Do They Really Save Energy, and How Do They Work?
While they may not save more energy than a very conscientious person, they operate 24/7 without days off. The National Lighting Product Information Program suggests a time delay of two to five minutes in most office settings. The time delay used is a trade-off between saving energy and avoiding occupant complaints from lights going off while the space is occupied by a sedentary occupant.
There are three technologies — passive infrared, ultrasonic and microsonic — that are used to detect occupancy. Systems may employ one technology or combine two of them, called dual technology. Combining technologies can provide better performance. Manufacturers usually advertise coverage at the maximum possible, but it can be severely altered due to location, sensitivity adjustment, height and location of furniture and distance of moving objects. Large or complex areas will often require additional sensor units for satisfactory performance.
Passive infrared sensors detect heat energy. It works by line of sight so will not work well in spaces with many barriers like restrooms or office areas with high cubicle walls. The sensors are more sensitive to cross motion than to motion approaching the sensor. They work well in enclosed offices, high ceiling warehouses and hallways or aisle-ways in storage areas.
Ultrasonic sensors transmit pressure waves at high, inaudible frequencies that cross the space and return. Motion in the space changes the pattern of the waves, activating the sensor. These units are sensitive to any motion, including inanimate objects, but can "see" around barriers so are often used in restrooms. Increased distance of the moving object from the sensor results in decreased or erroneous motion detection.
Dual technology sensors are also called hybrid sensors, blending the above systems. With dual technology it takes detection by both sensors to turn on the lights but detection by only one unit to keep them on. This system works well to prevent false on-off switching.
One manufacturer has combined infrared technology with a microphone to pick up sounds. The sound-responsive system is responsive to the sound of typing, an activity that may not involve much perceptible movement, and can be effective in partitioned spaces. The unit in operation at Seattle's Lighting Design Lab is reportedly working as well as other systems.
Yet another technology is the personal occupancy sensor that plugs into a standard socket. The big advantage is that no electrician or wiring is required. A disadvantage is the same thing — it is a small, very portable unit. It has provided some users with great savings by plugging in all the appliances in a personal office that is intermittently unoccupied.
Adapted from information compiled by Western Area Power Administration.