Analysis of Threshold Conditions for Cementation of Soiling on PV Modules and Telescope Mirrors

To enhance the understanding of the high adhesion of dust particles on photovoltaic modules and telescope mirrors, glass, and mirror samples were subjected to soiling tests in laboratory and field experiments. The specific objective of this study was to investigate glass contamination on a microstructural level using light and scanning electron microscopy (SEM). In addition, the dusts were analyzed for their main mineral components using X-ray diffraction. The study exposed samples to natural dust from Qatar, Chile, Morocco, and the standard Middle East Test Dust. The tests showed that the adhesive strength increases abruptly due to cementation when the surface temperature falls below a critical temperature difference to the dew point (ΔT dp/critical / K). This work found values of ΔT dp/critical from 1.9 K to >7 K. The analysis shows that cementation is initiated by precipitation of the main soluble component available and promoted by surface humidity. The contribution of the water solubility of the dust components to the cementation process is more significant than the hygroscopic properties of the dust. SEM images show different cementation structures on the sample surfaces. These structures range from needle-shaped deposits <1 μm to rod- and plate-shaped structures from 3 μm to 30 μm to cubes with edge lengths from 2 μm to 50 μm. All appearances of cementation lead to significantly increased particle adhesion so that wind speeds less than 10 m/s do not remove particles from the surface. This investigation compared, at a microstructural level, laboratory test samples with outdoor samples from Morocco, Qatar, and Chile. The findings showed that the laboratory tests are very well suited to reproduce the cementation effects from the outdoor tests. This article quantified cementation conditions for the first time in dependent on operating environments. These results will support the development of suitable prevention strategies against soiling on functional surfaces.