Evaluation of dynamic sky-type using novel angular sky luminance measurement system

Standard sky-types of the established Commission International de l’Eclairage (CIE) sky-models are employed for generating sky-luminance distribution datasets in most building daylight simulation packages, which are used for predicting indoor illuminance distribution. However, sky-type can vary on diurnal and seasonal basis; and there is a need for identifying closest sky-type at different time-points for better indoor illuminance predictions. This paper aims at recommending closest sky-model for measured angular sky-luminance distribution sets through comparison with established CIE standard skies. A novel design of sky-luminance measurement device using calibrated Light Dependent Resistors is proposed to measure incident illuminance from 265 discrete sky-positions every 5 min. Statistical analysis is performed on 143,100 distinct data-points corresponding to 540 representative datasets measured across 5-month period at Mumbai, and the closest CIE sky-type for various hours is recommended based on frequency of occurrence. A range of 9 sky-types of the CIE sky-model was determined as closest model sky-set, with sky-types 2, 4, 5, 6, 7, 8, 10, 11 and 13 assessed as closest sky-type for different time-points across the measured datasets. CIE sky-type 8 is identified dominating over 50% of measured datasets — 51.6% in November, 53.6% in January, 52.6% in March and 71.5% in April. Measured sky distribution is observed to generally follow trend of overcast to intermediate to clear sky-types in morning hours, and opposite in evening hours. This approach of recommending closest sky-type as simulation input — as against using generic sky-types — can enhance the accuracy of building indoor daylight predictions.