Lighting

Advanced tab on Model Options dialog.

Daylighting method

The options for calculating daylighting in EnergyPlus are:

• 1-Detailed where the zone daylighting illuminance levels are calculated and then used to determine how much the electric lighting can be reduced. The daylight illuminance level in a zone depends on many factors, including sky condition; sun position; calculation point; location, size, and glass transmittance of windows; window shading devices; and reflectance of interior surfaces. Reduction of electric lighting depends on daylight illuminance level, illuminance set point, fraction of zone controlled and type of lighting control. This is the default method implementing the EnergyPlus Daylighting:Controls object.
• 2-DElight. The DElight method of analysing daylighting in buildings is very similar to that used in the Detailed method. For each point in time, DElight calculates the interior daylighting illuminance at specified reference points and then determines how much the electric lighting can be reduced while still achieving a combined daylighting and electric lighting illuminance target. The daylight illuminance level in a zone depends on many factors, including exterior light sources; location, size, and visible light transmittance of simple and complex fenestration systems; reflectance of interior surfaces; and location of calculation reference points. The subsequent reduction of electric lighting depends on daylight illuminance level, illuminance set point, fraction of zone controlled, and type of lighting control. There are two primary differences between the Detailed and DElight methods of calculating interior illuminance levels. The first is that DElight includes the capability of analysing complex fenestration systems that include geometrically complicated shading systems (e.g., roof monitors) and/or optically complicated glazings (e.g., prismatic or holographic glass). The second key difference is that DElight uses a radiosity method to calculate the effects of light reflection inside a zone. These methods are discussed in more detail in the engineering documentation. There are other important differences between the two methods. One is the inability of DElight to perform the type of dynamic shading controls possible using the Detailed method at each point in time during the thermal simulation (e.g., changes in electrochromic glazing transmittances and blind slat angles). Another is the DElight ability to include more than two reference points in its interior illuminance and electric lighting reduction calculations. A third is the current lack of visual quality (e.g., glare) calculations performed by DElight. Fourth, the modeling of interior obstructions is different in the two methods. In the DElight method interior obstructions block inter-reflections but do not block the intial direct illuminance. In the Detailed method, interior obstructions block the initial direct illuminance but do not block inter-reflections. Input for invoking the DElight method involves three object types: Daylighting:DELight:Controls, Daylighting:DELight:ReferencePoint, and Daylighting:DELight:ComplexFenestration. DesignBuilder uses the first 2 of these.

Warning: We understand from EnergyPlus that the 2-DElight option is not implemented in EnergyPlus v8.2. Because of this, because of the above-mentioned limitations and because generally we believe that the 1-Detailed method is much more widely used and better tested, we strongly recommend that you avoid using the DElight option unless you have very good reason to do otherwise.