|LED taxiway prototype
In recent years, the Federal Aviation Administration (FAA) has targeted LEDs as a promising option for many airport and airfield lighting applications, especially those requiring colored light. Airport taxiways, which guide pilots between the runway and the airport terminal, are denoted by blue light fixtures. Traditionally, these fixtures have used incandescent lamps topped by optics made of blue borosilicate glass. The colored glass domes lower the fixture’s efficiency tremendously, to less than five percent.
LEDs offer the opportunity to increase the fixture efficiency greatly while at the same time saving energy and providing long life. According to U.S. Department of Energy estimates, 50 million kilowatt hours of electricity could be saved each year in the United States if all taxiway luminaires were converted to LEDs. Yet, LED-based taxiway fixtures have one drawback: they do not radiate enough heat to melt snow and ice buildup on the fixtures. As a result, LED-based taxiway lights often incorporate electric heaters to meet FAA requirements for weatherability. However, this significantly reduces the energy savings potential of LEDs.
In a project sponsored by the FAA and its Center of Excellence for Airport Technology, the LRC explored new taxiway fixture designs that could transfer heat from the LEDs to the optics, eliminating the need for heaters during winter months.
Fixture design criteria and solutions
Former LRC graduate student Alex Baker, now with the U.S. Environmental Protection Agency working on residential lighting initiatives with the ENERGY STAR® program, began this study as part of his master’s thesis. His literature survey uncovered the history behind airport taxiway lighting. He found that most LED-based taxiway fixtures mimic the designs of incandescent models with top-mounted glass domes. These designs were based on kerosene railway signals developed during the 1800s. However, presently there is no requirement for this particular mechanical design. All taxiway lights must meet FAA performance specifications in outdoor temperatures ranging from –40°C to +55°C. During his thesis work, Baker found that it is possible to use the convection heat from LEDs to prevent snow and ice buildup on the glass dome.
New design eliminates need for heaters
As a continuation of Baker’s thesis study, LRC researchers investigated additional fixture concepts. They concluded that the design criteria should include temperature requirements for both the optics, in order to melt ice and snow, and for the LED junction, to preserve good performance.
“We concluded that the most efficient design would produce the lowest ratio of LED pin temperature over optics surface temperature,” said LRC Director of Research N. Narendran, Ph.D., principal investigator for the study. LED pin temperature, which is usually measured on the LED circuit board, is a good indicator of LED junction temperature, which directly correlates to LED life, Dr. Narendran explained.
Using this ratio as a guide, LRC researchers produced a novel prototype design that used several blue LEDs distributed around an aluminum heat sink. This design reduced the size of the optics surface, as well as the path length for the heat to travel from the LED junction to the optics for melting ice and snow.
Laboratory tests at room temperature showed that the prototype met FAA photometry requirements. At 10 watts, the prototype was projected to produce enough heat to melt ice and snow at –40°C.
Narendran said, “Overall, we found that it is possible to use LED junction heat to raise the surface temperature of the fixture optics to melt ice and snow, and this can be achieved using 10 watts or less and no heaters.” A traditional taxiway light uses 30 watts for incandescent models and 45 watts for tungsten-halogen models. He also noted that during the summer, the current to the LEDs can be reduced to save energy further and reduce the junction temperature to extend life.
More information about the LRC’s design for blue airport taxiway fixtures can be found in the paper “Investigation of thermal management technique in blue LED airport taxiway fixtures,” published in the SPIE Proceedings of the Seventh International Conference on Solid-State Lighting, held in August 2007.
For more about the LRC’s work in solid-state lighting, including other airport lighting applications, visit the Solid-State Lighting program online.