Energy and Building, 1 (1977) 65 - 67 65 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands

Energy Conservation Opportunities in Luminaire Design and Installation

KENNETH LIM

Wellmade Metal Products Co., Oakland, Calif. 94621 (U.S.A.)

EDWARD DEAN

Department of Architecture, University of California, Berkeley, Calif. 94720 (U.S.A.)

Fluorescent lamp operating characteristics are discussed. Energy consumption and light output are sensitive to ambient temperature in such a way that lamps installed in lumi- naires operate far from the optimum condi- tion. The effect is worse for surface-mounted fixtures than suspended fixtures. Fixture ratings are inaccurate because it is assumed that the lamp rating for the installed lamp is the same as the bare-bulb at the fixture ambient temperature. In practice this results in the over-design of lighting levels. Energy efficiency can be improved by (1)improved lamp design, so the lamps achieve optimum light generation at higher temperatures, (2) improved fixture design, such as air-cooling, and (3) improved fixture mounting, such as chain- or stem-suspension.

Fluorescent lamps are sensitive to ambient temperature, an effect not shared by incan- descent or high-intensity discharge light sour- ces. Nearly all fluorescent lamps are designed to produce maximum light output for an ambient temperature range of 70 °F to 80 OF in still air. Figure 1, adapted from the IES Handbook [1], shows that this corresponds to an optimum temperature of about 105 OF at the coolest spot of the lamp wall. As can be seen from this figure, the total efficiency of the fluorescent luminaire is adversely affected when the operating lamp temperature departs from the optimum. This departure occurs when the lamps are installed in the luminaire. In fact, the operating temperature profiles of the most widely used indoor fluorescent luminaires, namely the surface-mounted enclosed type (20% of the total luminaires manufactured) and recessed luminaire static types (50% of the total luminaires manufac-

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Fig. 1. Typical fluorescent lamp temperature charac- teristics. Exact shape of curves will depend on lamp and ballast types; all fluorescent lamps have curves of the same general shape, however, since the shape depends only on the mercury vapor pressure.

tured) exhibit much higher lamp operating temperatures (see Fig. 2). Note that for the surface-mounted enclosed luminaire, the lamps operate at much higher temperatures because of the geometry. The ballast also does not cool as readily, thereby creating a higher internal ambient temperature.

The particular type of luminaire design can therefore cause a large shift from optimum lamp performance. This shift is caused not only by an increase in lamp wall temperature but by a change in the temperature distribu- tion on the surface of the lamp as well. That is, the lamp "bums" differently in different types of luminaires. Hence, the lamp rating obtained from measurements of a bare-bulb burning in free air is not the same as that for a lamp installed in a particular fixture. For

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Fig. 2. Typical four-lamp enclosed luminaires. (Above) surface mounted, (below) suspended, recessed in suspended ceiling.

example, a lamp rated at 3000 lumens for a particular temperature at the coolest spot of the lamp wall may produce only 2500 lumens when installed in a fixture and operating at the same "coolest spo t" temperature. The mounting condit ion of the luminaire further alters this light output . In fact, manufacturers give a fixture rating on a photometr ic data sheet based on measurements of one particu- lar installation condit ion, namely suspended in free air at 77 ~F ambient air temperature. No indications are given on the data sheet as to variations from this rating for different types of installation. Surface-mounting, for instance, would result in a lumen ou tpu t less than the data sheet rating.

The result in practice, curiously, is not an installed footcandle level lower than predic- ted, but generally a higher one. (The reason for this is that lighting designers, aware of this discrepancy, typically derate the fixture by a factor that gives a more correct value.) This factor is a conservative one, however, and though reasonably accurate for surface- mounted fixtures, for example, it leads to over-design for other types of mounting where the fixture rating is closer to actual conditions. What is needed, therefore, is a set of fixture ratings given on the photometr ic data sheet that distinguishes among common mounting condit ions and that accounts for

the change in lamp rating when installed in the fixture. This set should include the laboratory test condition, surface mountings for an array of ceiling-types, as well as suspended and recessed mounting conditions. Data for each of these t y p e s should specify ambient temperature, lamp cavity temper- ature and the lamp "coolest spo t" wall temperature.

An additional point is that this percent reduction of light ou tpu t is greater than that of the power consumed. Under normal ope- rating conditions, a typical surface-mounted four-lamp enclosed luminaire produces, ac- cording to standard measurements, approxi- mately 75% of the fixture-rated light output while consuming 87% of the fixture-rated input power; in a typical installation a static recessed four-lamp luminaire produces approx- imately 80% of the fixture-rated light ou tpu t while consuming 90% of the fixture-rated input power.

In order to improve the luminaire operating efficiency it is necessary to provide an envi- ronment in which the fluorescent lamp will produce its rated lumens. Controlled spot cooling or water cooling of fluorescent lumi- naires are effective means of achieving a high efficiency of light ou tpu t over a wide range of temperature. However, due to the require- ment of additional hardware in the luminaire

TABLE 1

Comparison of standard lamps and new 'low-energy" lamps (data from General Electric Co.)

67

Lamp Watts Lumens Efficacy Improvement

GE F40 Watt-Miser 86 5700 66 5%

GE F40 Mainlighter 100 6300 63

GE F96 Watt-Miser 140 11200 80 2.5%

GE F96 Slimline 175 13660 78

and difficulties in the installation, these ap- proaches are considered impractical at present. A group of special lamps could be designed for optimum light generation at temperatures other than 105 °F, but this is also considered impractical because of the increase in lamp inventories that it would entail.

The following are more practical means of improving luminaire efficiency in the system without additional expense:

1. Chain- or stem-suspended fixture instal- lations perform approximately 10% more efficently over the same fixtures installed directly on the ceiling, while consuming approximately 3% more energy; thus, the efficacy* improves 7%.

*"Efficacy" is defined in this case as the ratio of lumen output to rate of energy consumption (lumens/watt). This is distinguished from "efficien- cy" by the convention that efficiency is a dimension- less quantity and always some fraction less than or equal to 1.

2. Air-cooled recessed luminaires produce approximately 10 - 15% more light compared with the static recessed luminaires, while consuming only 3 - 5% more energy; thus the efficacy improves 7 - 10%.

3. Use of the low-energy ballast {rated at 300 mA) improves the fixture efficacy by approximately 10%. These ballasts, which cost about the same as standard ballasts, provide cooler ambient conditions within the luminaire.

4. Use of low-energy lamps (GE Watt-Miser, for example) reduces energy consumption with a correspondingly smaller reduction in light output, thus yielding a higher ratio of lumens/watt. Table 1 compares two such lamps with their predecessors, assuming a typical two-lamp open fixture.

REFERENCES

1 IES Lighting Handbook, Waverly Press, Baltimore, 5th edn., 1972, pp. 8 -26.