Optical thin film stacks integrating spectral and angular control of solar energy and thermal radiation

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The objective of this project is to enhance the efficiency of photo-thermal conversion by improving the optical and other properties of solar-absorbing surfaces. Designing a suitable coating for these surfaces involves a delicate balance between thermal stability, reflectance and emittance. As an added complication, it is necessary to have a coating with a spectral response that switches from highly absorptive in the visible and near-IR range to reflective at longer wavelengths. Despite extensive prior investigations in this area, there are still several problems that remained unsolved — in particular the maintenance of structural integrity and optical response of solar absorbers operating at higher temperatures (>500°C). The results of the present work are highly relevant to various kinds of high temperature concentrated solar power (CSP) applications as well as to thermo-photovoltaic (TPV) systems. A number of advanced new spectrally-selective solar absorbers: Al/AlN/Au-AuAl₂:AlN/AlN/SiO₂, Al/AlN/Au-AuAl₂:AlN/AlN/ Au-AuAl₂:AlN/AlN/SiO₂, Pt/AlN/TiAlN/ AlN/SiO₂, Pt/Ta:SiO₂/Ta:SiO₂/AlN/SiO₂ and Ta/Ta:SiO₂/ Ta:SiO₂/AlN/SiO₂ were investigated. All were produced by magnetron sputtering, and their optical properties and thermal stability assessed. This work has shown that the Au-based solar absorbing structures are strongly oxidation-resistant, however, their exploitation in CSP applications is currently limited due to coarsening and agglomeration of the Au inclusions in the dielectric host temperatures greater than 400°C. A solution to this problem is proposed : the Au nanoparticles in the cermet layer are allowed to alloy with Al. This converts them to the intermetallic compound AuAl₂, which is considerably more resistant to coarsening than pure gold. This was achieved by an introduction of the Al substrate to serve both as an IR-reflecting layer and as a source of the Al species to form more structurally and temperature stable AuAl₂ nanoparticles in the AlN host. The alloying process was thermally induced at 200°C and was finalised at 500°C, where alloying of all Au inclusions present in the matrix was achieved. The resultant new structure was able to endure 168 hours annealing in vacuum at 500°C without major change. Such stability has apparently not been achieved before for Au-based solar absorbers. Furthermore, the AuAl₂ formation was shown to be also beneficial for the solar absorptance (𝛼𝑠) enhancement, leading to an increase in 𝛼𝑠 by 3%, from its initial 92% to a final 95%, while preserving low emittance. Spectrally-selective coatings based on the TiₓAl₁₋ₓN system were also considered due to their known diffusion barrier properties, high thermal tolerance, and very suitable optical properties. The composition of TiₓAl₁₋ₓN, (effectively, the Ti/Al ratio) was selected to achieve a maximized solar absorptance of the overall stack. A tandem absorber, which included top anti-reflective layers, was tested on a stainless-steel substrate in order to see how the stack design would serve in parabolic trough-based power plants that used stainless steel pipe to carry the heat-transfer fluid. The diffusion of the Fe present in stainless steel into the coating is known to normally start at 600°C but this was successfully suppressed in the present work by an application of an AlN diffusion barrier. The whole TiₓAl₁₋ₓN-based stack, despite some structural modifications upon heating up to 900°C, preserved its optical integrity with solar absorptance remaining unchanged at 92%. Finally, a new algorithm for designing a nearly ideal cermet-based spectrally-selective absorber was developed. This enabled achievement of 𝛼𝑠> 97%. There are only a few structures known to absorb solar energy with 𝛼𝑠 in the 97-98% range, however, their optical performance is degraded in the range 250°C-500°C due to surface oxidation, diffusion of the back reflector into the coating, shape and/or phase transformation of the nanoparticles. The result may be a significant drop in solar absorptance down to 84%. The new algorithm was exploited in the present project to design a novel spectrally-selective coating, the heart of which was composed of two absorbing Ta:SiO₂ layers with different Ta content, which showed not only an efficient light absorptance with 𝛼𝑠 = 97.6%, but also preservation of its value up to 900°C with simultaneously improved spectrally-selective performance due to recrystallization of the Pt or Ta back-reflectors. These effects lowered thermal emittance to 0.04 and 0.15 from initial values of 0.18 and 0.21, respectively. The Ta:SiO₂ cermet-based absorber on a Pt reflector showed good thermal stability up to 1000°C, with a minor solar absorptance reduction to 95%, but high enough for an enhanced photo-thermal conversion. This would appear to be an unprecedented degree of stability at 1000°Ç for a cermet-based solar absorber. In summary, this project has resulted in the development of new stack designs for use in high temperature conversion devices. A new way of enhancing thermal stability in a Au-based coating has been discovered, and a new procedure for designing coatings demonstrated.
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