Cloud Enhancement Phenomenon and its Effect on PV Generators

Due to the cloud enhancement (CE) phenomenon, global horizontal irradiance can exceed the clear sky irradiance value. Enhanced irradiance areas are formed when photons are scattering from the water droplets in clouds near the direct irradiance path. The CE is well-known as a phenomenon, but its effect on photovoltaic (PV) generators has not been thoroughly studied. Output power of a PV generator is almost directly proportional to the irradiance. Therefore, due to the CE phenomenon, output power of a PV generator can exceed the nominal power, which affects the requirements set to the inverter. We used irradiance measurement data of 440 days from an array of pyranometers to calculate the speeds and movement directions of cloud shadows to deduce the diameters of enhancement areas. During the CE events, the irradiance values were occasionally more than 150% of the expected clear sky irradiance value. We discovered that the diameters of these extreme irradiance areas are tens of meters and, therefore, they can have a full effect on residential or commercial scale PV systems. Utility scale PV generators are in practice affected more by less extreme CE events with larger diameters, since spatial smoothing by the large generator area decreases the effects of the extreme events.