Solar selective coatings based on carbon: transition metal nanocomposites

The design of an efficient and stable solar selective coating for Concentrating Solar Power central receivers requires a complex study of the materials candidates that compose the coating. Carbon-transition metal nanocomposites were studied in this work as absorber materials because they show appropriate optical properties with high absorption in the solar region and low thermal emittance in the infrared. Furthermore metal carbides are thermal and mechanical stable in air at high temperatures. In this work a solar selective coating was grown by a dual source filtered cathodic vacuum arc. The complete stack consists on an infrared reflection layer, an absorber layer of carbon-zirconium carbide nanocomposites and an antireflection layer. The aim of this research is optimize the absorber layer and for that, the metal content was controlled by adjusting the pulse ratio between the two arc sources. The elemental composition was determined by Ion Beam Analysis, X-Ray diffraction measurements show the crystal structure and the optical properties were characterized by spectroscopic ellipsometry measurements. The reflectance spectra of the complete selective coating were simulated with the optical software CODE. Bruggeman effective medium approximation was employed to average the dielectric functions of the two components which constitute the nanocomposite in the absorber layer. The optimized coating exhibited a solar absorptance of 95.41% and thermal emittance of 3.5% for 400°C. The simulated results were validated with a deposited multilayer selective coating.

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