Occupancy sensors are a proven strategy to reduce lighting energy consumption. As such, they are mandated by commercial building energy codes.
Current codes require a maximum 30-minute time delay. Time delay is a field-adjustable setting that determines the amount of time between last detected occupancy and the lights switching or dimming. Newer codes may reduce that to 20 minutes.
Shorter time delays translate to higher energy savings. However, very short time delays can produce more frequent switching, which can shorten fluorescent lamp life. Meanwhile, longer time delays serve as insurance against nuisance switching by ensuring the space is truly unoccupied.
The advent of LED lighting creates an opportunity to increase energy savings by reducing time delays. LED sources are instant-ON and do not experience an appreciable reduction in lamp life due to frequent switching. Theoretically, time delay could be reduced to 5 minutes or less, though a majority of current sensors do not offer settings that low.
To address false triggering that leave occupants in the dark, multiple cheap sensors could be deployed to ensure reliable detection. This is a strong potential for that with networks of luminaire-integrated sensors installed in open office plans.
National Research Council Canada (NRC) put these ideas to the test in a simulation study of an ideal office lighting control system. Occupancy data was recorded in a space consisting of six 6×8 workstations in a windowless room over 10 days between 7 AM and 7 PM (12 hours). The lighting consisted of luminaires mounted over each workstation. NRC applied three control scenarios to this application:
Scenario #1: Timer Control. This benchmark scenario represents the traditional approach of centrally controlling all luminaires via scheduled ON/OFF, with the lights operating the full 12 hours. Energy consumption over the 10 days was calculated at 7.2 kWh.
Scenario #2: Adaptive Central Control. This scenario features central control but with a single local occupancy sensor set with a 10-minute time delay. The lights are ON from the time the first occupant arrives until the last occupant leaves. Energy use: 6.1 kWh, 15% energy savings compared to Scenario #1.
Scenario #3: Multiple Sensors and Time Delays: Each workstation has dedicated occupancy sensing. Time delays are set at 30 (Scenario #3a), 20 (3b), 10 (3c), 1 (3d) and zero (3e) minute(s). Multiple sensors for reliable detection are required at the lower time delay settings of 1 and zero minute(s).
NRC evaluated energy savings, comparing these options against Scenario #1, and found:
• Scenario 3a (30 minutes), 22% energy savings
• Scenario 3b (20 minutes), 26.4% energy savings
• Scenario 3c (10 minutes), 31.9% energy savings
• Scenario 3d (1 minute), 45.8% energy savings
• Scenario 3e (0 minutes), 48.6% energy savings
In “A Quick Timeout” (LD+A, December 2014), NRC researchers Dr. Erhan E. Dikel and Dr. Guy R. Newsham wrote, “Overall, Scenario 3-d seems like the best balance between maximizing savings, with some protection against false negatives.”
They added that a detailed cost analysis and human factors study of the acceptance of this frequency of switching are needed before application in a commercial building.
Regarding lamp life, shorter time delays are ideally suited to LED, though there may be potential for fluorescent. While shorter time delays results in more frequent switching, with associated reduction in lamp life, actual operating time is greatly reduced.
Deployment of LED lighting also provides another opportunity for greater resolution. The researchers offered a scenario where lighting is modularized within the workstation by task, with advanced sensing detecting not just occupancy, but also task being performed. The lighting would instantaneously raise and lower light levels based on location of the occupant and task.
The proliferation of LED lighting is opening the door to new energy-saving opportunities with occupancy sensors by reducing time delays.