach which include optically selective covers. Thin film depositions of ZnS-Ag-ZnS and tin doped indium oxide were proposed as a solution to make common cover materials perform selectively, with their optimum film parameters depending on collector operating conditions. In outdoor tests with the optimum materials/designs defined herein, it was found that DACs made from current materials are still unable to compete with selective coatings like Black Chrome and TiNOx.
Overall this research provides engineering tools, valuable insights, and identifies key areas for improvement in the field of solar thermal energy harvesting. It is hoped that these tools can be utilised and refined further as materials develop to further optimise DACs.