Analysis of Daylighting Performance and Energy Savings in Roof Daylighting Systems

An investigation has been made of potential lighting electricity reductions and associated thermal impacts of replacing electric light with daylight admitted through rooftop glazing. Horizontal apertures on flat roof (skylights) and vertical apertures facing in two opposite directions (roof monitors) are examined in a prototype office building. A computer simulation tool that integrates lighting simulation (RADIANCE) and energy simulation (EnergyPlus), DIVA-for-Rhino has been used. Lighting simulation determines the fraction of the solar radiation entering the aperture that reaches the work plane as useful illumination; whole-building energy simulation predicts reductions in lighting electricity and the impact on energy consumption for heating and cooling the building. Building operation costs are calculated as a function of aperture to floor area so that the optimum roof daylighting design could be identified. The results indicate that rooftop glazing contribute to savings in building operation if a modest amount of glazing is used in the roof. In some climates either heating or cooling energy is more sensitive to increasing the area of glazing, but the potential cost benefits of daylighting are larger when an optimum aperture area is used. The design implications of the results are discussed and important parameters that influence the rooflit building operation costs will be outlined.