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Soiling of PV modules decreases their energy yield by blocking sunlight and is a serious problem in regions with high aerosol concentrations and little rain such as arid and semi-arid climates. Due to many influencing factors, which are often site-dependent, the complex processes of soiling are not fully understood. In order to approach this goal, detailed knowledge and analysis of the various influencing parameters at a microscopic level are necessary.

In this study we present results of outdoor experiments to reveal the physical mechanisms of soiling-layer formation on exposed glass surfaces. The experiments were conducted at a PV test site in Doha, Qatar, and included sample cooling, heating and cleaning. Detailed microstructural investigations of the soiling layers were performed to study cementation of dust particles. To achieve a better understanding of the underlying processes, we provide an analysis of environmental parameters (relative humidity, ambient and module temperatures, wind speed) with special focus on dew formation, as well as dust characteristics (chemical composition, mineralogy, size distribution), which influence cementation and optical losses. In addition, we compare results of different methods for soiling-rate determination including light microscopy, spectroscopy, gravimetry and PV power analysis to determine the relation between micro-scale soiling measurements and actual PV energy losses.

The results show that in Qatar, dew formation frequently occurs and leads to particle cementation by needle-shaped crystals of the clay mineral palygorskite, which directly precipitates at the glass surface. It is shown that preventing dew formation by sample heating inhibits cementation and can significantly reduce dust accumulation. Based on these findings, new mitigation approaches for PV soiling are proposed.