By Craig DiLouie
Properly executed daylighting is both a best-practice lighting tool, with highly desirable lighting effects, and a frequently promoted sustainability strategy, with high potential energy savings and carbon-emission reduction.
Consider a study conducted by Heschong Mahone Group in 2003 in which researchers studied a major retailer's 73 store locations in California from 1999 to 2001 and found a 0- to 6-percent average effect of daylighting on monthly sales with a maximum of 40 percent. In fact, researchers concluded that daylight has as much explanatory power in predicting sales as traditional retail potential predictors such as local demographics, parking area, and number of local competitors.
The presence of daylight, while potentially beneficial, does not make a lighting design sustainable, however. Energy savings come when lighting can be adjusted according to daylight contribution.
A basic daylight harvesting control system consists of a photosensor that monitors task light levels and signals a dimmable ballast or switch to adjust light output according to the amount of daylight entering the space. The New Buildings Institute says daylight harvesting systems can potentially save 35 to 60 percent but can "easily save 10 to 50 percent of annual lighting energy" in suitable spaces.
After all, what sounds more sustainable than shutting off or dimming the lights in response to free, renewable, ample sunlight?
The strategy itself is sometimes not sustainable.
Heschong Mahone Group researchers set out to determine if daylight harvesting control systems in toplighting (e.g. skylights) and sidelighting (e.g. windows) applications were delivering positive and persistent energy savings.
In the toplighting study (2004), researchers conducted site surveys of 32 daylight harvesting systems under skylights and found they were producing energy savings very close to savings that had been predicted using modeling software. What's more, two-thirds of the surveyed systems were working or working well. They concluded: "Photocontrols with toplighting is a cost-effective and sustainable method of saving energy."
The site staff, however, often did not understand how to adjust the control system, resulting in occupants manually turning off the lights before the control system's setpoint had been reached and thereby eliminating some of the utility of having automatic instead of manual controls. "This finding that staff doesn't adjust the control settings highlights how important initial commissioning is to long-term energy performance of the systems," concluded researchers.
Subsequently, the researchers conducted a study examining daylight harvesting control system performance in 123 sidelighted spaces. While top-performing systems in sidelighting applications produced persistent, high energy savings comparable to control systems in toplighted spaces, only 25 percent of the sidelighting control systems were functioning well, and about one-half were not working at all - most commonly because they had been intentionally disabled due to occupant complaints.
This was not a technology or equipment problem but a problem in design, application, and commissioning. The research suggests it's not enough to specify a daylight harvesting control system to increase the sustainability of a project: The system itself must be sustainable.
Craig DiLouie (firstname.lastname@example.org) is principal at ZING Communications Inc.