One approach to reducing office lighting energy consumption is using occupancy sensors.
What is an occupancy sensor?
Occupancy or motion sensors are devices that turn lights and other equipment on or off in response to the presence (or absence) of people in a defined area. Some sensors also control lighting based on the amount of daylight available in their coverage area. More sophisticated sensor units now offer users a variety of adjustment capabilities; manufacturers have also introduced sensors that can be integrated into a building's automation and control system.
A complete sensor unit consists of a motion sensor, an electronic control unit, and a controllable switch/relay. It also has a timer which signals the electronic control unit after a set period of inactivity. Some units also incorporate an optional daylight (or light level) sensor.
Units are available in wall-mounted switch configuration for use in offices or other small areas and in ceiling and wall-mounted configurations for large, open areas. There are also sensors specifically designed for bathrooms, stairwells, and hallways.
Available types of sensor technologies: infrared or passive infrared (PIR), ultrasonic or ultra sound (US), microwave, and audio.
Available types of sensor products.
|Hybrid or Combination Sensors
||Integrated Daylight Sensors
||Wall Switch Sensors
||Wall or Ceiling-Mounted Sensors
||two or more technologies to minimize false detection, usually PIR and ultrasonic, or PIR and audio||PIR or ultrasonic sensors with a light-level sensor.||PIR, ultrasonic, or combination/hybrid sensor and control circuitry packaged into one unit, sized to fit in a standard wall box.||PIR, ultrasonic, or hybrid sensors designed to be mounted separately from the control unit(s), usually in high locations.||PIR or ultrasonic sensors designed specifically for bathrooms, hallways, and stairwells.|
|Advantages||Can be very foolproof, allowing wide coverage and applications.||Can be wired to a dimming circuit to control room lighting based on available light and occupancy.
||Small, inexpensive, and easy to install.
||Can cover wide areas effectively; switching units can control a variety of equipment.||Specifically designed for these spaces.|
|Disadvantages||They can be more expensive (for small area applications), and may require more adjustments since sensors contain more than one sensing unit.||They can be difficult to adjust and require a dimming ballast or special wiring.
||Their range can be limited, and depending on the location of the switch, they can easily be obscured.||They tend to be more expensive and often necessitate rewiring.
||Rewiring may be necessary if certain lights need to stay on.|
|Recommended for||Large, open areas and areas with unusual occupancy patterns or work requirements.
||Areas that receive large amounts of daylight.||Smaller meeting rooms, individual offices, and store rooms.||Large areas.
What are the benefits of an occupancy sensor?
They are inexpensive and effective devices that can quickly and easily be installed on a wall or ceiling.
There may be utility rebates available to purchase occupancy sensors - check with your local utility. Where utility rebates are available, sensors pay for themselves in less than one year, but most pay for themselves in two to three years without rebates.
The chart below shows the possible percentage reduction in on-time for lights in a given area type.
|Large Work Room/Office
|Small Work Room
Compiled by MyTech Corp. in a survey of large, corporate headquarters.
Savings will vary depending on the area size, type of lighting, and occupancy pattern. Manufacturers claim that in some applications, savings can approach 75%. The CA Energy Commission estimates that typical savings range from 35-45%.
However, savings can be achieved without the use of sensors. If the occupancy pattern in an area is regular and predictable, a more effective choice is a timer system to turn lights and other equipment on and off at predetermined times. Also, installation of sensors may not provide a payback if extensive rewiring is required. In this case, more effective conservation may come from lighting retrofits or other conservation measures.
Generally, the most effective areas for sensors are areas that are not frequently used, areas with irregular use patterns, or areas where lights are inadvertently left on. The lights are more likely to be left on overnight in individual offices, conference rooms, copier rooms, bathrooms, and storage areas. To identify other potential areas for sensors, start where lights are often on, but where there is no continuous or permanent user presence.
Device sensitivity/accuracy and capability for multiple adjustments are the two most important characteristics to look for in a sensor. Selection should be a function of the type of activity(ies) in the sensing area. Distinct types of motion that occupancy sensors typically key on are: desktop-type motion such as page turning or mouse and keyboard motion, torso motion such as reaching for objects, and whole body-type motion, such as walking. Depending on type and sensitivity setting, sensors can also respond to false signals (or Afalse triggering@), such as air movements from HVAC vents, or motion on the desktop due to HVAC flows, or the movement of warm air in front of a sunny window.
Lighting controls work only when they are appropriate and unobtrusive. Occupants have disabled or defeated lighting controls when they interfered with their daily routine, and there are specific areas such as hallways or stairs that should not be controlled by sensors.
How do I select an occupancy sensor?
For use in small, individual offices, the coverage area is not as important as the feature requirements. Use the following criteria to select sensors.
- Daylight-Level Equipped Sensors
- Failure Mode
- Manual Controls
- Minimum Load
- Timer Settings
In 1991, the state of Connecticut began an energy efficiency program for state-owned office buildings in partnership with NE Utilities. One of the buildings selected for this program was 55 Elm St. in Hartford. This 188,000 square foot building houses the offices of the State=s Attorney General, Treasurer, and Comptroller. About 2/3 of the building is open office space and its lights often stayed on for 14-16 hours a day, shut off only when the building was totally empty.
The State used a contractor to outfit the building with hybrid occupancy sensors (dual technology PIR/microphonic). The installation was carried out over a 6 week period. All lights not intended for 24 hour use were wired to be controlled by occupancy sensors. Because of the building=s open space, sensors were selected over other control methods such as computer controlled or timed systems.
With a reported annual savings of $24,000 in direct electricity cost, the project paid for its $51,000 cost in just over 2 years. Just as important is the fact that the sensors were readily accepted by the buildings occupants - high-level lawyers and executives - without complaints.
For more information, contact the Connecticut Department of Public Works.
- California Energy Commission (916) 654-5200
- http://www.lrc.rpi.edu/ Lighting Research Center is a university-based research center devoted to the latest testing and information on lighting.
- http://www.Lamprecycle.org is a resource for any light bulb ("lamp") user seeking details on recycling spent mercury-containing lamps.