Transparent domed skylights: Optical model for predicting transmittance, absorptance and reflectance

Daylighting and thermal loads are very important design issues for skylight design, especially in large spaces such as atria. However, the trade-off between daylighting and thermal performance of skylights has been difficult to solve, due to a lack of daylighting and thermal design tools. A mathematical model was developed to predict the visible/solar transmittance, absorptance and reflectance of multi-glazed domed skylights for both direct and diffuse radiation- The model is based on tracking the beam and diffuse radiation transmission through the dome surface. Since all building energy simulation and fenestration rating tools are limited to planar skylights, the model was translated into a simple method in which domed skylights were substituted by optically equivalent planar skylights. The results showed that domed skylights yield slightly lower visible/solar transmittance at low sun zenith angles, and substantially higher visible/solar transmittance at high sun zenith angles, or near the horizon, than do planar skylights having the same aperture. The absorptance of domed skylights is higher than that of planar skylights, particularly at high sun zenith angles, or near the horizon. The model was compared with the IESNA transmittance calculation procedure for domed skylights and with the Wilkinson model. The IESNA transmittance calculation procedure overestimates by 19% the transmittance of a dome at low sun zenith angles and significantly underestimates the transmittance of a dome at high sun zenith angles, or near the horizon. However, the Wilkinson model significantly underestimates the transmittance of a dome for both low and high sun zenith angles.