Daylighting Calculation in DOE-2

Lighting accounts for about 20% of total electrical energy consumption in the United States. Using natural lighting is a cost-effective way to reduce this consumption and, at the same time, enhance the quality of the indoor environment. For several years, architects and engineers have used scale models, hand calculator programs, and sophisticated main-frame computer programs (such as LUMEN-11) to determine levels of interior daylight for different building configurations. However, none of these tools determines the annual energy savings from daylighting, information which could have an important effect on design decisions.

For this reason, a daylighting simulation has been added to DOE-2. Taken into account are such factors as window size, glass transmittance, inside surface reflectances of the space, sun-control devices such as blinds and overhangs, and the luminance distribution of the sky. Because this distribution depends on the position of the sun and the cloudiness of the sky, the calculation is made for standard clear- and overcast-sky conditions and for a series of 20 solar altitude and azimuth values covering the annual range of sun positions. The calculations are performed prior to the complete simulation, and the resulting daylight factors are stored for later use. Analogous factors for glare are also calculated and stored.

For the hourly envelope simulation, the illuminance from each window is found by interpolating the stored daylight factors (using the current-hour sun-position and cloud cover), then multiplying by the current-hour exterior horizontal illuminance. If the glare-control option has been specified, the program will automatically close window blinds or drapes to decrease glare below a pre-defined comfort level. Adding the illuminance contributions from all the windows gives the total number of foot-candles at each reference point.

This report describes the equations and algorithms used to perform the daylighting calculations in DOE-2.LB, and is intended as a supplement to the DOE-2 Engineers Manual, Version 2.EA, LBL-11353. Supporting user documentation may be found in the DOE-2 Reference Manual, LBL-8706, Rev.2, LA-7689-M, Ver. 2.1A, the DOE-2 BLD Summary, LBL-8688, Rev.3, the DOE-2 Users Guide, LBL-8689, Rev.2, the DOE-2 Sample Run Book, LBL-8578, Rev.1, and the DOE-2 Supplement, LBL-8706, Rev.3.Suppl.