Investigation of soiling effects, dust chemistry and optimum cleaning schedule for PV modules in Lahore, Pakistan

We investigate the photovoltaic (PV) power losses due to soiling for Lahore, Pakistan for solar panels. Optimized cleaning schedules are proposed incorporating the effect of solar panels' tilt angle and the method (manual vs. automatic) for cleaning. Output power losses and dust accumulation on solar panels were measured at variable tilt angles for a period of 120 days at an open roof top location in Lahore. The relative soiling losses for monofacial vs. bifacial (constructed by stacking two back to back monofacial) solar panels were compared for two different panel orientations, i.e., south faced tilted panels vs. East/West faced vertical panels. We found that the soiling rate for Lahore was consistently around 0.8% per day for 30 degrees tilted panel (for the measurement period between October to January), which is among one of the highest soiling rates reported for various urban locations across South Asian and Gulf regions. A dust accumulation rate of 0.01 - 0.02 mg/cm(2) per day was recorded for panels that were fixed at 30 degrees tilt. The variation for soiling/dust deposition rates was found to be negligible for different dry periods spanning between October and January. The chemistry and composition of the dust were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), and, electron dispersive x-ray (EDX) spectroscopy. Large contents of carbon and quartz were found in the dust collected from the samples through EDX and XRD analysis. High carbon contents in the accumulated dust are attributed to air pollutants and could be a contributing factor for the high soiling rate. For manual cleaning, the optimal cleaning schedule was calculated to be about once per week for panels at 30 degrees tilt, and, once every three weeks for panels at 90 degrees tilt. (C) 2020 Elsevier Ltd. All rights reserved.