The U.S. Department of Energy (DOE) recently published the evaluation report from a GATEWAY demonstration of LED street lighting in Kansas City, Mo., which was conducted in support of DOE's Municipal Solid-State Street Lighting Consortium (MSSLC). Nine different LED products were installed in February 2011 as replacements for incumbent high-pressure sodium (HPS) luminaires. Following are some key conclusions:
• As a group, the LED products tended to be slightly more efficacious than their HPS counterparts, but provided more substantial energy savings by reducing overall light levels and limiting spill light. The reduced light met the desired performance levels in some cases but not in others.
• For most of the luminaire types, it appears that seasonal variables in this location, such as temperature and foliage, drive as much as a 20% swing in measured illuminance, and that this may significantly outweigh any temporal lumen or dirt depreciation — at least during the early stages of product life.
• As LEDs are typically not expected to reach terminal light output levels (e.g., L70) until well into the future (perhaps decades in some cases), basing initial design decisions on these widely variable values may be impractical. A better approach might be to select a timeframe beyond which luminaire lifetime projections become increasingly tenuous — for example, 12 to 15 years — and recalculate expected illuminance levels for each product at that point.