Detection and characterization of cloud enhancement events for solar irradiance using a model-independent, statistically-driven approach

Abstract

Cloud enhancement events are instances in which the cloud pattern increases global horizontal irradiance on theground above levels higher than would otherwise be expected during clear sky with cloudless conditions, and ingeneral, these events are characterized by high irradiances occurring during highly changing transient conditions.Even though it is a well-known although infrequent phenomenon, currently no consensus exists as to itsformal definition. However, several studies point out to a need for a better understanding of this phenomenondue to their effects on mathematical models or photovoltaic systems behavior during these instances, as resultsshow significant deviations with respect to the expected results in normal conditions. Due to these issues, thepresent work intends to explore cloud enhancement definitions and detection methods, for which a novel modelindependent,statistically-driven approach is proposed and compared to three other models found in the literature.Assessments are made regarding the main aspects of the phenomenon: frequency of occurrence, clearnessindex, diffuse fraction and its relationship with solar resource variability. Data from eight measurements sitesthroughout Chile, with different climatic characteristics is used. To further support the analysis made, monthlymaximum expected clear sky irradiance for each site was estimated through satellite-based data as well as withestimation from ground measurements. Finally, the highest irradiances are presented for each measurement sitereporting, the record irradiances for one-minute data thermopile and photodiode pyranometers.