Soiling of Photovoltaic Modules: Size Characterization of the Accumulated Dust

 Abstract —Soiling significantly reduces the energy production of photovoltaic (PV) modules. The reduction is not only determined by the amount and composition, but also by the size distribution of the particles. This study investigates the particle-size characteristics of dust accumulated on horizontal and inclined glass surfaces used in PV modules. The accumulated dust is compared to the ambient airborne dust. Effects of tilt angle and wind speed are investigated. Variations in particle size over the glass surface are also studied. Dust accumulating on a photovoltaic module is finer than ambient airborne dust, except for a combination of forward tilt AND low wind velocity. For wind velocities large enough to initiate wind erosion the accumulated dust is finer than the airborne dust even in the case of forward tilt. For backward tilt the accumulated dust is always finer than the airborne dust. Reasons for the finer dust are the preferential accumulation of the finer particles in the wake of the module due to their lower response time compared to coarse particles and the preferential removal of the coarsest fractions by the wind. At forward tilt accumulated dust is finest near the leading and trailing edges of a module whereas at backward tilt the particle size distribution over a PV module is more uniform. Energy prediction models should incorporate these internal variations and the differences with airborne