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When I first started my electrical design career 20 years ago, a lighting design with a lighting power density (LPD) of 3.0 W/sq ft or higher was common and a design of 2.0 W/sq ft was considered “efficient.” Today, lighting power densities that high would never be accepted; 1.0 W/sq ft and lower is typical in most building design applications. That’s a 66% energy use reduction compared to systems installed 20 years ago. Few other design sectors can claim such a dramatic improvement in efficiency in that timeframe.
Over the past 20 years, not only have lighting technologies, lamp sources, and controls improved considerably, but energy codes and green building standards have also driven what we consider to be efficient. We no longer simply lay out 2 x 4-ft, 4-lamp troffers on an 8 x 10-ft grid spacing. Lighting designers, architects, and engineers work together to balance aesthetics, lighting quality, and energy for a better total lighting solution that performs well and complies with energy codes. We do not have to sacrifice the quality of lighting designs or reduce lighting levels just to meet energy codes.
Lighting as an energy reduction target
Buildings use a lot of energy. And a lot of that energy is used for artificial lighting. According to the U.S. Energy Information Administration, 21% of the total energy used in commercial buildings and 38% of all electricity used in commercial buildings is used for artificial lighting (see Figure 1).
In the “original” articficial lighting source developed by Thomas Edison more than 100 years ago, incandescent, visible light was merely a by-product. Incandescent lamps produce light by passing an electrical current through a filament of tungsten metal until it gets so hot it glows. Incadescent sources are essentially resistive heaters with 10% of the input energy producing visible light and 90% of the energy producing heat. Modern lighting sources such as fluorescent and LED are much more energy efficient but still produce heat as a by-product, which has to be removed from the building by adding more cooling capacity to the building’s HVAC system. For every 100 W of lighting that is NOT put into a building, approximately 50 W of cooling energy is saved (depending on the region), making energy-efficient lighting a very attractive target for overall building energy reduction.
Lighting and energy codes
Lighting is a primary component of a commercial building’s electrical system. In the United States, there are a number of energy codes and sustainability standards that help drive overall building energy performance including lighting efficiency.
Each of these codes and standards has its own goals, focus areas, and applications. It can be hard to keep them all straight. Table 1 summarizes various building performance standards and compares their lighting energy requirements for typical hospital/inpatient healthcare, commercial office, and school/university educational buildings.
ASHRAE Standard 90.1 is generally considered the industry accepted baseline standard for building energy performance and is incorporated by reference or otherwise integrated into most energy codes and green building standards. ASHRAE Standard 90.1 addresses lighting energy in two ways:
Some engineers consider the additional step of calculating the LPD to prove compliance with energy standards to be time consuming and burdensome. However, the use of ASHRAE’s Building Area Method makes the calculation simple by applying a uniform LPD for the building and calculating the overall wattage allowed by multiplying the LPD by the overall building area. For example, if a 500,000-sq-ft hospital is allowed an LPD of 1.2 W/sq ft, the total lighting power budget is 500,000 sq ft x 1.2 W/sq ft = 600,000 W. Designing a building’s lighting systems without considering the impact on the building’s overall energy performance is akin to an architect designing a building without taking the structural systems into account.
You can do it, but you will end up redesigning in the end. Lighting designers, engineers, and architects must design with LPD in mind.
The ASHRAE 90.1 requirements are based, at least in part, on Illuminating Engineering Society (IES) lighting level recommendations and current energy-efficient technologies proven to be cost effective. Essentially, this means the baseline design parameters for interior lighting for most commercial spaces are T8 or T5 fluorescent lamps and compact fluorescent lamps (CFL), although the use of light-emitting diode (LED) sources continues to increase. It also means the widespread use of occupancy sensors and other building-wide lighting control systems to automatically turn off lights after hours or when the lighting is not needed.
Quality lighting and energy efficiency
Some lighting designers and engineers may look at energy codes as a hindrance or an obstacle to good lighting design. I believe the opposite to be true: Energy codes help designers apply individualized approaches and design solutions to each building space in order to meet that space’s specific needs. By selecting appropriate lamps and fixtures, integrating daylighting, and applying automatic lighting controls, designers can easily meet and exceed energy code requirements. Some strategies to balance energy efficiency and lighting quality may include:
But even with new state-of-the-art lighting technologies and implementing synergistic design approaches, how low can we go? What is the low limit of LPD? 0.5 W/sq ft? 0.2 W/sq ft? 0.1 W/sq ft? There are diminishing returns for ultra-low LPDs; designers still need to use some power to produce artificial light. Until we see a new paradigm-shifting light source or new technology developed, energy codes are not likely to require much lower LPDs than they currently do. Still, seemingly incredible low LPDs are being achieved through good, synergistic design.
In addition to energy performance, many codes and green building standards, including LEED, also address light pollution reduction, light trespass, and site lighting controls. Light pollution reduction not only reduces glare, but also the concept of keeping site lighting directed downward (and not up into the sky), and keeping the light on the building’s property (and not spilling over onto your neighbor’s property) inherently reduces energy usage and cost.
Many local codes also include requirements for site lighting uniformity, with max: min or avg: min footcandle ratios that must be met, and dusk-to-dawn or other lighting control requirements. These are strategies designers and engineers should already be implementing. Again, energy codes are not a burden to good lighting design; they help prevent bad lighting design.
Energy-efficient lighting designs do not have to result in reduced lighting quality. Energy codes ensure that energy performance, lighting power density, daylighting, and lighting controls are design considerations brought into the discussion along with light levels, color rendering, and aesthetics. We can have both energy efficiency and quality lighting. We just have to change the way we approach lighting design.
Mark A. Gelfo, principal, is director of sustainability at TLC Engineering for Architecture. A graduate of Penn State’s Architectural Engineering program, Gelfo has 20 years experience in lighting design, electrical engineering, sustainability, and commissioning.
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