- Occupancy sensors save energy by automatically turning off lights in areas of variable occupancy.
- Primary types include infrared and ultrasonic, and both wall- and ceiling-mounted units are available.
- Careful placement and tuning are necessary to ensure sensor performance and optimize savings.
Lighting accounts for 40% of the electricity used in commercial buildings. Despite the attention paid to efficiency upgrades, great opportunities exist for reducing energy use by simply turning lights off where and when they are not needed. While energy savings can be achieved through education and incentives, it is often difficult to get staff or building occupants to cooperate. An automated lighting control system using occupancy sensors is more effective in many cases.
Selection and Placement
Proper selection and placement of occupancy sensors can result in a system that building occupants view as an improvement; automatically turning off lights after occupants leave a room and automatically turning them on as occupants enter. However, inappropriate application of this technology can limit the amount of energy savings and lead to poor, or even unsafe, lighting conditions.
Commonly used sensor types include infrared and ultrasonic. Infrared sensors detect motion from a heat source (such as a person), while ultrasonic models detect motion from objects using a form of radar. Each has strengths and weaknesses that should be considered when making a selection. Infrared sensors need to see the occupant, so they may not perform well in restroom stalls or office cubicles. Also, minimal motions (such as typing on a keyboard) may not always be detected. Ultrasonic sensors are good at sensing small movements and do not need to see the occupant directly. However, since ultrasonic waves bounce off room surfaces, any movement will alter their return patterns.
Occupancy sensors may not be a good fit for every part of your facility. Start by identifying spaces that are unoccupied on a regular basis. Spaces to consider include executive offices, copy rooms, restrooms, and conference rooms. Selection of appropriate spaces requires an accurate understanding of how the spaces will be used. For existing buildings, it may be worthwhile to measure and document space occupancy. This can be done with sophisticated data recorders or by simply having someone walk around at regular intervals to see which spaces are occupied. Occupancy sensors should not be used with high-intensity discharge (HID) lamps, unless they are used with stepped-dimming systems specifically designed for use with occupancy sensors.
Installation and Coverage
Occupancy sensors are available as wall-mounted and ceiling-mounted units. To avoid false detection with ceiling-mounted sensors, it is important to specify a viewing range that matches the application. For example, a hallway sensor should look in two directions but not into an office, while a conference room sensor should pick up motion from anywhere in the room. Some of the most common failures of occupancy control systems can be attributed to inadequate sensor coverage or not tuning a sensor's sensitivity appropriately for the application.
Coverage area of sensors depends on the room arrangement, room geometry, the presence of partitions, type of sensor, location of sensor, the sensor’s sensitivity setting, and type of motion. Ultrasonic sensors can cover a wider range than infrared sensors, but are more prone to false triggering from air motion. Ceiling-mounted models may cover 250 ft² to 2,000 ft², while wall-mounted sensors may cover 300 ft² to 7,500 ft². Larger areas can be covered by integrating multiple sensors. Each sensor has controls to adjust the time interval before lights are turned off—typically ranging from 1 to 15 minutes.
Saving Energy with Sensors
Occupancy sensors can result in a wide range of savings, depending on the occupancy pattern of the room and the habits of the occupants. Alcoa Composites in Monrovia, California enjoys $26,000 in annual electricity savings as a result of installing ultrasonic sensors in offices, work areas, and hallways. The installation paid for itself within one year.
Frequent on-and-off switching may shorten lamp and ballast life. Generally however, the energy-cost savings from turning lamps off more than makes up for any increased maintenance and replacement costs.