Concentrating Solar Thermal (CST) Technologies rely on mirrors or lenses to focus and concentrate solar irradiation to heat a receiver with the end of exploiting the created high temperature energy for a variety of purposes (electricity, chemical processes, heating, etc.). The most commonly employed concentrators are silvered-glass reflectors. Their high reflectance determines the optical quality of the system and with that its energy conversion efficiency. In commercial projects a high number of these reflectors, covering a large area, comprise the solar field for projects delivering energy in the order of hundreds of megawatts. As degradation of the reflectors during their service life may decrease their reflectance, the detection of possible degradation is crucial for operation and maintenance in commercial projects [1]. In addition, for the development of new, superior reflector materials, as well as quality control, detailed knowledge of degradation processes and evolution, is a key factor. Nowadays much of the research in this field is based on the analysis of small samples (around 10x10 cm2 as a standard measure) with advanced techniques (microscopy, reflectance measurements, etc.) in the laboratory [2]. The here presented work takes a different approach, investigating the use of a portable USB-microscope for in-field measurements of microscopic defects.

In a campaign performed on the CIEMAT-Plataforma Solar de Almería in southern Spain, images of microscopic defects are taken on outdoor samples and entire facets of different types of solar reflectors, followed by the analysis of these defects. The campaign includes the measurements during several years under realistic exposure conditions. Main questions addressed during this campaign are: the possibility to detect different types of defects and the limits of detection, evolution of the degradation in time, modelling growth of different defect types. It is shown that this kind of analysis technique is not a substitution but an important supplementary approach to traditional laboratory based techniques, with the benefit of allowing in-situ measurements without the removal of samples or facets.