Occupancy Sensors

Dec. 3, 2001
Take Control - Part 2 of 4

Improved technology has made occupancy sensors the most popular component light control solution on the market. Both ceiling-mounted or wall-box occupancy sensors survey a room through passive infrared, ultrasonic, or a combination of the two technologies (dual-technology) to determine if a space is occupied. If the sensor does not detect the presence of anyone after a preset amount of time, the sensor will turn off the lights. When the space detects an occupant, lighting will resume.

"Occupancy sensors are very effective at what they do, if they are applied properly," says Baselici. Selection of the right type of sensors and their mounting location can mean the difference between a major annoyance and a valued energy saver. Passive infrared (PIR) occupancy sensors respond to the heat energy emitted by humans and adjust lighting according to the detection of occupants in a space. PIR sensors must have an unobstructed "line of sight" to determine occupancy. For example, a wall-box PIR occupancy sensor in a restroom will not "sense" individuals behind partition walls. These sensors are most often employed in corridors, lobbies, or copy areas where large movements are common.

Conversely, ultrasonic occupancy sensors detect occupancy by emitting sound waves (at high, inaudible frequencies) that are reflected from a room's surfaces back to the sensor. When there is motion in the space, the frequency of these reflected waves is altered and lights are activated. Ultrasonic sensors are more capable of detecting occupancy in areas with partitions and other obstructions. However, the situation has to meet certain requirements (such as an enclosed space with hard surfaces that will easily reflect sound waves). The benefit of ultrasonic sensors is their ability to detect small movement, like a person sitting at a desk typing. However, small air currents moving a curtain can also trigger the system to activate lighting in an unoccupied space.

Dual-technology occupancy sensors combine both infrared and ultrasonic technology at a higher cost. Thought to be "the best of both worlds," dual-technology sensors minimize false on/offs.

Retrofit in some applications can be as simple as replacing a manual on/off switch with a wall-box occupancy sensor. "If you can replace a wall switch with an occupancy sensor, you can typically pay that back in three years," explains Alfred R. Borden IV, IALD member and president of The Lighting Practice, Philadelphia. Regardless of whether wall-box or ceiling-mounted sensors are selected for a space, serious consideration needs to be given to the sensors' area of coverage and coverage pattern. Borden suggests that 30 days after an installation, sensors should be checked and adjusted to make sure that they are working as intended.

Figure 2: Operating Cost Comparison: Private Office, 128 sq. ft.PerformanceBase CaseOccupancy SensorsDaylightingOccupancy Sensors + DaylightingAnnual Energy Use*450 kWh 340 kWh 330 kWh250 kWh Annual Energy Cost$33$24$24$18Annual Energy Cost Savings-$9$9$15* Average daily "on" hours for wall switch is 14.7. Average daily occupied hours for the office is 12.9.Source: US Dept of EnergyFigure 3: Operating Cost Comparison: Open Office Area, 1,000 sq. ft.PerformanceBase CaseTime SchedulingOccupancy SensorsDaylightingTime Scheduling + DaylightingAnnual Energy Use*5,700 kWh 5,100 kWh 5,000 kWh4,200 kWh 3,700 kWhAnnual Energy Cost$340$305$300$250$220Annual Energy Cost Savings-$35$40$90$120* Average daily "on" hours for wall switch is 9.1. Average daily occupied hours for the office is 6.8.Source: US Dept of Energy

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