Comfort Considerations in Net ZEBs: Theory and Design - Modeling, Design, and Optimization of Net-Zero Energy Buildings - page 134

3.3.3.1 Daylight Autonomy and Continuous Daylight Autonomy
Daylight autonomy (DA) indicates the fraction of time when the workplane
illuminance meets or exceeds a set threshold (e.g., 500 lux for typical office
environments), by daylight alone (Reinhart, Mardaljevic, and Rogers,
2006). The metric can be expressed as a fraction of occupied hours, daylit
hours or occupied hours that are daylit, over a given period of time (e.g., a
year). For nondimmable (i.e., on/off) electric lighting that is automatically
controlled by occupancy and/or a daylight sensor, DA can be directly
translated into potential energy savings.
An extension to DA is continuous Daylight Autonomy (DA cont ), which
indicates the fraction of time when the minimum workplane illuminance
requirements are partly or fully met by daylight alone. This is suitable for
quantifying the energy savings from automatically dimmable lighting.
(3.22)
where n is the total number of time steps (e.g., hours), E min is the minimum
workplane illuminance requirement, and E daylight is the workplane
illuminance provided by daylight only, at a given time step t .
3.3.3.2 Useful Daylight Illuminance
Useful daylight illuminance (UDI) categorizes workplane illuminance levels
for typical office spaces, due to daylight only, into three bins as shown in
Table 3.7 (Nabil and Mardaljevic, 2006). UDI offers the advantage over DA
and DA cont that it quantifies the duration of high illuminance and thus,
may indicate cause for concern from chronic visual discomfort. However,
there is some discussion in the research community about whether the
thresholds for the bins are appropriate (Mardaljevic, Heschong, and Lee,
2009). All metrics are relatively straightforward to obtain from BPS tools or
measurements(e.g.,onscalemodelsandfull-sizemock-ups)andprovidean
indication of year-round daylight quality for the building design and climate
of interest.