Quality Control of Concentrating Collector Components for the Optimization of Performance
W. J. Platzer, A. Heimsath, C. Hildebrandt, A. Georg, G. Morin
Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany
Corresponding Author, werner. platzer@fse. fraunhofer. de
In order to accompany the demonstration of a full-size Linear Fresnel Collector (LFC) Fraunhofer ISE has developed characterization techniques to qualify optical and thermal key components within the projects »FRESQUALI« and »FRESNEL2« funded by the German Ministry of Environment, Nature Conservation and Nuclear Safety (BMU).
One key component for the LFC is the absorber tube with a Cermet coating, which has to be stable under atmospheric conditions for temperatures up to 450°C. Spectral emissivity and absorptivity changes have to be evaluated. Experiments show that coatings can improve due to heat treatment compared to freshly sputtered coatings.
Mirror optics using nearly flat primary glass mirrors and a secondary concentrator for the receiver construction are used to achieve a geometrical concentration in the order of 30-50 with respect to tube circumference. Deformations due to construction, internal stress of glass, wind load and gravity may lead to a distortion of the narrow focus line on the receiver. The acceptance angle of the secondary concentrator may deteriorate due to shape imperfections. Deterioration of specular reflectance due to dust and degradation also reduces performance. Several qualification techniques were used to support quality control for production and technical development.
Collector performance and possible improvements are discussed together with a comparison to parabolic trough collectors.
Keywords: Optical Testing, Thermal Testing, Durability, Optimization, Quality Control, Parabolic Trough, Linear Fresnel
Having shown in previous feasibility studies [1,2] that the Linear Fresnel Collector (LFC) has a potential to generate direct steam sufficiently cheap to reduce the expected levelised electricity costs (LEC) of the solarthermal power plant by about 20% when compared to the parabolic through, the Fraunhofer Institute for Solar Energy Systems started developing key components for this new collector type with the support of the German Ministry of Environment, Nature Conservation and Nuclear Safety (BMU) within the project »FRESNEL2«. The aim was to produce an absorber coating being stable under atmospheric conditions up to temperatures of 450°C. Another point of concern was the bent mirror for the secondary concentrator which has to withstand temperatures between 250°C and 280°C. It is important for a economical viability of the LFC that the optical and thermal efficiency of the collector can be achieved in practice as simulated in the previous studies. Also the performance
should be stable over a long period of time, if not for the complete lifetime of a solar field. In our finished project mirror and absorber coatings with these specifications could be developed.
With the help of a second project »FRESQUALI«, also funded by the German Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU), we developed further characterization and qualification methodologies which were used and tested using a local Fresnel process heat collector in Freiburg. They have been used also to assist the building of a LFC prototype on the Plataforma Solar de Almeria (see Figure 1). This paper describes the general approach and gives an overview of methodologies for quality assessment for the linear Fresnel collector and its subcomponents in laboratory or production phase. The design, construction, on-site quality control, commissioning and performance testing of the collector is presented elsewhere. [3,4]
Figure 1: View of the Fresnel demonstration collector by Solar Power Group and MAN Ferrostaal Power
Industry at the Plataforma Solar de Almeria.