In August 2007, Cree’s management made a bold decision to replace the existing fluorescent lighting technology at its Durham, NC facility with new energy-saving, environmentally friendly LED lighting technology. The group started with the company’s headquarters building, but eventually plans to replace more than 7,000 existing fluorescent light fixtures covering more than 800,000 square feet campus-wide. Phase one replaced 200 existing fixtures in 7,000 square feet of space in the visitor lobby, waiting areas and conference rooms. All the exterior parking lot lighting was also converted from high-pressure sodium to LED lighting. Both parts of phase one had to be completed by early November 2007, just 3 months later.
To make phase one a reality, the facilities team needed vendors that had the capability to use emerging LED technology; to produce commercially available LED lighting fixtures fast; and to incorporate Cree lighting products — specifically XLamp lighting products — into the fixture design. This was challenging since many LED lighting vendors had not focused on LED lighting retrofit solutions to replace existing commercial grade 2×4 T8 fluorescent troffers, concentrating instead on LED lighting for under-counter applications, residential cans, and architectural decorative lighting. Cree needed to find vendors that could make commercial-quality LED 2×4 T8 troffer fixtures.
After networking with the sales and marketing teams and doing extensive research and phone interviews, the facilities engineering team identified five vendors out of an initial pool of 30 companies with the technical expertise to provide commercial-quality LED lighting that could complete phase one within the limited time frame. These five vendors were asked for product samples for testing to determine if they were the high-quality, application-appropriate LED lighting solutions with the “form, fit and feel” needed to assure the installation would be successful. This selection effort also involved a visit to each of the vendors’ manufacturing locations to conduct source inspections, quality control checks, and answer LED light fixture designers’ questions. This process was completed over a six-week period.
Designing LEDs into any illumination product requires a decision between designing a complete luminaire based on LEDs, or an LED-based lamp meant to install in an existing fixture. Generally, a complete luminaire or fixture design will have better optical, thermal, and electrical performance than the retrofit lamp, since the existing fixture does not constrain the design. In the end it is up to the fixture designer to decide whether the total system performance of a new luminaire or the convenience of a retrofit lamp is more important. If the target application or the customer’s need is better served by creating a new LED fixture, then designing the light output to match or exceed an existing fixture has several advantages. First, an existing design is already optimized to target a known application and can, therefore, provide guidance for setting the design goals around light output, cost, and operating environment. Second, an existing design is already in an accepted form factor. Switching to LED technology is easier for the end user or customer if the form factors are the same.
Unfortunately, some LED fixture manufacturers are misreporting or inflating claims of LED efficacy and lifetime characteristics. The lighting industry saw similar problems during the early years of CFL replacement bulbs. The lack of industry standards and wide variations in early product quality delayed the adoption of CFL technology for many years. The United States Department of Energy is aware that the same standards and quality problems may exist with early LED fixtures and that these problems may delay the adoption of LED lighting in a similar fashion. In response, it launched the DOE SSL Commercial Product Testing Program (CPTP) to test the claims of LED-luminaire makers. This program anonymously tests LED-based luminaires for the following four characteristics:
- Luminaire light output (lumens)
- Luminaire efficacy (lumens per watt)
- Correlated color temperature (degrees Kelvin)
- Color-rendering index
DOE’s CPTP sets a good precedent for LED luminaire design by focusing on the usable light output of a luminaire — not just the light output of the light source.
The LED fixture must meet or exceed the lighting requirements for the target application. Therefore, lighting requirements must be defined before establishing design goals. For some applications, there are existing lighting standards that will define the requirements directly. For other applications, a good approach is to characterize an existing fixture. The best LED lighting companies can provide data or IES files or documentation that detail the “critical” characteristics for each of their fixtures.
Cree discovered through the evaluation and selection process that there are many vendors that sell LED lighting fixture solutions, but the team was searching for one-to-one retrofits, which was a bit more challenging. From a facilities perspective, the team wanted a solution where they could remove the current conventional lighting and rapidly and easily install replacement LED lighting. The facilities engineers wanted to use all of the existing wiring and circuits, but as an industrial campus, Cree has a 277VAC electrical lighting system. Consequently, some of the electrical installation required adapting the electrical circuits to match specific LED fixtures, meaning fixtures were refitted with new circuits that were 110V.
For any organization considering converting to LED lighting, some challenges in selecting the right LED lighting solution and vendor are:
- Understanding the fixture manufacturer’s claims regarding the LEDs used;
- Quality of the LEDs used;
- Performance of the LED lighting fixture itself;
- Installation requirements for the LED lighting fixture;
- Claims for “light” performance;
- Claims for “energy” performance.
“Lighting-class” LEDs are a subset of high-power packaged LEDs, which are LEDs that consume 0.5 W of power or more and have the right combination of brightness, efficacy, quality of light and reliability to enable the replacement of incumbent lighting sources with LED light. LEDs are available in a variety of colors, including blue, green, amber and red. Most white light LEDs are created by applying a yellow coating to blue LEDs. The combination of blue light from the LED and the yellowish light combine to create white light.
White LEDs are composed of the LED chip or die, the yellow coating, and the LED package. In general, the radiant power of the LED chip is the primary factor that influences the brightness and efficacy of the complete packaged LED. The yellow coating and LED chip are the primary drivers when it comes to lighting characteristics, including the quality of emitted light, LED beam angle, lifetime, and reliability. While brightness and efficacy of the LED are very important, the entire packaged LED system must also be taken into account. A vendor’s claim to make LED lights does not mean they are a quality LED fixture manufacturer. In today’s market, quality LEDs are sourced by only a handful of major players and a technically competent fixture manufacturer will select one of these major LED manufacturers for their LED fixtures.
Key considerations for LED fixture manufacturers in choosing the correct LED includes the lumen value the LED is capable of providing; whether the LED will consistently provide that same output and color quality over time; and the efficacy (conversion of electrical energy into visible light) of the LED, measured in lumens per watt, the preferred LED junction temperature, and the LED thermal management.
There is a limit to what can be done regarding thermal management on the component level. When considering thermal management, a lot of what must be done is beyond the LED—such as the circuit board on which it’s mounted, the heat sink, and the light fixture housing.
LED Lighting Fixture Design
The facilities team learned there are many claims for LED lighting fixture solutions, either for a retrofit or for new construction. Some solutions are good and some are not so good. For example, some poor solutions analyzed included “filling” a 4 foot tube with several hundred 5mm LEDs (like those in a cell phone); others had poor heat sink designs, high LED junction temperatures, poor power or energy/wattage reductions, and poor power factors. Some solutions stated total lumens inconsistent with their actual performance.
As previously mentioned, a quality fixture requires good thermal management using a quality heat sink design to deliver the expected performance. (The fixture manufacturer is responsible for designing the heat sink, so examination of this design can be undertaken during the quality control check during the prototyping stage.) The majority of LED failure mechanisms are temperature related. Elevated LED junction temperatures cause light output reduction and accelerate the LED’s package degradation. Junction temperature is primarily affected by three parameters: ambient temperature of the LED’s immediate surroundings; the thermal path between the LED junction and ambient conditions; and the power dissipated by the LED.
In addition to these challenges, there were other specifications for the LED conversion fixtures, which included:
- Retrofit solutions to satisfy the facility maintenance and operation team;
- 30-50 percent energy savings over traditional lighting sources;
- Equivalent foot-candle lighting performance versus the baseline florescent lighting fixture;
- Performance measurement that is not lumens as the end-all, be-all factor;
- Solutions that did not require significant electrical rework (like new circuits);
- Installation that was “easy” with a DIY (do-it yourself) slant.
When considering the energy efficiency of a lighting fixture, the entire system must be considered and not just the light source itself. Any lighting fixture will contain at least the following three basic parts: a ballast or driver, a light source, and a fixture. The fixture includes the housing as well as any optical elements, such as a diffuser, a lens and/or a reflector. The fixture and optical elements can greatly affect the efficiency of the overall system. For example, CFL downlights can range in fixture efficiency anywhere from 70% down to just 30% efficiency. Some of these lights are wasting over two-thirds of the light inside the fixture!
Lighting-class LEDs create energy savings relative to traditional lighting in two ways. First, LED light is already directional, so the losses associated with using an omni-directional light source with a reflector are greatly reduced. Second, LED light has quickly progressed to become one of the most efficient artificial light sources currently available. LED lighting systems, like all lighting systems, have the same three basic parts: driver, LED and fixture. The efficiencies of the driver and fixture are high relative to the LED efficacy and are not likely to change much in the near future. Therefore, the brightness and efficacy of an LED lighting system is driven mainly by the quality of the LED itself.
Certain architectural and space constraints may limit the choice of a lighting technology used in a specific lighting application. For instance, the lights in an elevator are usually halogen lights because they are relatively small, not very tall and provide a pleasing color of light output. Other types of lights would require either much taller fixtures or not offer the same quality of light.
LED Design Options
There are three main optical options in LED fixture design. The first utilizes the bare LEDs and an existing lamp reflector, using no secondary optics. This option provides the lowest cost and lowest optical loss for the system. Using fewer components and less labor makes the fixture easier and cheaper to assemble. The drawback is a multiple-source shadow effect.
A second option is using LEDs with secondary optics and an existing lamp reflector. Secondary optics are optical elements used in addition to the LED’s primary optic to shape the LED’s light output. The general types of secondary optics are reflecting (where light is reflected off a surface) or refracting (where light is bent through a refractive material, usually glass or plastic). Secondary optics are available either by buying a standard, off-the-shelf part or by designing a custom optic through ray-trace simulation with an optical source model. By using a secondary optic per LED, the beam angle of each LED can be customized to provide the exact light output pattern necessary. For instance, the beam angle of each LED can be narrowed to make the luminaire optimized for spot lighting instead of general lighting. There are several drawbacks to this approach. First, the luminaire will have higher cost because of additional components and more complicated assembly. Second, since the optics are attached to each LED, there may still be multiple-source shadowing. Finally, the secondary optics will reduce the optical system efficacy.
The last option is using bare LEDs, an existing lamp reflector, and a diffuser. Instead of using one optic per LED, a diffuser can be used over the entire LED array to spread the light. The benefits of this approach are a wider beam angle than is possible with the bare LEDs and eliminating the multiple-source shadow effect. As with Option 2, the drawbacks are higher cost and reduced optical system efficacy. This is also not an option if the light distribution must be narrower than the bare LED, since diffusers can only spread light, not collect it.
Finally, the Installation...
The first phase of Cree’s LED lighting conversion covered 200 fixtures, 12 rooms and 7,000 square feet of space, all to replace old technology fluorescent lighting. Each conference room required two to four workers, working six to eight hours to complete the installation. As for costs, labor was billed at $40 to $60 per hour per worker, and the fixtures cost between $300 and $600 apiece.
Comparing the energy usage of the old fluorescent lighting technology to the new energy-saving and environmentally friendly LED lighting technology, in terms of before and after values for both watts and foot-candles, verified the energy savings for the total conversion was 48 percent. The combination of energy savings, reduced maintenance and disposal costs, and reduced environmental impact demonstrates that LED lighting is a viable alternative to traditional lighting solutions.