New Maintenance Study Reveals Unexpected Source of Lighting Cost Savings

April 5, 2004
Lighting Strategies - Online Exclusive

Yes, a three-year EPA-funded study about dirt may be able to save you money in both new construction and retrofits of existing buildings.

The interNational Association of Lighting Management Companies (NALMCO) recently completed a comprehensive study of luminaire (lighting fixture) dirt accumulation that may significantly impact lighting design and energy-saving upgrades.

The findings: Existing light loss factors related to dirt and dust build-up on fixture surfaces overestimate the extent of light loss that is traditionally estimated.

What does this mean?
It means that lighting systems are being over-designed - with additional capacity for producing light that can be trimmed to generate significant cost savings, up to 10 percent, according to experts. Specifically…

New construction: In lighting designs, this offers the opportunity to reduce the number of fixtures required to achieve the target maintained light level - reducing fixture initial and operating costs for the owner.

Existing buildings: In retrofit situations, it offers the opportunity to select components that produce less light output while saving more energy. Significant energy savings can also be gained through facility-wide dimming.

The Luminaire Dirt Depreciation (LDD) study results are being incorporated into a new Illuminating Engineering Society of North America (IESNA) Recommended Practice (RP) on maintenance (RP-36) and a future IESNA Handbook chapter by the IESNA and NALMCO Joint Committee. To make the LDD factor easier to calculate and use, a new calculation procedure has also been developed. I had the pleasure of writing the RP on behalf of the IESNA Maintenance Committee.

The RP, which also includes troubleshooting, procedures, lamp disposal, planned maintenance operations, and other information, is available from NALMCO by clicking here.

In this column, we’ll take a detailed look at how lighting system initial and operating costs can be reduced through application of this study’s findings. But first, we need to understand the impact that dirt and dust have on lighting fixture performance.

Lighting systems are designed to provide a sufficient amount of light output so that - when depreciated over time by various light loss factors - required light levels on the workplane result.

The basic process looks like this:

Light leaves fixture >>Light is degraded by environment>> Light reaches where it’s needed

Dirt and dust build-up is a primary cause of degradation; as dirt and dust accumulate on luminaires (light fixtures) and lamps, the particles absorb light rather than reflect it, resulting in a light loss factor called Luminaire Dirt Depreciation (LDD).

To account for LDD, lighting professionals design extra capacity into the lighting system in the form of fixtures and/or lamps and ballasts. The idea is that if a percentage of light output is going to be lost over time, the lighting system needs that extra capacity to ensure that, over the long haul, the occupants of the space have enough light to perform tasks efficiently, accurately, and safely. This equipment costs money to buy, install, and operate for a period of many years.

What is the impact of the new maintenance study on the lighting design profession - and on commercial buildings?
The study indicates that in a typical commercial environment, the extent of dirt depreciation on light fixtures is not as high as the accepted norms used by the lighting community for more than 50 years.

In the timeframe of a two- to three-year cleaning cycle, for example, the new findings recommend an approximate 10-percent loss, for many areas, vs. present lighting standards, which recommend allowing for light loss on the order of 20 percent.

“Test results indicate that in very clean locations, about 8-10 percent fewer fixtures are required to provide a specific light level compared to using design calculations with earlier LDD values,” says Norma Frank, CLMC, chair of the IESNA Maintenance Committee and vice president of Colorado Lighting. “Renovation projects in older facilities would result in the order of 15-20 percent fewer fixtures if this new data is utilized.”

“With more states mandating specific watts-per-square-foot limitations for new and renovated facilities, more accurate LDD factors will help achieve improved designs and lower capital expenditure and energy usage,” said Dr. Levin. “With this scientific data to support a change in design and maintenance standards, commercial facilities that collectively spend $27-$36 billion per year to operate their lighting systems can realize cost savings of 10 percent - potentially as much as $3.6 billion annually.”

The Study
The controlled LDD field study was conducted in 1996-1999 by NALMCO, promoted by the IESNA, and funded by the U.S. EPA. It included more than 200 sites at office, retail, and school facilities in the United States. Four popular recessed fluorescent lighting fixture types were studied: 2x4 lensed, 2x4 louvered, 2x2 louvered, and 2x4 air exhaust louvered - which collectively represented, based on some estimates, approximately 90 percent of recessed fixtures in operation in the United States in 1995. The split between lensed and louvered fixtures in the study was about 50-50. Technicians at 10 lighting management companies gathered the test data using an instrument specially designed to capture the total peak fixture light output emerging from the fixture.

Eight fixtures at each site were thoroughly cleaned and relamped. After six months, one fixture at each site was tested to record light output values: 1) when dirty, and 2) with the lamps and fixture cleaned. After 12 months, the test was repeated on another fixture at each site, and again after 18, 24, 30, and 36 months.

In Figure 1, the new LDD function, as determined by the test results, is contrasted with lensed and louvered fixtures in clean conditions (assumes better-than-average air filtration and some generated or ambient dirt). At 18 months, the LDD factor is 0.92 versus 0.84-0.85 using the traditional IESNA procedure, and at 36 months, the LDD factor is 0.89 versus 0.75-0.80. Lensed and louvered fixtures show virtually identical dirt depreciation and variable operating hours per year have negligible effect, according to the study.

“This is the first study ever on the subject of dirt depreciation to be completed in both a comprehensive and scientific manner,” says Dr. Robert E. Levin, senior scientist for OSRAM Sylvania and a primary technical advisor for the study. “Data was informally collected in the 1950s on maintenance, not as a controlled study.”

Figure 1. The LDD function for both louvered and lensed fluorescent fixtures determined by analysis of the LDD study test results (top curve). Previous IESNA procedures place louvered fixtures into Maintenance Category IV and lensed fixtures into Maintenance Category V, with their LDD curves shown for comparison.

Impact: The new LDD data can significantly impact lighting design of commercial facilities facilities where fluorescent, flat-bottomed, and either recessed or ceiling-mounted fixtures are installed.

New Installations. For new installations, the required number of fixtures is determined:
N = [(Lighted Area) * (Desired Illuminance)] / [(Bare Lamp Lumens/Fixture) * CU * LLF * LDD]

Assuming a three-year cleaning cycle, the LDD factor for a lensed fixture in an open office plan using the new LDD procedure is 0.89 and the LDD factor using the old procedure is 0.80. The below example illustrates fixture savings resulting from application of the new LDD factor. We’ll take a look at how many lighting fixtures we need using the old LDD and the new LDD:

Area:8,000 sq.ft.Target light level:50 fcOther light loss factors:Assume coefficient of utilization (CU) of 0.62 and the product of all other light loss factors (LLF) to be 0.75Fixture type: 4-lamp louvered fixture with 9,000 lamp lumens/fixture

The number of fixtures required to achieve the target light level is:

Old LDD (0.80):No. Fixtures = (50 * 8,000) / (9,000 * 0.62 * 0.75 * 0.80)
No. Fixtures = ~119 fixtures
New LDD (0.89):No. Fixtures = (50 * 8,000) / (9,000 * 0.62 * 0.75 * 0.89)
No. Fixtures = ~107 fixtures

In this early design phase, about 10-percent fewer fixtures are required to achieve the desired maintained light level of 50 footcandles (fc) in the open office. This saves initial costs and also operating costs in perpetuity.

Existing Installations. In existing spaces, lighting designs can be re-evaluated using the new data to retrofit or redesign to generate operating cost savings. Retrofits include a variety of ballast, lamp, and dimming upgrade options with reduced light output that can generate significant energy savings. In addition, flexible facility-wide dimming systems can be used to reduce light output at the fixture and save energy.

Money can be saved here, but remember the bottom line: The lighting system must produce enough light to satisfy the needs of occupants. For this reason, it may be wise to conduct light level measurements and also make sure the fixtures are cleaned periodically.

Back to our textbook example above, let’s see how much flexibility we can gain to reduce energy costs through replacement of existing lamps and ballasts:

Area: 8,000 sq.ft.Target light level:50 fcFixture type:4-lamp louvered fixtureBallast: Magnetic 120V, ballast factor = 0.86Lamps:F40T12/ES, 2,650 lumensTotal system/fixture wattage:67W/134W

Required Light Output/Fixture (Lumens) = (Desired Illuminance * Lighted Area) ÷ (Number of Fixtures * CU * LLF * LDD)

The number of lumens required per fixture is:

Old LDD (0.80):No. Lumens = (50 * 8,000) / (120 * 0.62 * 0.75 * 0.80)
No. Lumens = 8,961
New LDD (0.89):No. Lumens = (50 * 8,000) / (120 * 0.62 * 0.75 * 0.89)
No. Lumens = 8,055

Our present system produces about 9,100 lumens per system of two ballasts driving four lamps. Our new target is approximately 8,055 lumens. One method is a facility-wide dimming system, which can reduce the light output and energy consumption (requiring dimming ballasts as the typical option). Another is a retrofit of the lamps and ballasts. Due to the reduction in light output required, we now have more flexibility in choosing lower-power, lower-output equipment.

Two upgrade options include:
(1) electronic ballast powering (4) F32T8 lamps. Ballast factor = 0.77. Initial lumens per lamp = 2,850. Light output = 8,550 lumens/fixture. Total wattage/fixture = 100W. Power savings = 37W. In addition, one less ballast is required.
(1) electronic ballast powering (3) F32T8 lamps. Ballast factor = 0.88. Initial lumens per lamp = 3,100. Light output = 8,184 lumens/fixture. Total wattage/fixture = 85W. Power savings = 52W. In addition, one less ballast is required.

There are other options, but these two illustrate a good example. Suppose we go with the second option, and that our energy-saving picture looks like this:

No. Fixtures: 120
W/fixture (old): 134
W/fixture (new): 85
Hours of operation: 16 hours/day x 5 days/week x 50 weeks/year = 4,000 hrs
Energy savings: 4,000 * (134-85)/1000 = 208 kWh

Assuming an energy cost of 10 cents per kWh from the local utility, we would be projected to save $20.80 per fixture per year, which comes out to $2,496 per year for 120 fixtures. We may also reduce demand charges.

In the future, the LDD study will also likely have an impact on energy codes, starting with the ASHRAE/IES 90.1-2001 Standard (addendum g).

For more information about the study and the planned maintenance guide, click here:

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