Clearness-based sky taxonomy for one year irradiance data collected at BNL

The paper presents the analysis of one year irradiance data collected at Brookhaven National Laboratory during 2012, with the purpose to define a sky taxonomy mainly based on the calculation of the sky clearness index Kt. The evaluation is supported by additional meteorological variables like ambient temperature, wind, RH and atmospheric pressure. The analysis will additionally include the ratio of the diffuse-to-total irradiance for good weather conditions. Four sky categories are defined: cloudy, partly cloudy, sunny, and clear. The Kt parameter allows describing the solar climate of a particular location, providing also the basis for the estimation of solar radiation on horizontal as well as inclined surfaces. The statistical analysis defines the clearness index on the daily and yearly basis, but it also looks into hourly values to detect correlations between morning and afternoon conditions. The analysis herein developed is in the context of a statistical evaluation of the cloud coverage probability for application in a probabilistic production-oriented risk assessment analysis for photovoltaic systems.