A. Schuler, C. Roecker, J.-L. Scartezzini

Laboratoire d’Energie Solaire et de Physique du Batiment LESO-PB, Ecole Polytechnique Federale de Lausanne EPFL, Batiment lE, 1015 Lausanne, Switzerland J. Boudaden, P. Oelhafen

Institut fur Physik der Universitat Basel, Klingelbergstr. 82, 4056 Basel, Switzerland

Multilayered interference filters of dielectric thin films have been designed for the application as energy-efficient coloration of collector cover glasses. The optical behavior of the designed multilayers is analysed by computer simulations yielding the CIE color coordinates, the relative luminosity, the degree of solar transmission, and a figure of merit which is a measure for the energy effectiveness of the coloration. A high performance should be achieved with a number of individual layers reasonable for large scale deposition. Constraints on the refractive index of the dielectric films are given by the availability of suitable thin film materials. The challenge lies in finding the best combination of material choice and layer thicknesses. We describe several types of multilayer designs for which the computer simulations yield promising results.

Motivation

The issue of color becomes more and more important for thermal solar collectors, and has attracted interest recently [1-3]. This might be related to a generally growing attention towards architectural integration of solar energy systems into buildings [4-7]. A recent opinion poll [1] showed, that 85% of architects would prefer different colors besides black, even if a lower efficiency would have to be accepted. Thermal solar collectors, typically equipped with black, optical selective absorber sheets, exhibit in general good energy conversion efficiencies. However, the black color, and sometimes the visibility of tubes and corrugations of the metal sheets, limits the architectural integration into buildings.

One solution to this problem is to color the absorber sheets. Optical selective absorber coatings are usually deposited by processes such as magnetron sputtering [8-10], vacuum evaporation [11], electrochemical processes [12], sol-gel technology [13], or as selective paint (thickness-sensitive or thickness-insensitive) [3,14]. Niklasson and Granqvist described the pioneering work in this area within a comprehensive overview [15]. Modifying the parameters of the deposition process can result in a colored appearance. Following this approach, the absorber surface combines the functions of optical selectivity (high solar absorption/low thermal emission) and colored reflection. Tripanagnostopoulos reports a different solution: his group used non-selective colorful paints as absorber coatings for glazed and unglazed collectors, and compensated the energy losses by additional booster reflectors [16]. Alternatively, we propose to establish a colored reflection not from the absorber but from the cover glass. This approach has the advantage that the black, sometimes ugly absorber sheet is then hidden by the colored reflection. In addition to that, the functions of optical selectivity and colored reflection are separated, giving more freedom to layer optimization. No energy should be lost by absorption in the coating: all energy, which is not reflected, should be transmitted. Therefore, multilayer interference stacks of transparent materials should be ideally suited for this purpose. A recent feasibility study showed encouraging results [17]. By employing optical methods such as real-time laser reflectometry, spectroscopic ellipsometry and spectrophotometry, the deposition of multilayered interference stacks can be monitored very precisely [18]. In this article we
describe a variety of multilayer designs, which can be employed to achieve the desired characteristics.