Michael Lane LC

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HID vs. Fluorescent
October 1996

Back in July, when I was about to write this article, I had the opportunity to go out to a manufacturing facility with one of the Lighting Design Lab's sponsoring utilities. In the main production portion of the manufacturing facility the installed lighting system was 1000 watt Metal Halide luminaires. An energy service company had recommended that these luminaires be replaced with new 400 watt Metal Halide fixtures as an energy conservation measure. The facility manager had looked at the proposal's great savings and was ready to sign with the ESCO, but had decided to call his local utility first. After taking a tour with the facility manager I told him not to do the change out. Boy does the lighting industry need certification.

You see, the facility manager had already turned half of the luminaires over, pointing them at the ceiling. The measured footcandles at the production machinery was about 20 footcandles. I know the light levels sound low, but they were using strobes to check the registration of the printed material as it ran through the machinery. With the Metal Halides pointing down they washed out the strobes, and quality control went way down. To make matters worse the luminaires were only mounted at about 20 feet above the floor. I told the facility manager that he should either turn the remaining luminaires over, and have no energy savings, or replace the existing system with a new fluorescent lighting system.

Stop using Metal Halide luminaires in warehouse-storage-industrial facilities; fluorescent is more efficient! I know you are thinking that there is no way that a fluorescent system could be more efficient than a Metal Halide system when the light fixture s are mounted at 20 to 25 feet, or even higher. In many ways you would be correct, however from a true energy efficiency standpoint, a fluorescent system using electronic ballasts and T8/HO, T8/slimline, or 4-foot T8 lamps will deliver higher footcandles than a Metal Halide system.

Lets compare two lighting systems: a 400 watt metal halide "lowbay" luminaire and a 2-lamp F96T8/HO/RE70 industrial luminaire with a electronic ballast. The 400 watt metal halide luminaire uses 460 watts and produces 36,000 initial lumens, but only 27,700 (GE) mean lumens (60.2 lumens per watt) . A 2-lamp F96T8/HO/RE70 with electronic ballast uses around 160 watts and produces (16,000 - GE, 15,800 - Philips) initial and (14,400 - GE, 14,200 - Philips) mean lumens (88.75 lumens per watt). To produce the same light level you would need either a 4-lamp luminaire or two 2-lamp luminaires (28,800 - 28,400 mean lumens) compared to one Metal Halide luminaire (27,700 mean lumens). However the fluorescent system will be using 320 watts compared to 460 watts with the Metal Halide, a 30% reduction in energy consumption for the same lumen output!

Yes, there is the 400 watt high-output Metal Halide lamp that produces 40,000 initial, and 32,000 mean lumens at 460 watts (69.6 lumens per watt) . This would require five of the T8/HO lamps to provide the same lumen output, which would still uses 60 watts less energy. Additionally, there is no guarantee that when the Super Metal Halide burns out that another Super Metal Halide lamp will be installed.

Using a computer simulation and assuming a power density of 1.0 watt/sq.ft., a light loss factor of 0.7 for both systems, 60/40/20 reflectance, a 20 foot mounting height and no shelving in the space. The metal halide system produced 43 average maintained footcandles. The fluorescent system produced 52 average footcandles.

So, the fluorescent system will provide 20% higher light levels at the same wattage draw as the metal halide system, or a 20% reduction in energy consumption at the same light level. The lamp life for the two systems is comparable: 18,000 hours (12 hours per start) for the fluorescent and 20,000 hours (10 hours per start) for the metal halide. The lighting distribution (softer shadows) will be better with the fluorescent system due to twice the number of luminaires. The instant-on capabilities of the fluorescent eliminates the need, and cost, of the quartz restrike in the Metal Halide. Daylight control and scheduling scenarios should be cheaper and easier to implement with a instant-on light source. Color rendering and color consistency of the fluorescent is significantly better than with the Metal Halide.

On the negative side, there will be 4 times the number of fluorescent lamps to maintain and twice the number of fluorescent fixtures to purchase and install. The added cost for installation and maintenance may be offset by the lower cost of the of the fluorescent fixtures, lamps, and energy consumption. In unheated spaces the fluorescent system may have difficulty starting or providing full light output in cold environments.

So what is the answer? Well there are many options, and metal halide may not be the answer in low ceiling heights (20 feet mounting height or less; calculations show that the fluorescent system will still be 20% more efficient even at 35 foot mounting heights). But you must consider all the variables, number of luminaires, number of lamps, cost of equipment, type and quality of the luminaire, maintenance costs and energy costs.