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Specular reflectance of soiled glass mirrors - study on the impact of incidence angles

: Heimsath, A.; Lindner, P.; Klimm, E.; Schmid, T.; Nitz P.; Ordóñez Moreno, K.; Elon, Y.; Am-Shallem, M.; Nitz, P.


Rajpaul, Vikesh (Ed.):
21st SolarPACES International Conference, SolarPACES 2015. Proceedings : 13-16 October 2015, Cape Town, South Africa
New York, N.Y.: AIP Press, 2016 (AIP Conference Proceedings 1734)
ISBN: 978-0-7354-1386-3
Art. 130009, 9 pp.
International Conference on Solar Power and Chemical Energy Systems (SolarPACES) <21, 2015, Cape Town>
Conference Paper
Fraunhofer ISE ()
Solarthermie und Optik; Solarthermie; Gebrauchsdauer von Kollektoren und Komponenten; Solarthermische Kraftwerke

The accumulation of dust and soil on the surface of solar reflectors is an important factor reducing the power output of solar power plants. Therefore the effect of accumulated dust on the specular reflectance of solar mirrors should be understood well in order to improve the site-dependent performance prediction. Furthermore, an optimization of the CSP System maintenance, in particular the cleaning cycles, can be achieved. Our measurements show a noticeable decrease of specular reflectance when the angle of incidence is increased. This effect may be explained by shading and blocking mechanisms caused by dirt particles. The main physical causes of radiation loss being absorption and scattering, the near-angle scattering leads to a further decrease of specular reflectance for smaller angles of acceptance. Within this study mirror samples were both outdoor exposed and indoor artificially soiled. For indoor soiling, the mirror samples were artificially soiled in an in-house developed dusting device using both artificial-standardized dust and real dust collected from an arid outdoor test field at the Negev desert. A model function is proposed that approximates the observed reduction of specular reflectance with the incidence angle with a sufficient accuracy and by simple means for this soil type. Hence a first step towards a new approach to improve site dependent performance prediction of solar power plants is taken.