<p>This thesis deals with the preparation, optical characterization and analyses of experimental work and theoretical modeling on selective solar absorbers used in solar thermal collectors. The overall goal has been to obtain efficient absorbers by optimizing the optical properties, and to improve their durability using cost-effective techniques.</p><p>A Ni-Al<sub>2</sub>O<sub>3</sub> absorber was pyrolytically coated with SnO<sub>2</sub> to improve its coating quality. Undesirable increase of solar reflectance obtained as a result of the SnO<sub>2</sub> was reduced by applying a silica antireflection layer produced by a dipping technique from colloidal silica sol. Annealing of Ni particles in an Al<sub>2</sub>O<sub>3</sub> matrix was also carried out and compared to particles heated without the matrix. Due to the Al<sub>2</sub>O<sub>3</sub> matrix, a much slower oxidation rate was found for the embedded particles. In addition, the optical performance of commercial Ni-Al<sub>2</sub>O<sub>3</sub> and Ni-NiO<sub>x</sub> absorbers were experimentally compared at oblique incidence. A better solar-absorptance of the Ni-Al<sub>2</sub>O<sub>3</sub> at higher angles of incidence was found. This is due to enhanced optical interference in the double-layer structure of Ni-Al<sub>2</sub>O<sub>3</sub>, which could not be achieved in the graded index film of Ni-NiO<sub>x</sub>.</p><p>The optical properties of Si-Al<sub>2</sub>O<sub>3</sub> films of different thicknesses have been investigated by preparing the films using an integral coloration method. The solar-absorptance and thermal-emittance were found to increase with increasing film thickness. Due to high thermal-emittance, the Si-Al<sub>2</sub>O<sub>3</sub> coating shows non-selective absorbing properties. Its feasibility for a selective solar absorber was studied by modeling the coating as a function of coating thickness for different particle size and volume fraction using four-flux theory. The results indicated that the Si-Al<sub>2</sub>O<sub>3</sub> coating is not a suitable candidate for selective solar absorbers.</p><p>Scattering and absorption cross-sections of FeMnCuO<sub>x</sub> and black carbon pigments have been obtained from reflectance and transmittance measurements in the solar wavelength range. The cross-sections were determined by using pellets consisting of low pigment volume fractions dispersed in KBr matrix. The cross-sections exhibit linear dependence of the volume fraction, indicating that single scattering dominates. The cross-sections were used to model the optical properties of solar selective paints using four-flux model resulting in good agreement between calculations and experiments.</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:uu-1065 |
| Date | January 2000 |
| Creators | Tesfamichael, Tuquabo |
| Publisher | Uppsala University, Department of Materials Science, Uppsala : Acta Universitatis Upsaliensis |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, comprehensive summary, text |
| Relation | Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, 1104-232X ; 556 |
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