6.4.4 Daylighting Control

The method used to model daylighting. Daylighting credits must be calculated based on the local climate and daylight models of the space. Building descriptors are provided in this section for an internal daylighting model and two variations of an external daylighting model:

1. Internal daylighting model. With this method, the simulation model has the capability to model the daylighting contribution for each hour of the simulation and make an adjustment to the lighting power for each hour, taking into account factors such as daylighting availability, geometry of the space, daylighting aperture, control type and the lighting system. The assumption is that the geometry of the space, the reflectance of surfaces, the size and configuration of the daylight apertures, and the light transmission of the glazing are taken from other building descriptors. 
2. External daylighting model. An external daylighting model may be used in combination with an hourly simulation program to calculate daylighting savings as long as it produces consistent results and makes use of the key assumptions described below for internal daylighting models. Exterior daylight models include, but are not limited to, the following types of methods: 
   
   1. Schedule adjustments. With this method, a space is modeled in a stand alone daylighting program to determine the amount of interior daylight available different times of the year and for different times of the day. In addition this program has an electric lighting model that calculates the electricity savings by hour based on interior illuminance and the daylighting control type (switching, dimming etc.). These savings values are converted into a schedule of electric lighting power reduction multipliers. This lighting power reduction schedule is applied to the proposed design energy simulation model and results in reduced electric lighting energy consumption and reduced internal heat gain, both of which are reflected in the proposed design energy consumption. 
   2. Daylight ratio. With this method, an outside program pre-calculates a relationship between outdoor daylight conditions (illuminances or luminances) and interior illuminance. Within the rating software, interior illuminance is calculated from the daylighting ratios and the daylight conditions derived from data on the local weather file. The remainder of the calculations are the same as for an internally calculated daylight model where the interior illuminances are compared to an illuminance setpoint and electric lighting power is calculated based on control type. The two most widely used methods of pre-calculating daylighting ratios are the modified daylight factor method and the daylight coefficients method.
      
      1. The modified daylight factor method uses pre-calculated diffuse and direct illuminance daylight factors and multiplies these by diffuse and direct beam outdoor illuminance from the weather file to calculate interior illuminance.1  Daylight factors are calculated from a simulation of the space that relies on user entered information about the space modeled such as orientation, geometry, material properties (transmittances and reflectances) etc. For any given hour, the interior illuminance at the reference point is calculated by the direct beam angle specific daylight factor multiplied by the outdoor direct beam and clear sky illuminance and this is added to the overcast daylight factor multiplied by the overcast sky illuminance. Outdoor direct beam, clear sky and overcast sky illuminances are calculated from the weather data used in the proposed building energy simulation.
      2. The daylight coefficients method is essentially a similar but more accurate method that relates internal illuminance to the luminance of patches of the sky.2  The sky is divided up into patches as defined by altitude and azimuth. The daylight coefficients are ratios of interior illuminance to luminance for patches or areas on the sky dome. An outside daylight simulation program uses information about the space modeled: its orientation, geometry, material properties (transmittances and reflectances) etc, and calculates daylight coefficients for each sky patch. The precalculated daylight coefficients are then used to calculate interior illuminances for each hour. The illuminance for a location with the space at any point in time is the product of the luminance for each sky patch multiplied by the specific daylight coefficient for each sky patch integrated over the entire sky dome. The luminance for each sky patch is calculated from the weather data used in the proposed building energy simulation. 

Other methods may be used by software developers as long as they produce consistent results. Regardless of the method used, it is desirable that all methods have the same key assumptions.

The default primary daylight area for sidelighting is a band near the window with a depth equal to the distance from the floor to the top of the window. The default secondary daylight area for sidelighting is a band beyond the primary daylighted area that extends a distance double the distance from the floor to the top of the window. Other daylight areas may be defined with appropriate documentation.

The default primary daylight area for toplighting is a band around the skylight well that has a depth equal to the 70% of the ceiling height. The default secondary daylight area for toplighting is a band beyond the primary daylighted area that extends 140% of the ceiling height.

Daylighted areas may not overlap or extend beyond partitions higher than 5 ft.

Error checking includes ensuring that the following is true:

Sidelit depth is less than or equal to ceiling height. 

Total daylit area is no greater than space area.

The type of control that is used to control the electric lighting in response to daylight available at the reference point. The options are:

• Step Switching controls have discrete steps of light output, where the fraction of rated power matches the fraction of light output. See Figure 6.4.4-1.
• Step Dimming controls also have discrete steps of light output but typically the intermediate steps of light output are associated with higher levels of fraction of rated power. When the lights are fully off or fully on, the fraction of rated power matches the fraction of light output. See Figure 6.4.4-1.
• Continuous Dimming controls have a fraction to rated power to fraction of rated output that is a linear interpolation of the minimum power fraction at the minimum diming light fraction to rated power (power fraction = 1.0) at full light output. See Figure 6.4.4-2.
• Continuous Dimming + Off controls are the same as continuous dimming controls except that these controls can turn all the way off when none of the controlled light output is needed. See Figure 6.4.4-2.

Figure 6.4.4-1: "Example Stepped Daylighting Control"

Figure 6.4.4-2: "Example Dimming Daylight Control"