Lighting Controls: Reducing Cost, Saving Energy
By John L. Fetters - January 2008 - Energy Efficiency
The most effective way to reduce lighting costs is to turn lights off, and both manual and automatic lighting controls enable managers in institutional and commercial facilities to accomplish this goal. Controls also can reduce lighting costs, but deciding which controls to use can be challenging.
Manual lighting controls are most effective in occupied work areas. Occupants in areas with such applications usually are more responsible about controlling their lighting. Research at the Lighting Research Center shows that in areas featuring manual or individual control, occupants are more responsible about controlling their lighting. At one test site with individual control, occupants used 40 percent less lighting energy.
But managers and designers often overlook common building areas, such as corridors, restrooms and conference rooms, when considering savings with lighting systems. Automatic controls, such as sensors, can provide benefits when properly specified for these areas.
The Technology of Choice
Occupancy sensors have become the lighting control of choice for reducing wasted lighting energy in common-area applications. Occupancy sensors in new construction can provide the automatic shutoff required for state energy codes based on ASHRAE/IESNA 90.1, and they can fulfill the requirements to qualify for the commercial building tax deduction related to lighting projects.
When used properly, sensors can save energy and extend the life of lamps and ballasts. Using occupancy sensors lowers energy use by reducing:
• kilowatt hours of use
• power used during the peak demand period, either by automatically dimming lights or turning them off when they’re not needed
• a building’s internal heat gains; cutting lighting use lowers the building’s cooling needs.
Occupancy sensors are most cost-effective either when lighted spaces are unoccupied for two or more hours a day or in spaces where the lights commonly are left on when the space is unoccupied. Offices, classrooms, copy rooms, restrooms, storage areas, conference rooms, warehouses, break rooms, corridors and filing areas are ideal candidates such applications of occupancy sensors.
Successfully implementing a control scheme requires proper sensor location. For example, sensors must be located so they will not detect movement outside the desired coverage area, such as through an open doorway. Ultrasonic sensors are sensitive to air movement and should not be placed near an HVAC diffuser, where air movement also might trigger operation.
Many different mounting configurations and coverage patterns are possible for occupancy sensors. Sensor manufacturers provide coverage diagrams based on levels of activity and the sensitivities of each type of sensor. Managers can download this information from manufacturer web sites.
The challenge of effectively employing occupancy sensors involves selecting the right sensor technology — ultrasonic, passive infrared, sound, or a combination dual technology. Standard occupancy sensors require manual adjustment of their sensitivity and time delay to avoid false triggering.
Short time delays increase savings, but unless the application uses programmed-start ballasts, time delays of less than 15 minutes usually will reduce lamp life. For many applications, delays of 15 minutes or more are specified in switching instant-start ballasts. Codes often require a delay setting of 30 minutes or less.
Sensitivity controls determine the level of movement that will cause the sensor to activate the lighting system. Setting the sensitivity too high increases potential false-on triggering, while setting the sensitivity too low increases the possibility for false-off triggering.
Changing the sensitivity control also results in changes to the coverage pattern. Sensors are shipped with a factory-set sensitivity setting, and installers are expected to adjust them to meet individual application requirements. In this way, they are more likely to respond properly to the tasks in the space at the correct distance, while taking into account potential sources of nuisance switching, such as airflow.
To avoid these installation hassles and prevent false operations, sensor manufacturers now provide designs that use advanced detection capabilities called self-adaptive technology. These new-generation sensors continuously analyze real-time occupancy patterns and automatically adjust their sensitivity and delay settings to accommodate the current space activity. They can learn from their mistakes and adjust accordingly, resulting in true plug-and-play operation requiring no adjustments.
For some applications, such as open-plan offices and laboratories, managers should consider setting the light level lower, rather than turning lights off. This control scheme might be appropriate when occupancy sensors control separate zones of fixtures in larger spaces, such as in open-office areas or laboratories. In these applications, the lighting systems can be dimmed to a preset level in individual areas when the spaces are unoccupied.
Mangers can achieve this control strategy by using either multi-level switching with split-ballast schemes or bi-level ballasts that provide a choice of light levels. The main advantages of switching include lower initial cost and simpler design and commissioning.
The bottom line? Using sensors to turn down or turn off lighting systems can reduce energy costs. And when applied correctly, they can improve working conditions and comfort in the spaces in which they are installed.
Shedding Light on Daylighting
Occupancy controls can combine with daylight-harvesting controls to keep lights dimmed when enough daylight is present and the room is occupied. Managers can specify automatic, self-adjusting dimming ballasts to provide cost-effective daylight-harvesting solutions in small spaces with windows, such as individual offices.
These ballasts are equipped with individual photosensors that automatically and continuously adjust lamp output to take advantage of available daylight. Installers can adjust the initial light level, and the ballasts will compensate for lamp-lumen depreciation.
Occupancy sensors can control power to these ballasts and the rest of the fixtures located away from windows to minimize lighting energy use.
For larger spaces with larger window areas, installers can wire a separate photosensor daisy-chain fashion to the 0-10 volt control leads of the dimming ballasts in the fixtures located in the window row. The photosensor is mounted in the ceiling so the sensor “sees” the combination of daylight coming through the windows and the electric lighting. The control leads allow a control zone to be independent of the power zone, which can be controlled by occupancy sensors.
Many early daylight harvesting control projects have failed. To discover the reasons, The Weidt Group — which provides energy-design assistance, including daylighting as an energy-conservation strategy — reviewed the design intent, met with the project team, and made additional site visits to eight projects.
The team concluded that savings from automatic daylighting control systems often are not fully realized when a building is turned over to users. One major reason daylight harvesting control projects fail, according to the group, is that “users are not educated about the installed control systems; when something doesn’t work, users often disable the system instead of getting it fixed.”
The study cited several specific reasons daylight-harvesting projects fail:
• A lack of coordination or understanding exists between the design disciplines concerning the daylighting-control system.
• Controls systems, component parameters and sequence of operations are specified inadequately.
• Contractors’ shop drawings detailing the system are not checked, or the lighting designer does not know what to check.
• Field changes to adjust a system are not documented and taken back to the designer to complete the feedback loop.
Managers can use these examples to learn how to better use daylight-harvesting controls more effectively. Help also is available from the Lighting Research Center — www.lrc.rpi.edu — and sensor manufacturers’ web sites.
— John L. Fetters
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