The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Characterization and Aging Studies of Selective Solar C/Al2O3/Al Absorber Surfaces

Petri Konttinen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Engineering Physics and Mathematics for public examination and debate in Auditorium F1 at Helsinki University of Technology (Espoo, Finland) on the 23rd of April, 2004, at 12 o'clock noon.

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Dissertation is also available in print (ISBN 951-22-7002-1)

Abstract

Solar thermal collectors are mainly used for domestic water and space heating. They capture incident solar radiation, convert it to usable thermal energy, and transfer the energy into a heat transfer fluid. All of this should be accomplished economically with minimal energy loss. One of the most important components of the solar thermal collector is the solar absorber. To be effective, the absorber should exhibit wavelength selectivity, i.e. have maximum solar absorptance and minimum thermal emittance. Selective solar absorbers have been studied intensively since the 1950's. State-of-the-art sputtered selective solar absorbers have good optical properties and long lifetime. A drawback can be high manufacturing costs.

The main purpose of this thesis was the characterization and improvement of a mechanically-manufactured selective C/Al2O3/Al absorber surface. The manufacturing method is the only one based on solely mechanical treatment. The optical properties and microstructure of surface samples were analysed. Together with an industrial partner the manufacturing methods were refined. Comprehensive accelerated aging studies were carried out for the absorber surface.

As a result the solar absorptance and the thermal emittance were improved to 0.90 and 0.22, respectively. The microstructure of the surface is composed of microgrooves and unhomogeneous carbon, graphite or graphite/alumina clusters. Inside a glazed collector a service lifetime between 20 and 25 years can be expected. The main degradation mechanism found was hydration of Al2O3 if condensed water is present on the surface at an elevated temperature. For very humid climates, an additional moisture barrier would be advisable even for glazed collector applications. For non-glazed applications moisture resistance needs to be improved.

The price of the required manufacturing infrastructure for the C/Al2O3/Al absorber varies. It may be very low for manual manufacturing up to some tens of thousands euros for a more sophisticated mechanical workshop. Optical properties and energy yield of the C/Al2O3/Al absorber are in the same range as the best commercial spectrally selective paints, but lower than sputtered surfaces. Economically the C/Al2O3/Al absorber may compete with selective and non-selective paints in most glazed applications.

This thesis consists of an overview and of the following 7 publications:

  1. Konttinen P., Lund P. D. and Kilpi R. J., 2003. Mechanically manufactured selective solar absorber surfaces. Solar Energy Materials and Solar Cells 79, number 3, pages 273-283. © 2003 Elsevier Science. By permission.
  2. Konttinen P., Kilpi R. J. and Lund P. D., 2003. Microstructural analysis of selective C/Al2O3/Al solar absorber surfaces. Thin Solid Films 425, numbers 1-2, pages 24-30. © 2003 Elsevier Science. By permission.
  3. Konttinen P. and Lund P. D., 2002. Characterization of selective absorbers prepared through a mechanical treatment. Invited lecture, in Proceedings of the World Renewable Energy Congress VII. Cologne, Germany, 29 June - 5 July, 2002. © 2002 Elsevier Science. By permission.
  4. Konttinen P. and Lund P. D., Thermal stability and moisture resistance of C/Al2O3/Al solar absorber surfaces. Solar Energy Materials and Solar Cells, in press. © 2004 Elsevier Science. By permission.
  5. Konttinen P. and Lund P. D., 2004. Microstructural optimization and extended durability studies of low-cost rough graphite-aluminium solar absorber surfaces. Renewable Energy 29, number 6, pages 823-839. © 2004 Elsevier Science. By permission.
  6. Konttinen P. and Lund P. D., Physical interpretation of impacts from a low cost manufacturing process on the surface microstructure of a novel solar absorber. Solar Energy Materials and Solar Cells, accepted for publication. © 2004 by authors and © 2004 Elsevier Science. By permission.
  7. Konttinen P., Salo T. and Lund P. D., Corrosion of unglazed rough graphite-aluminium solar absorber surfaces in simulated acid and neutral rain. Solar Energy, submitted for publication. © 2004 by authors and © 2004 Elsevier Science. By permission.

Keywords: solar energy, solar thermal absorber, accelerated aging, mechanical manufacturing

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© 2004 Helsinki University of Technology

Last update 2011-05-26