In our homes we use electric lights only until the sun is high enough; the sun’s light is ‘free’ and we use it until the sun goes down, when we switch on the electric lights again. This is not what happens in industrial storage and production facilities, warehouses and factories, since there is often not enough illumination in the workplace area even during the day. In such places, therefore, electric lighting is left running all day long. This costly waste of energy could be reduced considerably, in some cases avoided altogether, by installing skylights over workplace areas to take advantage of the sun’s ‘free’ light.
Daylighting is becoming more popular in commercial buildings and manufacturing facilities in the United States. After Thomas Edison invented the electric light, most architects changed their building plans and designed for more and more artificial lighting. As a result, only a very small percentage of the light used in major buildings and facilities came directly from the sun. Recently, the Europeans have realized the importance of daylighting and begun to use a reasonable percentage of sunlight in their buildings. The Rocky Mountain Institute, Snow-mass, Colorado, has investigated the benefits of daylighting and reported that daylighting increased productivity and reduced absenteeism by 15 percent. Also, sunlight reduces the heating and cooling bill.
In 1993, Wal-Mart Stores Inc. opened a prototype store in Lawrence, Kansas, with nine special skylights designed by Andersen Corp., Bayport, Minnesota. The architectural firm of Leo A. Daly, Omaha, Nebraska, opened an office building in 1983 with l5ft high window walls and a glazed roof. About 50% of its electricity bill for lighting was saved as a result (Reno Gazette-Journal, November 27, 1995). Daylighting is an inexpensive way of lighting interiors since the sun is a ‘free’ light source which can be further exploited by installing skylights on the roof. On an overcast day, the light entering through a 2 sq. ft skylight area is equivalent to three 100-watt light bulbs.
There are two additional reasons for installing skylights. First, the cooling load of a building can be reduced by using daylight. The reason for this is that whereas about 80% of the power of an electric light is converted to heat, sunlight has a far lower heat content and therefore requires far less air-conditioning. Second, just at the time when there is the heaviest demand for electricity (and other utilities) from manufacturing facilities, namely during the summer, sunlight is at its most plentiful and available: in short, skylights can significantly reduce peak load stresses and costs.
An illumination level of 50 footcandles (540 Lux) at the work site is the design standard in industrial facilities. A large portion of this illumination level comes from electric lights of fluorescent and incandescent lamps. Almost 5% of electricity consumption in the US is used up to provide adequate illumination in commercial and industrial buildings.
In production facilities generally, there is a lack of awareness about the energy conservation potential of skylights. Some manufacturers are so unaware about the cost savings that can be achieved by daylighting that they do not have skylights in their production areas, and leave lamps on in work areas throughout daylight hours. In other places which do have them, skylights have been neglected to the extent that they are so dirty they block the incoming sunlight.
Since heating and cooling load are increased with increased skylight surface area, there is a limitation associated with this measure. Some authorities have suggested that the optimum skylight surface area should be reckoned at between 2 and 4% of roof surface area. However, since the measure depends upon local climate conditions, the range should be allowed to vary between 2 and 10%. In many potential sites, heating only (and not cooling) is the principal consideration. Generally, therefore, building designers with heating costs in mind tend to prefer 2% for cold climates and 10% for warm climates.
Heating load and costs will increase when skylights are installed. However, the increase in heating cost is considerably smaller than the saving from reduced lighting cost. The average unit cost of electricity is three times greater than that of natural gas (typically preferred for heating). In any case, heat loss from the skylights can be minimized by double glazing them. In view of the favourable financial balance and the productivity improvements to be expected from daylight working, the benefits from installing skylights generally offset any negative consequences of doing so.
The amount of savings in electric lighting consumption and costs depends on climate and operating periods. The payback period for installing skylights ranges from one to five years. They can be an expensive roof aperture, but it is relevant to note that the lifetime of a skylight is more than twenty years. The skylights need to be cleaned at least once annually, which means that service and maintenance costs are negligible. Skylights are most cost- effective in uninsulated ceilings in climates, like that of southern California, which have no heating season. In such climates, the workplace roof is typically covered with corrugated metal sheets making skylights both easy and cheap to install: corrugated fibreglass sheets can be cut and fitted in place of the metal sheets wherever the skylights are required.