Technologies > HIGH-BAY: FLUORESCENT VS. METAL HALIDE

Introduction: Traditionally, fluorescent lighting has been used at fixture mounting heights of <15 ft., and HID lighting has dominated spaces with higher mounting heights, due to the different characteristics of each type of lighting system. In spaces where color quality of the light is important, metal halide (MH) is generally the preferred HID source. At higher ceiling heights, 250W and 400W lamps are common.

In recent years, manufacturers have begun offering fluorescent fixtures designed to replace MH fixtures in low-bay and even high-bay applications. The fixtures house two, three, four, six or eight T8 or T5HO lamps to provide various levels of light output. A number of single-point pendant-mounted models are available for retrofit.

Switching to fluorescent can result in energy savings as high as 50 percent with a modest reduction in maintained light output, while gaining the advantages of fluorescent lighting.

Meanwhile, other choices have become competitive, including pulse-start MH and induction lighting systems.

Fluorescent lighting offers many advantages over MH lighting in high/low-bay applications, but can present trade-offs. As is always the case with lighting, there is no single best solution for all applications within a given space type. The best solution depends on the application requirements. Below is a comparison between probe-start MH and fluorescent in applications with higher mounting heights. Other choices not discussed below include pulse-start MH and induction lighting.

Standard MH (Probe-Start): Probe-start MH lamps are the most common installed MH lamp type. MH lamps are rugged, efficient, long-life, compact, powerful point sources that can cover a large area with few fixtures. They are able to operate reliably in a wide range of ambient temperatures, including very cold environments, and numerous fixture models are available for demanding environments such as hazardous locations.

MH lamps have several disadvantages. They experience color shift over time, which may result in poor lamp-to-lamp color consistency. They require several minutes to start, up to 10 minutes to re-strike after recently being shut off. Most significantly, their light output can experience a severe drop over time. A 400W MH lamp, for example, can see a 35 percent drop in light output at 40 percent of its rated service life.

Fluorescent: Fluorescent lighting can offer a number of advantages versus probe-start MH, including higher efficiency/energy savings, higher lumen maintenance, instant on and re-strike, emergency ballasting (eliminating need for auxiliary lighting), higher color rendering ability, negligible color shift, lamp-to-lamp color consistency, wide range of color options, and longer lamp life (versus 250W MH lamps).

As shown in Table 1 (below), a four-lamp F54T5HO fixture can replace a 400W standard MH fixture, for example, and save about 50 percent on energy while producing about 20 percent less maintained light output. A six-lamp F32T8HO fixture can replace the same 400W MH and save about 50 percent on energy while producing about 16 percent less maintained light output.

Table 1. Comparison of 400W probe-start MH system with competitive T5HO and T8 systems. Source: Advance.

	400W Probe-start MH	(4) F54T5HO Fluorescent	(6) F54T5HO Fluorescent	(6) F32T8 “High Lumen” Fluorescent
No. Lamps	1	4	6	6
Service life	20,000 hours @ 10 hours/start	24,000 hours @ 10 hours/start	24,000 hours @ 10 hours/start	28,000 hours @ 10 hours/start
Initial lamp light output	36,000 lumens	20,000 lumens	30,000 lumens	18,600 lumens
Ballast	Probe-start magnetic	Program start	Program start	Instant start
Ballast factor	1.0	1.0	1.0	1.18
Initial system light output	36,000 lumens	20,000 lumens	30,000 lumens	21,948 lumens
Lamp watts	400W	216W	324W	192W
System watts	458W	234W	351W	222W
Relative system watts	100%	51%	77%	48%
Initial system efficacy	79 lm/W	85 lm/W	85 lm/W	99 lm/W
Mean lumens	23,500 lumens @ 40% of lamp life	19,000 lumens @ 40% of lamp life	28,500 lumens @ 40% of lamp life	20,851 lumens @ 40% of lamp life
Lumen maintenance	65%	95%	95%	95%
Relative maintained light output	100%	81%	121%	89%
Maintained system efficacy	51 lm/W	81 lm/W	81 lm/W	94 lm/W
Color rendering	65 CRI	82-85 CRI	82-85 CRI	85 CRI
Starting Time	4 minutes	<1.5 seconds	<1.5 seconds	<1 second
Re-strike Time	10 minutes	<1.5 seconds	<1.5 seconds	<1 second
Color temperature	3000-4000K	3000-5000K	3000-5000K	3000-5000K

In addition, fluorescent lamps, as a linear source, offer potentially more uniform lighting, less shadows and less glare. Fluorescent is easily and inexpensively dimmable, can be dimmed the full light output range, and is friendly with switching and control strategies using devices such as occupancy sensors, photocells and scheduling systems due to its instant on and re-strike capability. These control strategies can generate significant additional energy savings while reducing peak demand, shortening the payback. New line-voltage occupancy sensors have significantly reduced the installed cost of these devices, making it economical to install one sensor per fixture.

Disadvantages include inability to start and operate efficiently at extremely cold temperatures, in addition to performance sensitivity to high ambient temperatures. Fluorescent will also require the installation of four to six times more lamps to be maintained (although, on the plus side, when one lamp fails, the fixture will still produce light, whereas if a metal halide lamp fails, the entire space it was lighting will become darkened). Typically, in an upgrade situation, switching to fluorescent will require new fixtures. While one-to-one fluorescent to MH fixture replacement is often feasible, potentially up to two to three times the number of fixtures may be required to purchase and install, which can lengthen payback in an upgrade scenario.

T8 Vs. T5HO: Regarding fluorescent, the choice is T8 or T5HO. Lighting professionals may hear that T8 fixtures are better at fixture heights <20 ft., T5HO at >20 ft., and either T8 or T5HO at 18-25 ft. mounting heights. However, while T5HO may produce “glare bombs” at lower mounting heights, both T8 and T5HO can be used in high- and low-bay applications.

T5HO lamps produce about twice the light output of a standard T8 or T12 lamp in approximately the same length and a fraction of the diameter, making them very bright. While they produce less light than a standard MH lamp, they have 95 percent lumen maintenance compared to 65 percent for a standard MH. This means a six-lamp F54T5HO system produces about 83 percent of the initial light output of a 400W MH lamp, but 121 percent of its light output at 40 percent of lamp life.

For bay applications, standard T8 lamps are generally not used. Instead, F32T8HO, or Super T8, are used. Super T8 lamps produce 3100+ lumens; when operated on a high ballast factor (1.15) ballast, this system can produce 3,565 lumens, 71 percent of the light output of a comparable F54T5HO system.

Besides light output, ease of replacement should be considered. To get higher light output from a T8 fixture, Super T8 lamps should be specified, but the owner may substitute to lower-output (and less expensive) T8 lamps that may be used elsewhere in the facility such as offices. This will result in lower light levels than expected. T5HO and Super T8 lamps are typically offered only by electrical distributors.

T5HO and T8 lamps are sensitive to ambient temperature in the fixture’s ballast compartment. T5HO is designed to operate optimally at higher temperatures (35 ºC or 95 ºF) and T8 at lower temperatures (25 ºC or 77 ºF).

In addition, the presence of occupancy sensors and other switching strategies is a factor in whether to choose T8 or T5HO. If occupancy sensors are used and the lamps are frequently switched, lamp degradation will occur after 12,000 to 15,000 switching cycles on a T8 lamp operated on an instant start electronic ballast, the most common T8 ballast type. Some lamp manufacturers may not warranty their T8 lamps when operated on instant start ballasts and controlled by occupancy sensors. Super T8 lamps, however, are rated to last 4,000 hours longer than T5HO lamps.

On the other hand, T5HO lamps are operated with programmed start ballasts, which are generally more expensive and are wired in series or series-parallel compared to parallel for instant start ballasts.

Project Examples: Timken Aerospace chose fluorescent when it upgraded its 142,000-sq.ft. manufacturing plant in Lebanon, NH. Because the facility makes precision ball bearings, high-quality lighting is required. The lighting operates around the clock, every day. In 1999, Timken upgrade 543 standard 400W metal halide low-bay fixtures with 2x4 232W T5HO fixtures from MetalOptics. The project, implemented by Timken personnel, cost $268,000 and benefited from a rebate of $91,500 from Granite State Electric, the plant’s utility. Timken reduced its demand by 120kW and saved more than 1,000,000 kWh, an annual savings of $70,200, resulting in a payback of 2.5 years. Timken also achieved its goal of high-quality lighting.

World Gym retrofitted eighteen 400W metal halide fixtures each with four T5HO biax lamps, reducing demand by 3.96kW and energy consumption by 54,810kW, or about 50%, resulting in $3,735 in savings per year. The investment yielded a simple payback of 1.8 years. Significant energy savings resulted from the installation of advanced lighting controls, which reduced the hours of operation by 55%. The new T5HO system provided 34% more light than the metal halide in addition to improved color quality.

 

Back to Main