The U.S. Department of Energy (DOE) has published a technical feasibility assessment of replacing existing high-pressure sodium and low-pressure sodium roadway lighting on San Francisco's Golden Gate Bridge. The goal was to identify solutions which would reduce maintenance and energy use without compromising the quantity or quality of existing illumination. However, the historic status and high efficacy of the existing luminaires—as well as their accepted color characteristics—present challenges for energy-saving alternatives.
Key findings include:
•Four luminaire types are currently used to illuminate the roadway, the two walkways, and the two towers. Although no suitable commercially-available LED retrofit kits were identified for installation within existing luminaire housings, it appears feasible to develop custom LED retrofit kits which would save energy while maintaining light levels. In doing so, issues surrounding color and color shift, thermal effects, vibration, and product weight may need to be addressed.
•Light emitted by existing luminaires is more yellow in appearance than light emitted by the alternative technologies, so mock-ups should be considered to determine whether a whiter light would be appropriate. Also, preliminary analysis suggests many of these alternative sources may appear somewhat greenish when installed in one luminaire type, an amber-lensed shoebox.
•Other options may ultimately merit evaluation, such as replacing existing luminaires with fully-integrated LED or ceramic metal halide (CMH) luminaires rather than replacing internal components. Several complete LED and CMH luminaires were found to offer energy savings, although the CMH products might increase maintenance due to the shorter rated lamp life. Alternately, the amber lenses might be reformulated to allow the use of off-the-shelf LED or CMH products.
•This analysis assumed that existing light levels must be preserved. If reduced illumination is determined to be acceptable, it could enable the use of lower wattage products, increase energy savings, and improve the feasibility of using LED technology in this application.
The study was conducted by the DOE GATEWAY demonstration program, with support from the Golden Gate Bridge Highway & Transportation District and Pacific Gas & Electric, and the report is available for download.