Daylight harvesting is a lighting control strategy that automatically adjusts electric lighting system usage based on available daylight. It relies on a photocontrol system to measure ambient light levels, then switch or dim the affected electric lighting to maintain the desired level of illumination. An effective daylight harvesting system will save energy while being virtually unnoticed by occupants.
Daylight can be captured in a space through sidelighting, such as through vertical windows, or toplighting, such as through skylights. After Heschong Mahone Group, Inc. conducted a study of toplit photocontrol systems and found them to be cost-effective and persistent sources of significant energy savings, which was used to justify new code requirements for toplit spaces in California, the firm next studied sidelit photocontrols.
“There was a general concern that the rumors of poorly functioning systems were commonly associated with sidelit spaces,” says Lisa Heschong, principal of Heschong Mahone. The resulting Sidelighting Photocontrols Field Study, funded by Southern California Edison, Pacific Gas & Electric and the Northwest Energy Efficiency Alliance, examined 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, but that when they were working well, they were capable of a similar level of energy savings, demand reduction and persistence as toplit systems,” says Heschong. “Unfortunately, half the systems in sidelit spaces were found to be not saving any energy, and were not functioning for a wide variety of reasons. Of the systems that were saving energy, only half of those were performing well. Thus there is a lot of room for improvement.”
The Field Study points to significant technical potential for this control strategy, finding the top-performing systems to be saving 1.1 kWh/sq.ft./year, or 51 percent lighting energy savings, while achieving a peak net demand reduction of 0.6W/sq.ft. in controlled daylit areas. Assuming this is the achievable potential for photocontrol systems in sidelit applications, Heschong Mahone estimated that 3,190 GWh/year could be saved nationwide based on the current commercial building stock of 58 billion sq.ft. Assuming an average commercial rate of $0.08/kWh, this equates to $80 million in annual energy cost savings.
“I was personally surprised that the best sidelit systems were performing nearly as well as the best toplit systems,” says Heschong. “This is very good news. Some of the best-performing systems were quite old—12 to 16 years old. Thus the technology from a decade ago was fully capable of success.”
The Field Study, however, revealed that only 25 percent of the sidelighting photocontrol systems in the studied buildings were functioning well, whereas almost 100 percent of the toplighting photocontrol systems were found to be functioning well in the previous study. What went wrong?
The researchers discovered that 52 percent of the photocontrol systems were not functioning at all. The most common reason: The system had been intentionally disabled, most often due to occupant complaints. The system also may not have been working because it had never worked (typically due to incorrect or incomplete installation), it had never been commissioned, there was not enough daylight, or the system was incompatible with a present building automation system.
The choice of dimming versus switching did not make a significant impact on the likelihood of success. Dimming systems failed less often than switching, but saved less energy when functional. “Overall, dimming and switching were equally likely to be saving energy, but the two systems have very different challenges,” says Heschong. “Switching systems seem to work best in simple, very well daylit spaces, where the source of electric light is not directly visible to occupants, such as in combination with indirect luminaires. Dimming systems are more likely to be ‘tuned’ down to low levels where they are not saving as much energy as they could, and left continuing to function at these low levels. Dimming also generally requires more knowledgeable designers and installers, and more money. It is important to know that both can be successful, and choose the appropriate application.”
Commissioning may have been a factor. Unfortunately, interviewed facility managers were unclear about the concept, and so it could not be correlated to a greater likelihood of system success or failure. The researchers did determine conclusively, however, that in buildings where the occupants had been trained on the performance of the photocontrols, the systems were more likely to be successful.
“The most common design error was trying too hard—putting photocontrols in spaces that do not have good daylighting,” says Heschong. “Open office spaces with low windows and high partitions are the most common case. Or private offices that are only used infrequently, where occupancy sensors would be far more cost effective. We also saw many cases where the circuited area for photocontrols—i.e., the ‘daylit zone’—was too large, and so the back or a far corner of the zone was not controlled well.”
Just as important: What went right?
The Field 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 it simple, without too aggressively pursuing energy savings, were also more likely to be successful. Systems in classrooms were least likely to fail, but saved the least energy when working.
And the lessons learned? According to Heschong, demand for photocontrol systems has been increasingly dramatically, but the supply of knowledge needs to catch up. She pointed out three rules of thumb to follow:
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,” says Heschong.
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. “Currently, there is not a deep reserve of common sense in the construction industry about these systems, thus you cannot rely on the contractor filling in the blanks or figuring it out on the fly,” she adds. “Substitutions are usually fatal. If you can’t clearly describe exactly how the system is supposed to work, chances are no one else will be able to figure it out either.”