The black absorbers of parabolic-trough solar collectors are surrounded by borosilicate glass tubes in the focal line of the reflector. These tubes reflect a total of about eight per cent of the sunlight. Scientists at the Institut für Nichtmetallische Werkstoffe of the Technical University of Clausthal have recently succeeded in applying a practical and functional antireflection coating which increases the light transmission of these glass tubes by up to seven per cent. If a larger fraction of the incident light reaches the absorber, the efficiency of the solar collectors is higher. The new antireflection coating is currently being tested at a large solar collector installation in California.
The project was executed in cooperation with Schott Rohrglas GmbH, Mitterteich, a subsidiary company of the Schott Glas Group, Mainz, which is active all over the world. Schott Glas had filed an application for a patent on the development, designated as "Glass body with porous coating", with the inventors in Clausthal, Dipl.-Ing. Marta Krzyzak, Dr.-Ing. Gundula Helsch, Privatdozent Dr. habil. Gerhard Heide, and Prof. Dr. Günther H. Frischat, in Europe, the USA, Israel, Japan, and China.
Broad-band antireflection coatings had already been available before, of course, but the insufficient adhesion strength and wiping resistance on borosilicate glass proved to be decisive disadvantages. The coatings did not withstand the rough operation in practice, and the desired effect was no longer present after a short time. The porous silicon dioxide coatings developed at the Technical University of Clausthal, with a thickness of only 110 nanometres (1 nanometre equals one millionth of a millimetre), are applied by the sol-gel dip coating method and burned into the glass at 500 °C. In comparison with other methods of application, the advantage offered by the sol-gel method is the possibility of flawlessly coating the inside of glass tubes, not only sheet glass. A specially developed sol-gel formulation imparts the adhesion strength and wiping resistance to the antireflection coating. Schott Rohrglas has continued the development from the laboratory scale to the production stage: Glass tubes with an antireflection coating and a length of four metres have already been installed in modern solar power stations and are being tested in California.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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