The cost of lighting industrial buildings

Abstract

With the growth of the science of light and vision, the engineer has been putting forth some effort to use daylight to a greater advantage. In our endeavor to use natural and artifical light in a more economical manner, it becomes necessary to know the initial and operating costs of each. The determination of the initial cost of natural lighting is made by computing the difference in cost between a building equipped with natural and artifical lighting and a building of similar construction equipped merely with artificial lighting. Though it may be possible to dispense with natural lighting, it would hardly be possible to dispense with artificial lighting if work is to be pursued for eight hours a day throughout the year; for natural light is of no greater constancy than daylight itself. The major items entering into the initial cost of natural lighting equipment per 100 sq. ft. of floor area under normal conditions vary from a minimum in one type of standard building to a maximum in another type as follows: 5 Minimum Maximum Windows and structure $ 0.00 to $45.00 Heating equipment 10.30 to 32.10 Light-courts 8.40 to 45.30 Total for all three items for standard buildings 28.56 to 79.90 The investment cost of heating equipment to supply heat losses incident to windows varies from $.32 to $.45 per sq. ft. of window surface for buildings having side windows only, and from $.41 to $.59 for buildings having windows in the roof also. The annual cost of maintaining and operating the heating system to supply heat losses incident to windows varies from a minimum of 7 cents per square foot of window surface in Building A to a maximum of 12.3 cents in Building C. The initial total cost of electric lighting equipment per outlet or per 100 sq. ft. of floor area with lighting-units spaced on 10-foot centers, varies from $10.20 for 5.5 foot-candles on the work-plane to $17.50 for 38 foot-candles. Under average conditions the ratio of the initial costs of natural to electric lighting equipment varies from 2.25 for a single-story flat roof building to 6.3 for a sawtooth building. Under the same conditions the ratio of the annual cost of natural lighting to the annual cost of electric lighting varies from 1.09 for a single-story, flat roof building to 4.03 for a sawtooth building. For a minimum intensity of illumination of 13 foot-candles upon the work-plane the average time a lighting-unit operates a year varies from 651 hours for a single-story, flat roof building to 253 hours for a sawtooth building. For a minimum of 13 foot-candles, the average time a lighting-unit operates per year in a six-story building surrounded by buildings of its own height varies from 918 hours for 150-foot light-courts to 1,762 hours for 30-foot light-courts. The combined cost of natural and artificial lighting per hour per minimum foot-candle upon the work-plane attains a minimum at about 38 foot-candles for most types of standard buildings. Since the cost of heat loss due to windows is generally offset by the cost of electric energy saved by the incoming light, it would seem usually justifiable to equip industrial buildings with simple side windows. However, it would seem unjustifiable from a viewpoint of economy in lighting to incur a great expense for light-courts, or windows in the roof of the building unless the minimum intensity of illumination upon the work-plane is to exceed about 15 foot-candles. It appears that a practice of washing windows less frequently than twice a year is uneconomical excepting when the minimum intensity of illumination upon the work-plane is maintained at less than 10 foot-candles. When determined merely by the cost of lighting, it appears from these results that for any type of standard building investigated and operating under average conditions that no appreciable expenditure for natural lighting equipment is justifiable when electric energy can be purchased as low as 1.4 cents per kw.-hr.