Characteristics of the cloud enhancement phenomenon and PV power plants
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The cloud enhancement (CE) of solar irradiance is a well-known phenomenon, but its effects on PV power plants are not thoroughly understood. Because of scalability, the diameters of PV generators can vary from some meters up to several hundreds of meters. The output power of a PV generator depends mainly on the irradiance to the PV panels. Therefore, if the irradiance is enhanced, the actual output power might exceed the nominal power. We created a method to estimate the average irradiance over typical land areas of PV generators. The average irradiance over the PV generator land area was used to calculate the frequency, duration and average irradiance of the CE events. The analysis was based on actual irradiance measurement data from an array of pyranometers on an area of 1400 m2, which corresponds to the land area of 0.1 MW PV generator. We found out that the focus areas of the CE events have diameters of the order of tens of meters. Therefore, up to 0.1 MW power range, the land area of the PV generator does not greatly affect the maximum average irradiances. In this power range, the average irradiance was measured to be up to 1.5 times the clear-sky irradiance. The CE events affecting utility-scale PV generators were estimated to have maximum average irradiances of 1.4 times the clear-sky irradiance, but the number of these CE events is considerably lower.