5.6.1 Energy aspects of daylight
By using daylight to its full potential, the electricity demand for lighting during daytime can be significantly reduced or even eliminated.
The Architectural Energy Corporation has stated (Architectural Energy Corporation, 2006) that “Daylighting can drastically improve the energy efficiency of a space with adequate control of electrical lighting and solar heat gain”.
In offices, the electricity demand for lighting can account for as much as
40-50% of the total energy demand (Walitsky, 2002), which can result in significant savings if replaced by daylighting. In order to quantify the energy savings on electric lighting, the number of hours for which daylight is an autonomous light source in the interior must be known. The relevant light levels for residential buildings were discussed in section 1.7.1.
The optimal use of windows in buildings to provide good daylight conditions with good energy performance requires careful selection of the window characteristics τv, g (and Uw). Due to the laws of physics, the g value will always be at least 50% of τv.
The best solution is often a combination of window and solar shading. A window with high g value and high τv value will generally provide a good result. High values of g and τv will perform well in that part of the year with least light; in parts of the year with excessive light, solar shading should be used. It is important that the design of the building and the placement of windows in it are planned as part of a holistic process where the requirements for daylight and energy performance are continuously evaluated and used as design
parameters (Moeck, 2006).
The following example illustrates that high light levels are achieved with daylight and that windows are very energy-efficient light sources.