Industry research reveals why daylight harvesting systems are only as strong as their weakest link.
In recent years, the sustainable design movement has returned daylighting to the fore of mainstream construction. In its simplest definition, daylighting is the use of daylight as a primary source of illumination to support human activity in a space. Effective daylighting has been demonstrated to save energy and increase the quality of visual environments, reducing operating costs while improving user satisfaction. Playing a strong role in programs such as LEED, daylighting has also received expanded recognition in California's Title 24 energy code.
“Design analysis often shows daylighting control to be one of the most promising energy conservation strategies for commercial buildings. Consequently, daylighting controls are more frequently installed,” says David Eijadi, FAIA, principal at The Weidt Group, a Minnetonka, Minn.-based firm that provides energy design assistance (including daylighting) as an energy conservation strategy to architects and engineers. “Because energy codes may eventually mandate the use of daylighting controls, it seems prudent to look for object lessons for success and failure from the set of early adopters.”
Why do some daylight harvesting projects succeed when others fail?
The study. To help answer this question, The Weidt Group conducted a study of daylight harvesting projects to find out if they'd met with expectations. Reviewing dozens of completed projects — most of which were side lit using windows — the team separated the success stories (some of them operating for 25 years) from those they considered to be failures. Eventually, they ended up focusing on eight projects that represented different components of the success/failure spectrum (Table) reviewing the design intent, meeting with the project team, and making additional site visits. In some cases, a project was largely successful except for a single element that caused it to fail. “Like a roof, success can be defeated by a series of small compromises or a single catastrophic failure,” says Eijadi.
Using this research, Eijadi cites common examples of why daylight-harvesting projects fail:
Lack of coordination or understanding between the different design disciplines concerning the daylighting control system.
Improper location of controls.
Inadequate specification of the controls systems, component parameters, and sequence of operations.
Shop drawings made by contractors that detail the system are not checked, or the lighting designer does not know what to check.
Field changes to tune a system are not documented and taken back to the designer to complete the feedback loop.
These problems result in common failure modes, such as:
Under-dimming, which results in less than expected energy savings.
Over-dimming, which results in user irritation.
Frequent cycling of dimming or switching, resulting in user irritation.
Lights left on at night, which results in less than expected energy savings.
A closer look. A review of the details for two of the eight projects studied by the group will help you better relate these findings to projects you may face in the future.
College dining hall project. Large windows on three sides of the room and high diffuse glazing on the south side were supposed to provide daylighting. The south windows had a deep interior light shelf, which functioned as a shading device. Photosensors were placed near the windows, linked to controllers by zone and a central control system. However, non-dimmable lighting systems were connected to the dimming system. Control zones were not matched to daylight patterns. The circuits were not wired as shown in the construction drawings, and the photosensor was not calibrated during construction, resulting in lack of detection of the wiring errors.
After noticing the problem, the staff called for calibration. The control system was reprogrammed as switching instead of dimming. In some cases, the control zones matched the daylight pattern, and the problem was resolved easily. In others, the control zones did not match the daylight, which resulted in rewiring work.
Office building project. This 300,000-square-foot office building featured a deep perimeter open office arrangement and ribbon glazing to a 10-foot ceiling height. A series of dimmable T5HO direct/indirect light fixtures were installed in rows parallel to the windows. Each row was controlled as a separate zone with its own photosensor.
At first, everything seemed to work perfectly. The controls were calibrated and responded well to changes in daylight levels. However, furnishing colors were not selected to support the daylighting conditions. Dark furnishings were installed, resulting in a light level of 25 to 40 footcandles on work surfaces when the daylight harvesting system was active. Accustomed to a brighter work environment, occupants generated so many complaints that the system was deactivated.
The Weidt Group also felt that the sensors were calibrated too aggressively in this case, considering occupant preferences. Most significantly, the users were not told about the control system and its benefits, so there was no buy-in. Because there was no problem-reporting process for the controls, the operations staff lost confidence in the system.
Ensuring success. To combat this type of situation, Prasad Vaidya, international associate, AIA, The Weidt Group, recommends that lighting designers should first develop a controls narrative that defines the behavior expected of the daylight harvesting system.
“A good control narrative can help the user and building operator understand the system, and it helps the controls contractor calibrate and test it adequately,” he says.
Eijadi and Vaidya advise control system designers to follow these steps to ensure a successful daylight-harvesting project:
Conduct a daylight simulation, and use these plans when designing the lighting system and its controls.
Prepare plans that document daylight zones, and establish independent control zones that work optimally with these patterns.
Locate the photosensor on the reflected ceiling plans and interior elevations.
Identify light fixtures that are controlled by individual sensors or controllers.
Write a daylighting controls narrative.
Require the contractor to submit shop drawings based on design documents and control narrative for review.
Include the requirement for calibration of controls in the specifications, and require calibration logs to be submitted by the contractor.
Provision building operator training by the controls manufacturer
Daylight harvesting need not be complicated with the proper attention and a commitment to coordination between disciplines. “The only way to have successful daylight harvesting is to make the whole team responsible for the outcome,” says Eijadi. “Anyone can drop the ball or pick it up and run with it.”
DiLouie is the communications director for the Lighting Controls Association and principal of Zing Communications in Calgary.
Switching and Dimming Strategies. Daylight must be properly integrated with the electric lighting system for its energy-savings potential to be realized. A primary strategy, called daylight harvesting, is to use lighting controls that switch or dim the lights either manually or automatically in response to available daylight.
Advantages of switching include a lower initial cost, simplified design, and less involved commissioning. The primary disadvantages of switching are less flexibility than continuous dimming and sudden changes of light level, which can be irritating to occupants.
The benefits of continuous dimming include the highest level of flexibility and user satisfaction, and also often the highest energy savings. Disadvantages include the addition of dimmable ballasts and potential wiring to the initial cost and commissioning.
Due to their advantages and disadvantages, switching is often recommended for spaces with non-stationary tasks such as corridors. Continuous dimming is often recommended for spaces where users perform stationary tasks, such as offices.