Any room with outside windows has daylight available to supplement the artificial luminaires. There are three types of sunlight that can enter a conditioned space: direct, indirect, and reflected. Direct light is the least desirable because it can cause problems from glare, and it also has the highest contribution to the cooling load. Reflected light can have similar consequences unless it is carefully controlled.
Some architectural features can be enhanced to increase the natural daylighting. Reflective sills and properly angled Venetian blinds can be used to amplify the flow of light through windows or opaque openings such as glass brick walls. Even directing the light straight up to a light-colored ceiling helps, as the light reflected from this ceiling reaches deep into the room.
Landscape features can be designed to direct more natural light through windows. A reflective area on the ground beneath a lower-level window—a gravel surface, a light-colored wood deck, or a concrete path—directs more diffuse light through nearby wall openings. Carefully trimming shrubs, trees, and ground covering around windows also helps. Such plantings may be especially desirable around windows facing south or west in the summer, to prevent direct light from heating up the building walls in the late afternoon. Many people, though, keep plantings near windows trimmed and use blinds or shutters when direct lighting is a bother.
The incentive for increasing natural lighting is that it’s the best for color discrimination and detailed work such as drawing, painting, or drafting. Artificial lighting can be designed to approach the natural ideal, but no further. Consequently, any method that can increase the natural light entering a space enhances the working environment, thereby improving the productivity and attitude of the occupants.
Another factor to consider is that windows with closed blinds or curtains have a higher insulation value. This is offset by the fact that the lights in a room also generate a portion of heat that must be removed by the air conditioning. This may be as much as 10 percent of the total load. With the exception of direct lighting, though, it’s generally an energy saver to take advantage of all available daylight and pay the small penalty in higher air-conditioning cost.
The insulation issue is more important during the heating season when the temperature difference from inside to outside is greater. The reason for this is that the rate of heat transfer is directly proportional to the temperature difference:
Heat flow = U-value * temperature difference * area
The significance of this function is that increased heating costs from opening up blinds or curtains normally closed in the winter (i.e., increasing the U-value or coeffcient of heat transfer) offsets most of the savings in electricity due to lower use of artificial lights. Also, since the lights add heat to a room, when they’re turned off in the winter the furnace must make up the difference.
The electricity savings made possible by increased use of daylighting can take several forms. In a space that can turn off all lights when outside conditions are favorable, the total lighting cost can be considered the net savings. Even though the available natural light varies through the day and from season to season, the annual electricity savings can be as much as 60 percent of the lighting bill.
A good practice is to require special circuiting for large rooms with exterior windows, so the outer rows of lights can be switched separately from the rest of the room. Since these perimeter lights serve the part of the room best able to use daylight, such an arrangement permits a portion of the lights to be turned off much of the day. The lights can be controlled by either a direct switch or a combination switch/optic sensor. A final alternative is to put the lights in a day-lit space on a multilevel switching system. If the space has three-lamp fluorescent fixtures, for example, the ballasts can be modified to permit one-, two-, or three-lamp operation of the lights for three separate ambient light levels.
This provides the flexibility of supplementing the daylighting with the minimum necessary to provide the proper total footcandle levels. One switch-leg can be controlled by a photocell for optimum savings and comfort. Each of these systems permits immediate savings, depending on the type of controls used and the extent to which the present switching must be modified.
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Excerpt from Energy Conservation Projects.