ASHRAE Journal - January 2010 - 64

Solid-State Lighting, Part 1: Technology By Matthew Wiggins, Ph.D., John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE This two-part series discusses solid-state lighting and its envisioned use and benefits in U.S. buildings. The second part will cover specific applications of general solid-state lighting technology. Benefits and Challenges of Inorganic LEDs s solid-state lighting (SSL) technology is introduced into the world’s buildings, the economics of their lighting systems will shift more to- ward the HVAC system model. A solid-state lighting system is an efficient, long-term solution with reduced utility cost, less maintenance, and fewer bulb replacements. The first type of solid-state light source discovered was inorganic light emitting diodes (LEDs), which have an inorganic semiconductor junction, and are frequently installed in consumer electronics. Organic light emitting diodes (OLEDs) are a more recent discovery being investigated mainly for displays and diffuse light sources. The U.S. Department of Energy (DOE) has programs that fund LED and OLED research and development. Inorganic LEDs are the focus of this column. They are efficient, long lasting, low maintenance and durable. LEDs contain no mercury, and do not emit infrared or ultraviolet light. Though justified in some applications, LED technology still faces several challenges before it can be used for all illumination purposes. While some LED luminaries produce sufficient light output, LED retrofit replacements for linear fluorescent bulbs, which represent the majority of lamps 64 ASHRAE Journal in commercial and industrial buildings, are relatively dim. Also, natural, warmcolored light is diff icult to achieve while maintaining high efficiencies. The effective removal of heat from the semiconductor is also important to maximize LED light output and life. LED research has resulted in consistent improvements in the last decade, suggesting that more breakthroughs are to come. The motivation for continued research is that the theoretical ceiling for LED efficacy is far beyond that of the mature incandescent and fluorescent illumination technologies. LED Benefits Traditional incandescent bulbs produce light by running current through a filament, heating the filament to a high enough temperature to emit visible light. The filament is housed in a glass enclosure to prevent oxidation and deterioration. Incandescent bulbs ashrae.org convert roughly 8% of the input power to visible light, while the rest is wasted as heat.1 Fluorescent bulbs, used most frequently in commercial buildings, contain a gaseous mixture of mercury and inert material in a phosphor-coated glass tube that is electrically excited to emit light. These bulbs convert roughly 21% of their input power to visible light and the rest to heat.1 LEDs emit light by passing electrons through the junction of two semiconducting materials, which, depending on their electrical properties, results in emission of photons of a specific wavelength. The light-emitting semiconductor material is typically small, making this a point source type of lighting. One of the most significant benefits of inorganic LED lighting is its efficient conversion of electricity to usable light. Inorganic LEDs currently can convert 15% – 25% of their input power to white light, with theoretical projections of higher than 50% in coming years.2 The metric for this is luminous efficacy, expressed in lumens per watt (lm/W). Current typical source luminous efficacies are 14 lm/W for incandescent, 83 lm/W for fluorescent, and 72 lm/W for warm LEDs.2 The best-in-class fluorescent lighting and LEDs on the market are currently about 100 lm/W, while LED products to be introduced this quarter will likely reach 130 lm/W. It is not sufficient to compare light sources based on the luminous efficacy of the bulb alone because the overall light delivery efficiency also depends on the efficiency of the light fixture, or luminaire. For this reason, the DOE’s CALiPER Program3 tests the output and luminous efficacy of the entire fixture. This shows what efficiency is achieved in actual operation with the true thermal and reflector characteristics in place. The latest CALiPER results4 show LED luminaire replacements for recessed downlights and 2 ft by 2 ft (0.6 m by 0.6 m) fluorescent troffers to be comparable with their fluorescent counterparts in output and efficacy. January 2010

ASHRAE Journal - January 2010

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