Lighting is a factor that provides a significant opportunity to reduce operating costs, improve occupant performance, and reduce greenhouse gases that contribute to global warming.
With the help of lighting manufacturers, you can reduce lighting power density by a minimum of 25- to 35-percent below ASHRAE 90.1-2004 guidelines while still meeting the New York City-based Illuminating Engineering Society of North America's lighting-design recommendations. As a starting point, you can use a space-by-space method to calculate lighting power density as defined in ASHRAE 90.1-2004, Section 9.6, to develop a base case against which to judge the lower power density goals.
The lighting power density table, which illustrates typical lighting power densities allowed in ASHRAE guidelines, shows lower lighting power densities that can still provide excellent visual quality and recommended footcandle levels for both positive physiological and psychological perceptions, which lead to improved productivity and efficient work-mission delivery.
In addition to lowering construction costs and total cost of ownership, interior finishes, texture, and color greatly affect visual quality, distribution of light, and lighting performance. Higher surface reflectance on ceilings and walls increases lighting levels by reflecting light from available light sources. Improved reflectance in the space allows for a reduction in light wattages to accomplish the same lighting levels.
Glare from specular surfaces (mirrors, glass, etc.) will reduce visual quality and occupant comfort. If daylighting is a goal, an 80-plus percent ceiling reflectance and 70-plus percent wall reflectance in daylight zones are recommended. (Reflectance values are available from paint and fabric manufacturers.)
To achieve recommended lighting power density (LPD) reductions, high-performance T8 lamps and electronic ballasts can be used for general lighting. High-performance T8 lamps are defined as having a lamp efficacy of 90-plus nominal lumens per watt based on "mean lumens."
Mean lumens are published in lamp catalogs as the degraded lumen output occurring at 40 percent of the lamp's rated life. Higher performance can be achieved either by increasing the output (3,100 lumens) while keeping the same 32-watt input as standard T8s, or by reducing the wattage while keeping the light output similar to standard T8s (e.g. 2,750 lumens for 28 watts or 2,850 lumens for 30 watts).
When selecting T8 and energy-saving T8 lamps, designers should carefully consider the lamp's color rendering index (CRI). The CRI is a scale measurement identifying a lamp's ability to adequately reveal color characteristics. The scale maximizes at 100, with 100 indicating the best color-rendering capability.
Depending on the mission of the facility, the CRI can very. For example, lamps specified for the ambient and accent lighting of retail merchandise or office environments should have a CRI of 80 or greater to allow consumers to effectively examine the color component of a product and provide office occupants with good visual comfort.
High-performance electronic ballasts, defined as a two-lamp ballast using 55 watts or less with a ballast factor (BF) of 0.87 or greater, are also necessary to achieve the desired performance. One-lamp, three-lamp, and four-lamp ballasts may be used, but should have the same or better efficiency as the two-lamp ballast. Dimming ballasts do not need to meet this requirement. Higher-output 3,100-lumen lamps are visibly brighter than standard T8s. The use of ballasts with a BF of 0.77 may provide more comfortable lamp brightness without sacrificing efficiency in direct luminaires where the lamp is visible.
- In addition to lowering the lighting power density through better design practices, the use of daylighting can also significantly reduce electrical consumption and greenhouse-gas emissions. Daylight in buildings can save energy if the electric lighting is switched or dimmed in response to changes in daylight levels in the space.
- Automatic lighting controls increase the probability that daylighting will save energy. It is also important to select glazing characteristics for specific climate zones to reduce heat gain and loss through glazing, and also control glare and contrast so occupants are comfortable and not inclined to override electric lighting controls.
- In hot climates, designers should use north-facing clerestories for skylighting to eliminate excessive solar heat gain, glare, and air-conditioning loads. Typically, north-facing clerestories have one-sixth the heat gain of skylights. If a north-facing clerestory is not an option, an alternative strategy would be to use smaller-aperture skylights in a grid pattern to gain maximum usable daylight with the least thermal heat transfer.
- In moderate or cooler climates, the use of either north- or south-facing clerestories for skylighting is a good strategy. Also, skylight frames that have a thermal break to prevent excessive heat loss/gain and winter moisture condensation on the frame are recommended. In more arid climates, or during swing months when outside humidity conditions are lower, clerestories with operable glazing can provide natural ventilation that significantly reduces the need for air-conditioning.
H. J. Enck is principal and founder of Atlanta-based Commissioning & Green Building Solutions Inc. (CxGBS) (www.cxgbs.com).
