Evaluation of reflectance measurement techniques for artificially soiled solar reflectors: Experimental campaign and model assessment

The soiling of heliostats is a highly detrimental process that affects the energy conversion efficiency of solar tower power plants as soiled heliostats suffer significant drops of reflectance. Tools that allow reliable soiling measurements and predictions are needed to accurately assess the induced power losses and to develop strategies that mitigate the related detrimental effects. The results of an experimental campaign conducted at the Fraunhofer ISE laboratories are presented and discussed in this paper. Four different techniques were used to measure the reflectance of artificially soiled mirrors: (i) measurements obtained with a portable specular reflectometer 15R-RGB (D&S), (ii) spectral analysis of the outcomes of a spectrophotometer, (iii) post-processing analysis of the measurements obtained with the device VLABS (very low angular beam spread), and (iv) microscope-captured image processing. A detailed comparison between the measurements obtained with each device is performed to evaluate the agreement among them and the motivations of slightly different trends in the results. Finally, the outcomes of the analyses are proved to give further strength to the validation of a previously developed solar reflectors soiling model.