Use of Measured Aerosol Optical Depth and Precipitable Water to Model Clear Sky Irradiance

Predicted clear sky irradiance depends on atmospheric composition as well as solar position and extraterrestrial irradiance. The effects on clear sky irradiance of year to year variations in atmospheric composition were studied using measurements of aerosol optical depth (AOD) and precipitable water (Pwat) at seven locations in the United States. Three clear sky models were evaluated, including one that uses Linke turbidity (TL). This model was evaluated using historical, static TL as well as updated values derived from real-time AOD and Pwat measurements. The average annual error in predicted clear sky irradiance using static TL did not differ significantly from year to year. Annual average error in predicted GM was less than 5% for all models with no significant difference between models. The model with static TL had the lowest DNI errors, and the Bird model had the smallest GM error but the largest DNI error. On average DNI and GM were under-predicted.