Study of Dust Particles on Solar Mirrors for Measurement of Soiling by Specular Reflectance and Imaging Assessment

During the life time of Concentrated Solar Power plants (CSP), optical performances of solar mirrors are affected by soiling phenomena and surface degradations. In order to provide an adequate cleaning strategy, operators must determine the performance loss induced by soiling. Several commercial instruments already exist to measure optical reflectance, but they are dedicated to a single wavelength range or angle, contact and punctual measurements or to laboratory analyses. CEA has developed a new kind of sensor to measure separately the loss of specular reflectance thanks to a CCD camera and photodiodes. In this study, we compared the cleanliness factor calculated with the specular reflectance measured by commercial devices with the image processing performed with our equipment on different artificially soiled solar mirrors. The aim is to ensure that different levels of dirt on the mirrors can be easily assessed with a camera and image processing. We conclude that the level of soiling and the calculation of the percentage of dirty surface are similar to the measurement of the absolute reflectance for all the mirrors tested. These combinations of non-contact, automated, fast and precise measurement with image processing are reproducible for all levels of soiling.