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Daylight Harvesting Sensor Fixtures Daylighting uses daylight as a primary source of general illumination in a space. Daylighting has become a more important feature of mainstream construction due to the sustainable design movement. Rapidlite Fixture Corporation offers the option of using Daylight Harvesting Sensors on most lines of fixtures (see Options in the Ordering Information). We use Wattstopper and Levitan sensors to control individual fixtures so that they perform at optimal energy saving levels by forcing the lights to shut off when there is enough natural light in the room. Numerous studies over the last 50 years attest to the importance of daylight in design. Research indicates that daylight can improve user satisfaction/performance and retail sales. These characteristics can make daylighted buildings more valuable and marketable. Daylighting also enables daylight harvesting, an innovative control strategy that can generate 35-60+% energy savings. A daylight harvesting system decreases electric light contribution as the daylight contribution increases.
Daylight harvesting, also called daylighting control or automatic daylight dimming or switching, uses a ceiling-, wall- or fixture-mounted light sensor to measure the amount of illumination at the task surface in the space or at the daylight aperture, then signals a switch or dimming ballast to adjust light output from the electric lighting system to maintain the desired level of illumination. An effective daylight harvesting control system saves energy while being virtually unnoticed by occupants. With a daylight harvesting control system, electric lighting is increased or decreased in direct or approximate proportion to the amount of natural light present, resulting in a minimum maintained illumination level in the controlled space. Daylight harvesting controls can be effective in virtually any type of facility where the lights operate much of the time and where a significant quantity of daylight is provided with windows and/or skylights. Spaces with skylights, and corridors, private offices and open cubicles near windows—particularly those with task lighting—are good candidates for daylight harvesting. If the entire space is uniformly skylighted, energy savings can accrue on the entire lighting load. More commonly, they apply only to the perimeter zone of a vertically glazed installation.
Figure 3. The result of daylight harvesting is energy savings. While the level of savings depends on the application characteristics, savings of 35-60+% have been demonstrated. Courtesy of Lighting Design Lab. Construction Automatic daylight harvesting control systems are comprised of:
Note that control components may be mounted in the application as separate units or can be consolidate into packages; some dimmable ballasts, for example, can be matched to photosensors that directly control the ballast without the need for additional controls.
Designing a System
Dimming Versus Switching The first step in designing a daylight harvesting system is to select the control method. Two control methods are available, dimming and switching.
Control Method: Open Versus Closed Loop Daylight harvesting controls may be “closed loop” or “open loop” systems. They measure the daylight contribution on the task surface differently.
Comparison: The primary advantage of open-loop systems is that they are able to control multiple zones from a single photosensor, as opposed to closed-loop systems, which require that each zone be controlled by a dedicated photosensor. (In review, a zone is a fixture or group of fixtures that are controlled simultaneously.) Because a single photosensor can be used for control of multiple control zones, open-loop systems are generally economical for control of larger areas with multiple adjacent control zones in dimming applications (e.g., an open office). Open-loop systems are also recommended for high-bay applications with skylights, as the photosensor can be mounted in the lightwell of the skylight, while with a closed-loop system, it may be difficult to find a good photosensor viewing location. Open-loop systems tend to be easier to adjust, requiring setup with a light level reading only during the daytime. In addition, open-loop systems provide greater calibration flexibility than most closed-loop systems, and are more “forgiving” to errors in placement of the sensor or its field of view. The primary disadvantage of open-loop systems is that they respond only to exterior daylight availability and not actual daylight contribution in a space; if an occupant closes the blinds, the system will not recognize that and dim the lights anyway. For this reason, local overrides are useful. In addition, in applications with a single zone, open-loop systems generally pose a higher initial cost than closed-loop systems. The primary advantage of closed-loop systems is that they pose a lower initial cost when only a single zone must be controlled, and, unlike open-loop systems, they measure actual light level by sampling the task surface, so they will respond to users opening and closing blinds and other changing conditions. As a result, closed-loop systems are generally economical for control of smaller spaces, or larger spaces with all the lights controlled in a single zone (e.g., a private windowed office). However, there is more setup required; closed-loop systems must be set up with light level readings under both daytime and nighttime (or approximating nighttime—i.e., with blinds closed) conditions.
Figure 5. Open loop control method. Courtesy of Watt Stopper/Legrand.
Figure 6. Closed loop control method. Courtesy of Watt Stopper/Legrand.
Selecting the Photosensor Automatic daylight harvesting control systems use a photosensor to measure light level on the task surface or entering the space—measuring reflected light but not direct sunlight. The photosensor is a small device that can include a light-sensitive photocell, input optics and an electronic circuit used to convert the photocell signal into an output control signal, all within a housing and with mounting hardware. The visible size of a photosensor on the ceiling ranges from a golf ball to a standard wall switch. While many photosensors are ceiling- or wall-mounted, photosensors are now available that integrate directly into open and louvered light fixtures by attaching to a lamp via a clip and to the fixture’s dimmable ballast via low-voltage wires. This allows individual fixture control; each fixture requires its own photosensor. Another form of integration with light fixtures is shown in the second photo, where a photosensor is integrated with a pendant direct/indirect light fixture.
Figure 7. Photosensor. Courtesy of Watt Stopper/Legrand.
For more information, go to www.wattstopper.com or www.leviton.com and choose the right daylight harvasting/occupancy sensor to fit your lighting application's needs.
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