Predicting daylight illuminance and solar irradiance on vertical surfaces based on classified standard skies

Solar irradiance and outdoor illuminance, particularly on vertical surfaces are crucial to energy-efficient building designs and daylighting schemes. In Hong Kong, only hourly horizontal global solar radiation data have been systematically recorded for a long period but no measurements of daylight illuminance exist. In 2003, the International Commission on Illumination (CIE) adopted a range of 15 standard skies covering the whole probable spectrum of skies in the world. Standard skies of the same category would have the identical well-defined sky radiance and luminance distributions. Once the skies are identified, the basic solar irradiance and daylight illuminance at the surfaces of interest can be obtained, involving simple mathematical expressions. This study presents a numerical approach to predict the vertical solar irradiance and daylight illuminance based on the CIE standard skies. Climatic parameters recorded between January 2004 and December 2005 are used in the analysis. The performance of the calculation method is evaluated against data measured in the same period. The annual RMSEs were found ranging from 17.7% to 20.8% for daylight illuminance prediction and 17.9%-19.8% for solar irradiance estimation. The findings provide an alternative to compute solar irradiance and daylight illuminance on vertical surfaces facing various orientations.