Characterization and aging studies of selective solar C/Al203/Al absorber surfaces

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.