[BLDG-SIM] Day-Lighting and DOE2.1E

California's Title 24 energy code has established values of energy credits for daylighting controls. These credits are used in both the prescriptive approach and the performance method which is based upon an energy simulation. For the LEED credits the performance method would be used as you are trying to estimate the energy consumption as a fraction of the consumption of a minimally compliant building. The current tool is EnergyPro which is DOE-2.1E with a user interface that places some limits on inputs and automatically creates a reference or minimally code compliance comparison case.

The lighting power of controlled lighting that is in the daylit zone (no more than 15 feet from a window and currently no more than 1 ceiling height horizontally from a skylight) is reduced by a Power Adjustment Factor (PAF). The factor is a function of the visible transmittance and the fraction of wall area for glazing. Though this doesn't capture peak reduction effects, the simplicity of the method provides some confidence in the results. Sensor placement has a huge impact on the results.

I don't know what rule set is involved for daylighting in DOE-2 when a geometry is not described. I wouldn't expect that this would be an acceptable method since you really do not know what you are modeling. The bottom line is that your model approximates what your daylighting design does and that the design would reasonably save energy - thus a lighting design that does not provide the desired light levels in all the task locations would not likely provide maintained savings. Modeling two sensors does not make any sense if your control system has only one sensor and is designed to switch or dim lights evenly across the entire control zone. "Virtual" sensors are usually placed in a relatively dark spot in the controlled zone so that electric lighting provides sufficient light in the entire zone on the lighting control not just next to the window or directly under a skylight.

Can day-lighting controls be considered for LEED certification under “Energy & Atmosphere – Optimize Energy Performance”? If so where should the light sensor be located in the space? Is there any energy code that allows credit for day-lighting controls? The DOE2 program allows two light sensors per space. Supposing the two sensors are located 5 ft and 10 ft from the window. Is the average daylight at these two points used in determining the reduction in artificial lighting?

The DOE2 program expects zones to have 6 surfaces defined with X,Y,Z coordinates and reflectance values for day-lighting analysis. Ignoring this produces warning messages but the results show a reduction in artificial lighting energy. For day-lighting analysis we only enter into DOE2 the X,Y coordinates of the window origin relative to the wall and also the wall and window dimensions. Can we assume that the energy savings results from the DOE2 program are on the safe side (the actual energy saved is more) when we use the program without all the surfaces and coordinates?

Are there any rules for day-lighting analysis for establishing energy savings and for comparing the results from different studies?

Varkie Thomas

Skidmore, Owings & Merrill LLP

Tel: (312) 360-4467 (direct)