Potential of quarterwave interference stacks for colored thermal solar collectors A. Schu ¨ler a, * , C. Roecker a , J. Boudaden b , P. Oelhafen b , J.-L. Scartezzini a a Laboratoire d’Energie Solaire et de Physique du Ba ˆ timent LESO-PB, Ecole Polytechnique Fe ´de ´rale de Lausanne EPFL, Ba ˆ timent LE, 1015 Lausanne, Switzerland b Institut fu ¨ r Physik der Universita ¨ t Basel, Klingelbergstr. 82, CH-4056 Basel, Switzerland Received 16 April 2004; received in revised form 14 October 2004; accepted 1 December 2004 Available online 25 February 2005 Communicated by: Associate Editor Sanjay Vijayaraghavan Abstract The architectural integration of thermal solar collectors into buildings is often limited by their black color and the visibility of tubes and corrugations of the absorber sheets. A certain freedom in color choice would be desirable, but the colored appearance should not cause excessive energy losses. Multilayered interference filters on the collector glazing can produce a colored reflection, hiding the corrugated metal sheet, while transmitting the non-reflected radiation entirely to the absorber. We investigate the potential of quarterwave stacks by simulation of their optical behavior, yielding the visible reflectance R VIS , the solar transmittance T sol , a figure of merit M = R VIS /R sol , and the CIE color coordinates. The necessary number of individual layers in the multilayer stack as well as the choice of refractive indices and thus of thin film materials are discussed. Finally, examples for realistic multilayer designs are proposed. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Colored solar collectors; Coatings for thermal collectors; Collector glazing; Dielectric thin films; Interference filters 1. Introduction Architectural integration of solar energy systems into buildings has become a widely recognized issue (Hestnes, 1999; Roecker et al., 1995; Hagemann, 2002; Scartezzini and Courret, 2002), which regards techniques ranging from photovoltaics and daylighting to thermal solar energy conversion. Thermal solar collectors, typically equipped with black, optical selective absorber sheets, in general exhibit good energy conversion efficiencies. How- ever, the black color, and sometimes the visibility of tubes and corrugations of the metal sheets, limit the architec- tural integration into buildings. A recent opinion poll (Weiss and Stadler, 2001) showed that 85% of architects would prefer other colors besides black, even if a lower efficiency was the price to pay. Various attempts have been made to overcome this drawback. One option is to color the absorber sheets. Optical selective absorber coatings are usually deposited by processes such as magnetron sputtering (Graf et al., 1997; Schu ¨ler et al., 2000, 2001), vacuum evaporation (Lazarov et al., 1995), electrochemi- cal processes (Tabor, 1955), sol–gel technology (Kaluza et al., 2001), or as selective paint (thickness-sensitive or thickness-insensitive) (Orel et al., 1986; Crnjak Orel 0038-092X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.12.008 * Corresponding author. Tel.: +41 21 693 4544. E-mail address: andreas.schueler@epfl.ch (A. Schu ¨ ler). Solar Energy 79 (2005) 122–130 www.elsevier.com/locate/solener