LED Technology
LEDs (light-emitting diodes) have been around since the 1960s. You've probably seen them used as indicator lights in consumer products. Recently, however, they have become practical for general lighting purposes. Although they cost more upfront than the bulbs they replace, LED lights use up to 70% less energy and last longer than conventional bulbs, resulting in big savings and short payback periods. Another advantage of LEDs is that they produce directional light. This provides more control over what is lit (i.e. the street) and what isn't (the night sky), reducing light pollution and wasted energy.
How LEDs Work → Energy Efficiency of Outdoor Area Lighting → Light Distribution and Glare →How LEDs Work
LEDs are semiconductor devices, while incandescent, fluorescent, and high-intensity discharge (HID) lamps are all based on glass enclosures containing a filament or electrodes, with fill gases and coatings of various types. LED’s also differ from traditional light sources in the way they produce light. In an incandescent lamp, a tungsten filament is heated by electric current until it glows or emits light. In a fluorescent lamp, an electric arc excites mercury atoms, which emit ultraviolet (UV) radiation. After striking the phosphor coating on the inside of glass tubes, the UV radiation is converted and emitted as visible light.
An LED, in contrast, is a semiconductor diode. It consists of a chip of semiconducting material treated to create a structure called a p-n (positive-negative) junction. When connected to a power source, current flows from the p-side or anode to the n-side, or cathode, but not in the reverse direction. Charge-carriers (electrons and electron holes) flow into the junction from electrodes. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon (light). The specific wavelength or color emitted by the LED depends on the materials used to make the diode.
Back to Top ↑Energy Efficiency of Outdoor Area Lighting
Energy effectiveness encompasses luminous efficacy of the light source and appropriate power supply in lumens per watt (lm/W), optical efficiency of the luminaire (light fixture), and how well the luminaire delivers light to the target area without casting light in unintended directions. The goal is to provide the necessary illuminance in the target area, with appropriate lighting quality, for the lowest power density.
Table 1 compares measured illuminance data from the recent installation of LED outdoor luminaires referenced in Figure 1, in which existing 70W HPS luminaires were replaced with new LED luminaires. The LED luminaires installed used three arrays containing 20 LEDs each.
Table 1: Comparison of HPS & LED Outdoor Luminaires for Demonstration Site | |||
|---|---|---|---|
| Existing 70W HPS | LED 3-array Luminaire | Optional LED 2-array Luminaire | |
| Total power draw | 97W | 72W | 48W |
| Average illuminance levels | 3.54 fc | 3.63 fc | 2.42 fc |
| Maximum illuminance | 7.55 fc | 5.09 fc | 3.40 fc |
| Minimum illuminance* | 1.25 fc | 1.90 fc | 1.27 fc** |
| Max/Min Ratio (uniformity) | 6.04:1 | 2.68:1 | 2.68:1 |
| Energy consumption per luminaire*** | 425 kWh/yr | 311 kWh/yr | 210 kWh/yr |
| Energy savings per luminaire | -- | 114 kWh/yr (26.8%) | 215 kWh/yr (50.6%) |
* Lowest measured or modeled for each luminaire. IESNA guidelines call for at least 0.5 fc.
** Modeled results.
*** Energy consumption for the HPS system is based on manufacturer-rated power levels for lamps and ballasts, multiplied by 4380 hours per year. Energy consumption for the 3-bar LED unit is based on laboratory power measurements multiplied by 4380 hours per year. Energy consumption for the 2-bar unit is based on manufacturer-rated power levels multiplied by 4380 hours per year.
Since HID lamps are high-intensity near-point sources, the optical design for these luminaires causes the area directly below the luminaire to have a much higher illuminance than areas farther away from the luminaire. In contrast, the smaller, multiple point-source and directional characteristics of LEDs can allow better control of the distribution, with a resulting visible improvement in uniformity. This overlighting represents wasted energy, and may decrease visibility since it forces adaptation of the eye when looking from brighter to darker areas.
1 National Voluntary Laboratory Accreditation Program (NVLAP) accreditation for LED luminaire testing is not yet available, but is in development. In the meantime, DOE has pre-qualified several independent testing laboratories for LM-79 testing.
2 Kinzey, BR and MA Myer. Demonstration Assessment of Light Emitting Diode (LED) Walkway Lighting at the Federal Aviation Administration William J. Hughes Technical Center, in Atlantic City, New Jersey, March 2008. PNNL-17407. Available for download from http://www.eere.energy.gov/buildings/ssl/techdemos.htm.
Figure 1. Several HPS fixtures (left) were replaced with LED pole-top mounted luminaires (right) to illuminate a pedestrian area at a Federal Aviation Administration facility in Atlantic City, NJ. A full report on this installation is available.
Light Distribution and Glare
LED luminaires use different optics than Metal Halide or High-Pressure Sodium lamps because each LED is, in effect, an individual point source. Effective luminaire design exploiting the directional nature of LED light emission can translate to lower optical losses, higher luminaire efficacy, more precise cutoff of backlight and uplight, and more uniform distribution of light across the target area. Better surface illuminance uniformity and higher levels of vertical illuminance are possible with LEDs and close-coupled optics, compared to HID luminaires.
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SOURCES:
United States Department of Energy
City of Ann Arbor