Accumulated soil on the surface of PV modules (“soiling”) is a universal phenomenon that can reduce annual energy production of PV systems by as much as 25%. The economics of this energy loss has implications across the entire PV system value chain, from module design to site selection, finance and operations and maintenance (O&M). This provides strong motivation to develop a deeper scientific understanding of the environmental processes that lead to soil accumulation, the detailed effects on PV performance and effective mitigation strategies. Due to the interplay of the various factors involved, multidisciplinary approaches are often required. In this presentation, we will present an overview of many of the challenges associated with soiling of PV panels that may be approached from the perspective of materials science.
Soiling is most commonly caused by deposition and adhesion of suspended atmospheric particulates. While influenced by the local soil, suspended particulate also contains pollutants, pollen and other organic matter, particulates generated by nearby construction, etc. Deposition strongly depends on particle size and is governed by the processes of particle diffusion toward the surfaces, of particular significance for very small particles, and of gravitational sedimentation, significant for larger particles. Adhesion of the deposited particulate to the module surface may be further influenced by factors such as tilt angle, surface texture and surface energy.
Composition of local suspended particulate affects the details of energy loss in addition to the soiling rate. Soils containing significant mineral content may act as a color filter, modifying the spectrum of the transmitted light that reaches the PV cell. Presence of minerals may also enhance cementation, hindering natural cleaning processes. In contrast, soils with a high soot content cause neutral attenuation of sunlight. However, soot can have as much as a 10x impact on transmission loss vs silica. As a consequence, PV systems deployed in areas with significant pollution from combustion may be more susceptible to soiling losses than systems located in rural or agricultural areas.
Many PV systems - particularly residential and commercial – are never cleaned, effectively leading to a permanent energy loss. The traditional mitigation strategy has been to clean systems periodically, either on a preset time interval or when monitored system data drops below a predetermined threshold. Cleaning is not always practical due to increased labor costs or local availability of water. Alternative mitigation strategies include the application of anti-soiling coatings and the use of dry, robot-based cleaning. However, these approaches may be mutually exclusive due to the potential for abrasive damage to the coatings during cleaning.