Occupancy sensors automatically save energy by reducing lighting after a space is vacated. After more than 10 years of tightening commercial building energy codes, they are a staple in new construction projects. The rapid development of radio-frequency (RF) wireless occupancy sensors is making them more economically attractive for existing construction, too. According to the 2012 Commercial Buildings Energy Consumption Survey, occupancy sensors now control lighting in some 15 percent of commercial buildings and more than 40 percent of floorspace.
With few exceptions, energy codes require automatic lighting shutoff. The main options are either an occupancy sensor or time schedule. Recent generations of energy codes specifically mandate occupancy sensors in a growing list of applications—typically smaller enclosed spaces, such as private offices. A majority of codes require a 30-minute time delay, which means the lights must be reduced within 30 minutes of the space being vacated. The latest codes are beginning to reduce that to 20 minutes. All of these codes define the maximum size for the control zone, which is a group of lights simultaneously connected to a single controller.
Theoretically, and since the code requirements are minimums, the time delay for shutoff could be zero and control could be zoned for loads as small as single luminaires, both of which should increase energy savings. The problem with very short time delays is the frequent switching increases energy savings but reduces fluorescent lamp life. The problem with very small control zones is cost. Therefore, highly granular control zoning increases responsiveness and is likely to increase energy savings but typically increases cost and complexity as well.
The advent of light-emitting diode (LED) lighting has made very short time delays possible because the light source is instant-on, and frequent switching does not appreciably reduce lamp life. Meanwhile, the development of RF wireless sensors reduces the installed cost of granular control zones.
Two studies recently examined the energy-savings potential of these possibilities. The General Services Administration (GSA) recently studied the installation of advanced RF wireless controls in two California federal buildings.
At the first building, LED luminaires replaced existing luminaires in a space that already had occupancy sensors and manual switches. Each luminaire featured a wireless controller, enabling individual luminaire control. At the second building, wireless controls were installed with existing fluorescent luminaires, occupancy sensors, time-based controls and manual switches. In open office spaces in each building, multiple luminaires (typically four to six) were grouped and assigned to a dedicated controller.
Photosensors were installed in daylight zones. Private offices saw installation of an occupancy sensor, dimmer switch and a photosensor if the space was windowed. The control system was connected to an Internet server, which allowed remote programming and monitoring.
The researchers discovered that advanced wireless controls produced an estimated 32.3 percent lighting energy savings in the first building and 32.8 percent in the second. In the first, installing LED luminaires reduced lighting power by 55 percent, increasing overall savings to nearly 70 percent. The control zoning could have been even more granular, suggesting higher energy-savings potential.
In the second study, Canada’s National Research Council (NRC) evaluated the effect of shorter time delays on energy savings associated with occupancy sensors. This simulation study involved installation of an idealized lighting control system in an office space with six 6-by-8 workstations in a windowless room. The study took place over 10 days from 7 a.m.–7 p.m. Luminaires were mounted over each workstation.
The NRC operated the lighting under three control scenarios. The first used time-based control with the lights turned on at 7 a.m. and off at 7 p.m., providing a baseline. The second scenario combined centralized control and a single, local occupancy sensor on a 10-minute time delay. This resulted in 15 percent energy savings compared to the baseline. The third scenario featured occupancy sensors installed at each workstation, set to various time delays: 30, 20, 10, one and zero minutes. These time delays respectively produced 22, 26.4, 31.9, 45.8 and 48.6 percent energy savings compared to the baseline. In short, limiting zoning to individual workstations combined with very short time delays produced high energy savings.
While very short time delays might seem like a good idea, there are some challenges. Most sensors don’t allow low settings, such as 5 minutes. Very short time delays also increase the risk of nuisance switching (the lights turn off when they are not supposed to). Multiple sensors may be required for zero or one-minute time-delay settings. Additional research may be needed to determine occupant satisfaction.
These studies confirm what has always been understood: shorter time delays and smaller control zones are likely to increase energy savings. The rapid development of LED lighting and RF wireless controls is reducing barriers to exploring these opportunities, potentially paving the way to the next generation of occupancy-sensing control.