by Jim Brodrick, U.S. Department of Energy
The U.S. Department of Energy (DOE) recently released a GATEWAY report on a parking-garage demonstration conducted at U.S. Department of Labor (DOL) headquarters in Washington, DC, which serves as a classic illustration of the savings that can result from switching to SSL — and how the use of occupancy-sensor controls can increase those savings. Parking garages often present attractive energy-saving opportunities when safety and security demand that they be lighted 24/7, regardless of actual building use. Like many commercial office buildings, DOL headquarters has a fairly predictable use pattern, with most of the activity in its two dedicated subterranean parking garages occurring between 8 am and 6 pm on weekdays, and much lower and more sporadic use at other times.
So in 2010 DOL joined forces with the GATEWAY program to begin a demonstration project. The two garages at DOL headquarters were previously lighted entirely by about 300 high-pressure sodium (HPS) fixtures, 19 of which were replaced one-for-one with LED luminaires in one section of a middle floor. Those LED luminaires, which have an integral occupancy sensor that can control their output through bi-level dimming, were monitored over a period of about a year to evaluate their performance.
The results showed energy savings of 52 percent from the initial conversion of HPS to the LED product, due solely to the reduction in power draw (129.5W for HPS, vs. 61.8W for LED). These savings were increased substantially with the use of the occupancy-sensor controls, which reduced the luminaire power draw to 10 percent (6.2W) while in the low state — amounting to a 95-percent reduction over the incumbent HPS system.
Moreover, after the first few months of operation, it was determined that the motion sensors’ default delay setting (i.e., the time between the last detected motion and switching to the low state) was much longer than necessary and was leaving considerable energy savings on the table. After all, the delay setting only needs to be long enough to cover the typical time required for a vehicle to enter the area and park, with perhaps a short additional period while occupants gather their things before exiting the vehicle.
When the time delay was reduced from the default setting of 10 minutes down to 2.5 minutes, it was found that the average period of high-state operation among the metered luminaires decreased by approximately two-thirds — from about 60 percent down to 25 percent or less during the work week. The energy savings of the LED luminaires in combination with the occupancy-sensor controls, relative to the incumbent HPS system, amounted to 76 percent at the 10-minute setting, but increased to 88 percent at the 2.5-minute setting. Notably, no complaints about the shorter delay setting have been received from users, possibly because few have even noticed the change.
Before the time delays were reduced from the factory-default 10 minutes to 2.5 minutes, more than half of the metered luminaires exhibited anomalous behavior at least once during the monitoring period, switching to a high state of illumination and remaining there for extensive periods. After the time delay was reduced to 2.5 minutes, however, this abnormal behavior diminished dramatically. No definitive cause was identified for these false tripping episodes. Overall, false tripping didn’t have a significant negative effect on the final results, contributing less than 5 percent to the cumulative energy use.
Because of the relatively high cost of the LED luminaires purchased for this project, the simple payback periods were 6.5 years and 4.9 years for retrofit and new-construction scenarios, respectively. Staff at DOL headquarters reported high satisfaction with the operation of the LED product.
Because occupancy sensors are a relatively recent addition to the parking-facility lighting market, growing pains are expected. Nevertheless, the DOL installation encountered relatively few challenges while offering nearly ideal conditions for a combined LED/occupancy sensor approach. The use of occupancy sensors at this site produced substantial energy savings while successfully demonstrating the incremental levels of savings available from different control settings. In the end, making the most of an occupancy sensor-based system is a balancing act between numerous elements, and careful attention should be given to all of them to maximize the performance and savings achieved from the investment.
Click here to download this GATEWAY report.