GLOBAL CHALLENGES

LEDs Replacing Venerable Street Lamps

Oak Ridge National Laboratory claims to have surmounted the final hurdle to retrofitting streetlights with LEDs. Until now, LEDs burned out early when enough were amassed to illuminate a street. ORNL's carbon-based graphite material remedies the problem by keeping the LEDs cooler, an innovation it recently licensed to spinoff company LED North America for use in street lights, parking garages, industrial lighting and offices.

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LEDs Replacing Venerable Street Lamps
Semiconductor manufacturers have been concentrating their development efforts on making brighter light-emitting diodes (LEDs), albeit at the expense of generating more heat in the process. Unfortunately, every extra 10 degrees of heat cuts the lifetime of an LED in half, resulting in early burn-out for large arrays.

Until now, the principal way to cool these high-intensity LED arrays was to bolt the assembly to a metal heat sink to distribute the thermal energy over a larger surface area and then blow air on it with a fan to dissipate the heat. Now, Oak Ridge National Laboratory (ORNL) claims to have fabricated a high-tech graphite foam that works better by "wicking" heat away from the LEDs.

The reliability of LEDs derives from their solid-state construction�high-energy positive and negative charge carriers enter their oppositely polarized electrodes to meet internally at the semiconductor junction, where they shed their excess energy as a photon of light. Since all of these actions are performed at the subatomic level, the resulting device has no moving parts to wear out. In fact, many LED indicators on equipment from the 1970s are still in good working order today.

Unfortunately, in order to create high-intensity LEDs, manufacturers had to optimize for power-handling capabilities instead of energy efficiency, and as a result their operating temperatures rose, thereby shortening their lifetime.

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