Fundamentals of soiling processes on photovoltaic modules

The topic of soiling of photovoltaic module (PV) and concentrated solar power (CSP) collectors has recently gained increasing attention due to its impact on solar power production, especially in arid and semi-arid areas with high concentrations of airborne dust. Soiling describes the deposition of dust and other contaminants on surfaces, reducing solar irradiation by absorbing or reflecting the sunlight, causing energy yield losses which can exceed 1% per day. The amount of soiling is influenced by complex interactions of many factors which can vary significantly from site to site. In this study we provide a detailed overview of macroscopic and microscopic factors influencing soiling. This includes a global analysis of key parameters including airborne dust concentrations, dust characteristics (mineral composition, size distribution), and particle deposition rates. A theoretical model for relevant particle adhesion and removal forces is presented to achieve a microscopic understanding of wind cleaning effects. Further, it was found that dew occurs frequently on PV modules in many soiling affected areas and that this can significantly increase particle adhesion. Therefore, a detailed analysis and model of the dew-driven soiling mechanisms of cementation, particle caking, and capillary aging are given on the basis of microstructural material and dust characterization of outdoor exposed glass samples. Furthermore, we study the complex interplay and dynamics of different environmental parameters (relative humidity, ambient and module temperature, airborne dust concentration, wind speed) and their correlation to dust accumulation, and provide explanations with the help of the developed models. Overall the study aims to provide a useful, in-depth but also global overview and fundamental understanding of soiling processes on PV modules down to the microscale, which could be used to develop appropriate soiling mitigation strategies.