Contact: Andreas Schueler
Thermal solar collectors exhibit high energy conversion efficiencies and short energy pay back times. A temperature of 80°C is a commonly achieved by thermal solar collectors. According to the Stefan-Boltzmann law, a black body would emit under these conditions some 900 W/m2, which is already in the order of the incident solar energy at ground level. The ideal behavior of an absorber surface is a high absorptance in the solar spectral region and a low emittance in the range of thermal radiation, which implies a step-like reflectance spectrum with R = 0 in the solar region and R = 1 in the infrared range. A surface approaching this property is called optical selective.
In addition to these optical requirements, a solar absorber coating has to withstand elevated temperatures and humidity inside the collector during a lifetime in the order of 25 years. The absorber sheet of unglazed solar collectors is directly exposed to acidic rain attacking especially the low emission properties of the substrate by wet corrosion. It is quite a challenge to create a corrosion resistant low emittance surface while keeping a high solar absorptance. Advanced nanocomposite coatings are developed for glazed collectors aiming at high optical performance and superior aging stability.
Metallic clusters: Selective solar absorber coatings
A chromium-free black selective coating for durable selective solar thermal absorbers has been developped in the framework of Dr Martin Joly’s PhD thesis. As compared to the most durable solar absorber coatings today, the novel black selective nanocomposite coating has the im advantage of a highly superior stability at elevated temperatures in air, combined with an improved corrosion resistance. Its remarkable properties make the novel nanocomposite coating especially interesting for solar thermal electricity generation in large-scale solar power plants.
Patent WO 2014045241 A2, published 27.3.2014 (Industrial partner Zettl GmbH, Munich, Germany) Method for hardening a coating of a solar collector element, and elements produced by means of said method
“Novel black selective coating for tubular solar absorbers based on a sol-gel method”. by Martin Joly, Yannik Antonetti, Martin Python, Marina Gonzalez, Thomas Gascou, Jean-Louis Scartezzini, Andreas Schüler, Solar Energy, Volume 94, August 2013, Pages 233-239 (Best paper award)
PhD thesis 5541, Martin Joly, 2012: Développement et optimisation de revêtements nanostructurés pour capteurs solaires thermiques et modules photovoltaïques