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:
- 1. Power consumption of lighting is addressed by setting limits on lighting power density (LPD), measured in W/sq ft, based on the specific use of the space.
- 2. Mandates the use of lighting controls to shut off lighting automatically when it is not needed.
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:
- Use energy-efficient light sources. Linear and compact fluorescent lamps have been standard for years, and LED technologies have improved significantly over the past few years. They offer good light (lumen) output throughout their rated useful life (lumen depreciation), are rated for long life, and use very little energy. Compare a compact fluorescent source that uses 18 W to produce 1200 lumens and with a life of 10,000 hours to an LED source that uses 11 W to produce the same 1200 lumens and has a life of 100,000 hours. Even though and LED may use less energy, LEDs are not the “silver bullet” to energy savings that many people believe they are; they are not right for every application.
- Don’t use incandescents. Even the most efficient of incandescent sources are only 10% efficient and have very low efficacy in the range of 10 to 20 lumens/W. Today’s compact fluorescent and LED sources can produce the same light output, and color temperature and nearly the same color rendering, as most incandescents at much higher efficacy and longer life. Typical fluorescent source efficacies range from 85 to 95 lumens/W; LED efficacies can be 100+ lumens/W and are continually improving.
- Put the light where you need it. Using more light where you need it and less where you don’t may sound like a simple, obvious approach, but it is often overlooked. IES publishes standards on appropriate lighting levels (footcandles) in various typical building spaces, based on the tasks performed, surface reflectance, contrast, and occupant age. For example, lower lighting levels can be used in a storage room or a corridor than in an office or classroom. Using less light—and therefore less energy—in these areas allows designers to use more lighting energy in other areas and still meet the building’s overall lighting energy requirements.
- Task-ambient lighting design. Continuing with the concept of “put the light where you need it,” Consider the following. Open office and other group occupancy spaces where visual “tasks” are performed can benefit from a task-ambient design: provide lower levels of general lighting and higher level task lights for individual users. This eliminates the need to light an entire large, open office area to 50 footcandles when it is only partially occupied. Plus, it gives occupants individual control of their area lighting.
- Control the lights. Significant energy savings can be achieved by applying appropriate lighting controls. Vacancy sensors—commonly referred to as motion sensors or occupancy sensors—can be used in virtually all spaces to turn off lights when rooms are vacant. Vacancy sensors require “manual on” instead of “auto on” and are required by ASHRAE 90.1-2010. Multi-purpose rooms such as conference rooms, training rooms, and even offices should be provided with dimming or multi-level switching to reduce lighting levels—and therefore energy—when full illumination is not needed or desired. You can dim general area lighting by 10% and most people won’t notice any difference. Do this during times of peak demand (i.e., afternoons during the summer) when electric rates are high (if you have time-of-use rates). This not only saves electricity but also money on the utility bill. And simple daylighting sensors used to dim or switch off lighting when adequate daylighting is available—also known as daylight harvesting—can save significant amounts of energy.
- Use light-colored surfaces. Architects and interior designers can help improve energy performance simply by specifying light colored, reflective surfaces. Spaces with dark colored walls and floors not only “feel” dark, but they also don’t reflect or bounce as much light around the room, decreasing the actual lighting levels in the spaces. Designers then have to overcome this darkness by using more lighting and more energy. Spaces with light colored surfaces simply require less lighting to meet footcandle and esthetic goals.
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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