Quantifying the influence of soiling on PV module energy production based on a coupled opto-thermal-electrical model: An international assessment in different climatic conditions

According to the IEA, soiling on a global scale has been identified as the second most significant factor affecting the energy production of photovoltaic (PV) systems after solar irradiance. Soiling directly affects the transmittance of PV glass and also disrupts the thermal balance (heat flows) within PV modules, thus impacting both the amount of solar irradiance received by and the temperature of the cells. In this work, we develop and validate a coupled opto-thermal-electrical model capable of assessing the impact of soiling on PV module performance. The model goes beyond other models, commercial and in the literature, by accounting explicitly for the effects of soiling on the optical transmission and the thermal balance through PV modules, while also being capable of accounting for electrical output limitations caused by non-uniform soiling. The validated model, which is shown to predict experimentally obtained losses due to soiling with a RMSE of 6 %, is then used to investigate the impact of natural soiling on yearly PV system energy production while considering the local weather conditions at different geographical locations (Oman, Nigeria, Iran, Indonesia, Australia and Spain), as well as the module tilt angle and module cleaning frequency. For a monthly cleaning frequency, and thus exposure period, maximum yearly soiling losses (relative to modules kept continuously clean) of between 5 % in Nigeria and 18 % in Oman are obtained for horizontally oriented modules. For a seasonal cleaning frequency, the yearly losses can reach values up to 27 % in Nigeria and 32 % in Oman, with the lowest predicted loss being 12 % in Australia. Based on the results, it is concluded that soiling losses can be significant (>30 % for horizontal, but also >20 % for optimally-tilted modules, in the worst case), and that the cleaning frequency can have a significant effect on yearly PV energy production, motivating the development of improved soiling mitigation practices.