Net ZEB Design Processes and Tools - Modeling, Design, and Optimization of Net-Zero Energy Buildings - page 207

early in design; daylight requirements can have a profound influence on
building form.
Rules of thumb and pattern guides for daylighting design remain among
the most efficient methods to support major daylight-related decisions
including window type, size, and position, room depth, room surface optical
properties, and static and dynamic shading devices (O'Connor et al. , 1997).
However, in order to accurately quantify energy savings, especially for new
daylight strategies, technologies, or unusual geometries, use of dynamic
daylight simulation is valuable. There are three predominant algorithms
used in daylight simulation in order of increasing model resolution: (i)
split flux method, (ii) radiosity, and (iii) raytracing (Hensen and Lamberts,
2012). The split flux method calculates illuminance on the workplane based
on direct views of interior points to exterior light sources (e.g., the sky)
and surface-reflected light, but neglects multiple light reflections and thus
it can underestimate daylight availability. It generally leads to inaccurate
results for rooms with a high aspect ratio, specular (mirror-like) surfaces,
or interior obstructions (Department of Energy, 2013b). Radiosity was
originally developed to calculate nonvisible radiative heat transfer between
surfaces, but has been adapted for daylighting applications. It uses surface
reflectancesandluminanceexitancetosimultaneouslysolvetheilluminance
on all interior surfaces. Its major limitation is that it treats all surfaces as
perfectlydiffusereflectors,andthusmaybeinaccurateforspecularsurfaces,
such as metals, glasses, and highly polished surfaces. This could be a
limitationforspecularsolarshadingdevicessuchaslightlouversandspaces
that are prone to glare.
Raytracing is a daylighting analysis method that - as its name suggests
- traces light from the source to interior surfaces (forward raytracing) or
vice versa (backward raytracing). Unlike the previously discussed methods,
raytracing can accurately treat specular surfaces with complex geometries.
Raytracing can yield photorealistic images, but such a detailed method may
not be necessary for Net ZEB design. Forward raytracing follows rays of
light from all light sources to a surface (Hensen and Lamberts, 2012). The
number of bounces that the ray of light is followed for is normally specified
by the tool user depending on the desired accuracy and complexity of the
geometry. Backward raytracing starts with a final surface and then traces
light back until (if ever) it reaches a source. Raytracing is generally
considered superior for complex geometries with small and/or specular
surfaces (e.g., see Figure 4.18 ) . Numerous comparative studies of daylight