A little more information on how lighting calculations are performed will help an understanding of the lighting projects to be discussed. The fundamental equation for the zonal cavity method is as follows:
RcR = 5 * (ceiling height – work surface height) * (L + W)/(L * W)
L = length of room
W = width of room
RcR = room cavity ratio
The room cavity ratio is used to extract the coefficient of utilization from the table of photometric values published by fixture manufacturers. This number is then used to calculate the average footcandle level in the space according to the equation
FC = no. of lamps/fixture * lumens/lamp* no. of fixtures * CofU X DF/area
Cof U = coefficient of utilization
DF = lumen and maintenance depreciation factor (0.85 for T12 lamps, 0.92 for T8 lamps)
area = L * W
Light-colored walls have a higher reflectance than paneled walls or bookshelves; carpeted floors reflect less light than tile or linoleum coverings, and so forth. Any of these items with a higher reflectivity increases the coefficient of utilization, which in turn increases the ambient footcandles of lighting in a space. Recognizing the importance of surface colors and gloss is important in any circumstance.
Take a large classroom that was originally designed with a light level at desktop height sufficient to read books and take pencil notes. If this room is remodeled with dark carpeting and patterned wallpaper, the room light level will be reduced. The exact reduction can be determined from the fixture photometries and the new material reflectance values. As another example, consider a large open space lit by overhead fluorescent fixtures. When movable partition walls are set up in the space, the effective room area for each workplace is diminished (i.e., the room cavity ratio decreases) and the overall light level is also reduced.
Each person in a newly partitioned space will, therefore, need a task light to increase the light available.
These examples show how a minimal areawide lighting level can provide walkway and common area lighting, but may need to be supplemented in each work area according to the performed task, the age of the occupant, and other factors. The designer may even stipulate that fluorescent task lamps are used, so the net change in energy use will be minimized.
The overall quality of the lighting environment is much greater than if all the cubicle light requirements were:
• Fixture modifications to increase light output
• Efficient electronic ballasts
• Occupancy and photocell sensors
• Digital controls
The first two projects have been discussed as maintenance projects that require little fiscal commitment. The return on the investment is quick and constant, so that the savings can be used for subsequent projects that are more complex and costly. Consequently, they should be considered as the first steps in a comprehensive conservation strategy. The projects have been listed in the general order in which they should be evaluated for expected cost, savings, and payback. There are dependencies among the projects that affect these calculations and the effectiveness of the projects themselves.
For example, installation of an occupancy sensor to turn off the lights in a dining hall when not in use might be easily justified on the basis of existing lamps. A reduction in the energy use of the fixtures by the installation of energy-saving lamps, however, increases the payback considerably, perhaps beyond a typical 5-year maximum. So, it is wise to first install the most efficient lamps and fixtures before using the more sophisticated and costly controls.
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Excerpt from Energy Conservation Projects.