Daylight harvesting is a lighting control strategy that automatically adjusts electric lighting system usage based on available daylight, which can be captured in a space either through sidelighting, such as through vertical windows, or toplighting, such as through skylights. However, reports of poorly functioning systems are commonly associated with sidelit spaces. In response to these claims, a Sidelighting Photocontrols Field Study — funded by Southern California Edison, Rosemead, Calif; Pacific Gas & Electric, San Francisco; and the Northwest Energy Efficiency Alliance, Portland, Ore.; and performed by Heschong Mahone Group, Fair Oaks, Calif. — was conducted to determine photocontrol system performance in 123 sidelit spaces in West Coast buildings. “We found that the performance of systems in sidelit spaces was far more variable than in toplit spaces,” says Lisa Heschong, principal, Heschong Mahone Group.

In fact, the analysis revealed that only 25% of the sidelighting photocontrol systems in the studied buildings were functioning well. Moreover, the researchers discovered that 52% of the photocontrol systems were not functioning at all. The most common reason cited was that the system had been intentionally disabled — typically due to occupant complaints. Respondents cited other reasons for the systems not working properly, including incorrect or incomplete installation, the fact that they had never been commissioned, insufficient daylight, or incompatibility with a present building automation system. The choice of dimming vs. switching did not have a significant impact on the likelihood of success.

In addition, the study found that sidelighting photocontrols were most likely to be functioning in spaces with higher daylight levels and more uniform daylight distribution. The controls performed best in owner-occupied buildings, with large open spaces and no partitions, and with daylight entering the room from more than just one wall. Systems that kept things simple — without too aggressively pursuing energy savings — were also more likely to be successful.

To reap the maximum benefits of daylight harvesting, Heschong recommends following three rules of thumb when designing and installing sidelighting photocontrol systems.

Rule 1: Make sure the space will receive sufficient daylight. “There is really no point in installing photocontrols in spaces where the daylighting is marginal or causes visual comfort,” Heschong says.

Rule 2: Keep it simple. “Elaborate integration schemes or highly complex control protocols are more likely to fail sooner or later, when someone down the line does not understand the subtleties,” she says.

Rule 3: Communicate system specifications, design intent, and physical location very clearly. “If you can't describe exactly how the system is supposed to work, chances are no one else will be able to figure it out either,” says Heschong.

Editor's Note: Thanks to the Lighting Controls Association, Rosslyn, Va., for supplying information used in this report.