Stability of Absorber and Mirror Coatings
Modern selective absorber coatings usually are thin layer systems on metal substrates which selectively absorb short wavelength photons - a process which is enhanced by multiple scattering at small metal particles in a dielectric matrix (so-called Cermet layers). Long wavelengths average over the particle mix and do not resolve the particles of typical size (order of magnitude is 10nm). They experience an effective medium formed by the metal-dieelectric mixture. Interference effects are being used to maximize reflectance in the infrared range. Therefore the exact thickness, the volume fraction of the metal content of layers and the size of particles determines the optical and thermal performance - solar absorptivity and thermal emissivity. The high-temperatures as reached in concentrating collectors during operation on the other hand tend to increase the mobility of atoms in the layers. Therefore interdiffusion might deteriorate simple layer systems. Additional barrier layers therefore are used to restrict mobility. In some cases adhesive layers can improve the bonding stability of the system.
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Figure 2: Change of absorptivity and emissivity of prototype coating
Within the project Fresnel-II Fraunhofer ISE developed a sputtered air-stable absorber coating on stainless steel that resists temperatures up to 450°C. When heated at 500°C after a relaxation time the coating did not change the properties even after weeks of heating. The obtained solar absorptivity is 94% (AM1.5 global) with an emissivity of 18% in respect of a black radiator at 450°C. The application of such a coating is the single-tube absorber of a Linear Fresnel Collector. Of course, other applications are conceivable.
Other thin layer systems can be used to produce mirror coatings for secondary concentrators. These mirrors have to withstand elevated temperatures due to the proximity to the absorber tube. The solar reflectance shall be high, therefore first surface mirrors were the aim of the project. On the basis of highly reflecting silver an optical layer system has been developed and improved. Again barrier and adhesion properties have to be taken into account. Also mirror coatings change their properties slightly upon heating. It could be shown that the reflectivity changes ceased after a few hours when being heated at 150°C and 250°C (which seems to be sufficient for a secondary reflector in a linear Fresnel collector), but not at 350°C. Therefore the application e. g. for tower receivers seems to be problematic. Further work is needed to develop mirror coatings which are stable at even higher temperature loads.
Figure 3: Change of solar reflectance (AM1.5 global) for first surface mirror due to heating
at different temperatures
The durability and resistance against several environmental factors, not only temperature but also UV, humidity, salt and air pollution, is a main concern with regard to the cost effectiveness of solar technologies. Large investments are needed in order to harvest the sun at nearly no cost. Longevity, performance and cost have to be considered in parallel. Innovative complex and possibly expensive new components cannot to be cost effective if their performance degrades after short time. However, cheap products might be replaced when disassembly and replacement can be standardized.
In climatic chambers and similarly in outdoor exposition components can be checked and qualified. Certainly unsuitable materials can be excluded. Relative comparisons between various development options are possible. However it is not trivial to translate results from accelerated indoor testing (using higher loads than experienced in reality) to a quantitative lifetime estimation. Comparisons with actual outdoor weathering need very long time and are not available for new products. Thus risk can only be minimized by understanding the material degradation processes and the real load factors during application. Combining this material and process specific knowledge with carefully designed accelerated indoor tests at least can minimize the risk of failure.