EuroSun2008-5

Optical Concentration by Primary Mirror Field and Secondary Concentrator

The concentrator optics of the LFC consists of the mirror field of elastically bent primary mirrors having a focal length - depending on the actual collector - of several meter, and a secondary concentrator. Figure 4 shows schematically the configuration of a single tube collector. The primary mirrors may have different focal lengths and therefore different curvature radii. Radiometric flux density measurements at the aperture of the secondary concentrator may reveal the optical efficiency of the mirror field for a certain sun position. An alternative method we used for characterizing the incident flux is the photometric evaluation of the focal line on a calibrated white target at the receiver aperture. However, it is quite time consuming to evaluate sufficient daily and seasonally variing sun positions. Moreover, a measurement does not easily give the reason for inadequate focusing.

Therefore we adapted the so-called Fringe reflection Technique (FRT) already used for smaller specularly reflecting objects like lenses and optical glasses to large mirrors. With FRT one may determine the exact shape of a primary mirror as well as local slope and local curvature deviations. Several variations of this methodology have been investigated. One is working with passive (printed) patterns, another with dynamically generated active patterns using a computer LCD display or a beamer plus projection area. The principle can be used indoors in the laboratory or production, and outdoors with more variable light conditions and is described in detail by Heimsath [5]. After having qualified a statistically significant number of mirrors one may simulate by raytracing the optical efficiency of the mirror field for every sun position.

In order to characterize the secondary receiver we developed a method yielding the acceptance for radiation coming from each different primary mirror. Due to the reversibility of light paths one may take pictures of the absorber images from the primary mirror position and use these for the evaluation of acceptance as a function of incidence angle. For unambiguous evaluation we developed a colour - coded cover for the absorber tube. Weighting the reflected images by the reflectivity of the secondary mirror material we arrived at an angular acceptance plot which we compared with raytracing.

A series of optical tests were described which can be used to evaluate the optical performance of a Linear Fresnel Collector. An example calculation of the levelised electricity costs for a fictitious solar power plant using the LFC was performed using different parameters for optical concentration quality. An average statistical error including mirror shape, scattering, tracking and torsion was taken to show the influence of optical quality.