The core component of a solar thermal power plant is the receiver, with a selectively coated absorber tube on which the solar radiation is focused. Using magnetron sputter processes, at Fraunhofer ISE we are developing absorber coating systems that meet the high standards of temperature resistance required in the collector.
In collaboration with the company Schott, we have developed a highly selective absorber coating for evacuated receivers in parabolic trough power plants. We have also developed a coating system that is stable in air and at temperatures of up to 450°C. This system has been subjected to field tests since 2007 on a 100 m long absorber tube in the Fresnel demonstration collector at the Plataforma Solar de Almería center.
In sputtered, selective, cermet absorber coatings, a metal reflector ensures low emissions in the IR radiation range and thus low thermal losses from the receiver, while a cermet (ceramic-metal compound), together with an anti-reflective layer, is responsible for the high absorption of solar radiation.
By carrying out optical simulations of these thin-film systems, we investigate the potential of different materials regarding their absorption and emission. However, it is equally important that the individual layers are stable at the high operating temperatures. In addition to diffusion processes between the substrate material and the absorber coating, the oxidation of individual layers or constituent materials can also result in degradation of the selective absorber.
We can trace degradation processes very accurately using durability tests and parallel optical measurements. More information about the nature of the degradation is provided by surface and material analyses using SEM (scanning electron microscopy) or AES (Auger electron spectroscopy). This information is important in determining the next steps to be taken in the development process and, if necessary, to counteract degradation processes by means of additional adhesion and barrier layers.