Material and collector development

To achieve a high optical efficiency, the reflectors in a concentrating collector must track the sun as precisely as possible. It is even possible to optimize the level of illumination across the surface of a receiver. The services offered by Fraunhofer ISE in this area comprise:

• Creation of algorithms that calculate optimal orientation, taking into account the collector’s geometry and orientation, the desired location and other site-specific parameters
• Setup and programming of the tracking control system
• Development of processes to detect and correct stray light on the receiver during operating conditions
• Analysis of influences on the optical and thermal efficiency using ray tracing software, e.g. tracking intervals and inaccurate tracking
• Calculation of the mechanical loads to be expected, and design and selection of tracking components

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.

In addition to the absorber tube, the receiver of a Fresnel collector comprises a secondary concentrator. This guides rays, which do not strike the absorber tube directly, onto the tube by means of additional reflection. The secondary concentrator should therefore be as highly solar reflective as possible. At the same time it must be dimensionally stable at the high temperatures of up to around 300°C which prevail in the receiver, without losing any of its optical properties.

We are developing coating systems which meet these requirements. As with absorber development, durability tests and parallel measurements using AES and SEM provide information about degradation processes and subsequent development steps.

In recent years, Fraunhofer ISE has developed a methodology for modeling and optimizing the technical and economic characteristics of collectors used in concentrating solar power (CSP) plants to tailor and develop them for specific project requirements. In doing so, it is essential that the technical changes made to the collectors also have a positive economic impact.

Ray-tracing simulations are used to model the collector's yield in a process using the Raytrace3D simulation tool developed at Fraunhofer ISE. The optical efficiency is calculated according to the system's geometry, the material parameters measured and its optical accuracy or tolerances, the latter of which are calculated depending on the sun's position. The heat loss model is prepared using computational fluid dynamics (CFD) calculations or by taking measurements.

Taking the validated collector model as a basis, the energy yield of a collector working at a specific location can now be calculated. By linking this to a power plant model, it is also possible to deduce the electrical energy yield, which is a crucial factor when deciding which collector technology to use.

When optimizing collector design, it is essential that any changes made to the parameters (e.g. changes to the geometry) are accompanied by an appropriate effect on the system's costs. In addition to criteria such as optical, thermal or electrical efficiency, the specific costs of generating electricity and heat may be drawn upon when making design-related decisions, provided that plausible cost functions are taken into account.