Controls will compound the advantages of LEDs in street lighting

June 9, 2010
LED street lights designed for a long service life can be operated at reduced power levels and still deliver the needed light output, plus controls can dim or extinguish lights as appropriate.

There's an ongoing transition to LED-based street lighting globally as municipalities try to reduce energy and maintenance costs. LEDs will last far longer than legacy street lights and can use less energy as well, although the fact that LEDs can be dimmed accounts for part of the savings. Now Edward Smalley, director of the newly formed DOE Municipal Solid-state Street Lighting Consortium, points out that municipalities will be able to save even more energy by never operating the lights at maximum output early in their service life.

About new LED street light designs and deployments, Smalley stated, "You are designing for a time in the future." The point is that LED street lights must be designed so that the light output 50,000 hours (perhaps 11 to 12 years) in the future is still acceptable. The design must account for the projected decline in light output that all LED lighting products exhibit over time.

Smalley suggests that at install time, "You are putting out 30% more light than is required." So a street light with a controller could cut back drive current by 30% at install time both saving energy and potentially lengthening the service life of the LEDs and the drive electronics. Smalley stated, "We can save at least another 20% in energy."

In the past, most of the discussion of controls and energy reduction has primarily focused on dimming street lights late at night when there is little automotive or pedestrian traffic. LEDs offer a significant advantage in dimming capability compared to alternatives such as high-pressure sodium (HPS) or metal-halide lamps.

Ironically, the question of controls came up during the May 6 DOE webcast on the new Consortium. During the Q&A session following the presentation, an attendee asked Bruce Kinzey whether the Consortium would focus on control issues. Kinzey is working with Smalley on the Consortium and is the DOE Gateway project demonstration manager. At the time Kinzey stressed that the Consortium would focus primarily on street lighting although he acknowledged that controls would invariably be included in some of the demonstration projects.

In a one-on-one interview subsequent to the DOE webcast, Smalley initiated the discussion of controls and the potential for additional energy savings. Smalley stated, The reason for the consortium is to get these other municipalities up to speed." And Smalley meant up to speed both on LED lighting and the advantages afforded by controls.

The concept of operating LED street lights at a reduced power level and raising that level over time as light output declines does introduce logistics problems. A networked installation of lights would allow for remote control of drive current and light output -- at significant added cost. Of course municipalities could manually check light output sporadically and adjust the lights, but that goes against the selling point of little to no maintenance of LED luminaires.

Asked whether municipalities might use networks or manual adjustment techniques, Smalley replied "All of the above." Smalley pointed out that the city of Los Angeles, CA is in a multiyear street light upgrade program and is using the Roam remote monitoring system to monitor and control the lights.

Smalley also pointed out that Virginia Tech has developed a mobile unit that uses a camera to accurately measure light output without stopping traffic. Presumably, a municipality could survey street light performance on an annual or periodic basis and only make manual adjustments when required.

Both Smalley and DOE Lighting Program Manager Jim Brodrick have noted that zero maintenance is not a realistic expectation. In his keynote presentation at Lightfiar, Brodrick noted that dirty lenses could be just as problematic in an outdoor lighting application as LEDs with depreciated light output. So municipalities may have to perform periodic preventative maintenance in any event.