Towards a More Sustainable City: The Role of the Daylight Factor in Evaluating the Energy Requirements of Buildings

Abstract

The Daylight factor is the reference parameter used in Energy Standard EN15193:2008 to assess daylight contribution in the energy performance of buildings. But its efficacy in putting in relation buildings energy performance, daylight availability and visual tasks is now a subject of discussion in literature. In fact, the daylight factor is a static indicator independent from building geometry and environmental parameters. From the energy point of view, it has still to be assessed if it can ensure when the switch of artificial light is needed, representing therefore the most disadvantageous outdoor illuminance conditions. The goal of this study is demonstrating if the daylight factor calculated in standard conditions (Overcast sky) is conservative respect to that calculated using the diffuse component of the horizontal illuminance under CIE (Commission Internationale de l'Éclairage) Clear sky. If so, Daylight factor (D) might be applied, possibly with suitable corrections, to estimate the energy requirements for lighting, keeping the results on the safe side. 

Simple correlations between point daylight factor, average daylight factor and vertical daylight factor under all sky conditions and building design implications

“…As the PDF depends on C and D, the U o should vary slightly among the 15 CIE standard skies. Table 6 lists U o under the 15 CIE Standard Skies using equations ( 14) and (15), respectively. Based on equation ( 14), the PDI depends on VDF and D that vary under different sky conditions.…”

“…Some correlations have been shown between the PDF and the electric lighting energy reduction from the daylight-linked lighting controls. [13][14][15] The average daylight factor (ADF) indicates the visual adequacy of the daylighting in space as a whole rather than any particular point. 16 The ADF strongly relies on window openings and requires fewer input data than those required by the PDF, which could be helpful to building façade designs during the initial design period.…”

“…Daylight illuminance uniformity (U o ) under the 15 CIE standard skies using equations (14) and(15).…”