daylight harvesting control systems have become a common feature in green buildings, and with ASHRAE/IES 90.1 2010 now the baseline energy design standard, it also is expected to become a staple in mainstream construction.
Daylight harvesting is a lighting control strategy based on the use of light sensors (e.g., photosensors, daylight sensors) to automatically reduce electric lighting when sufficient daylight is present, saving energy. According to a study by the Lawrence Berkeley National Laboratory—which analyzed 240 energy savings estimates reported in 88 studies—daylight harvesting results in average energy savings of 28 percent.
Like all sophisticated control systems, which are becoming increasingly common due to energy codes and owner acceptance, this strategy comes with a risk: complexity increases the margin of error in design and installation. For this reason, commissioning—defined fully in ASHRAE Guideline 0 2005, The Commissioning Process, and specifically for lighting in IES Design Guide 29—is recommended to ensure the delivered system satisfies the design intent and the owner’s operational needs. The latest generation of energy codes now requires elements of The Commissioning Process, including a written narrative, functional testing of controls, and turning over as-built drawings and other documentation to the owner. The rationale: If sophisticated building energy systems are commissioned, they will satisfy the owner and occupants while producing higher energy savings.
With a grant from the Minnesota Department of Commerce’s Division of Energy Resources, the Energy Center of Wisconsin (ECW) tested this idea in a study of 20 office and public assembly spaces in Minnesota and Wisconsin. ECW collected subhourly measurements of light levels, lighting power and heating/cooling data over six months (Jan. 13 to July 10, 2012) for daylight harvesting controls already in place and compared those measurements to results after the system was recommissioned. Collected data included current, light levels, voltage, power factor, window treatment, position and heating, ventilating and air conditioning (HVAC) supply air temperature.
The results were published in “Commissioning for Optimal Savings from Daylight Controls,” in February 2013. For daylight harvesting controls as they were found in existing spaces, ECW determined a median control system lighting energy savings of 20 percent, or 809 kilowatt-hours (kWh), for every controlled lighting kilowatt. This increased to 23 percent, or 915 kWh, for every kilowatt, when HVAC savings were included.
ECW then evaluated each space based on a new metric, “controls effectiveness,” calculated as actual savings/ideal achievable savings. The actual savings had been measured. To determine ideal achievable savings, the researchers examined the operation of the controls and potential benefit from recommissioning, taking into account space limitations, such as glazing size and properties, furniture and material finishes.
ECW estimated the average controls effectiveness for the systems installed in the 20 studied spaces as 51 percent, meaning these systems were achieving only half of their potential energy savings. (Four of the systems were not producing any energy savings at all.) The spaces with the greatest controls effectiveness had been subject to continuous commissioning, while the spaces with the lowest controls effectiveness were never commissioned. Interestingly, there was no correlation between controls effectiveness and system age. The main problem, ECW found, was in execution.
Problems included a failure to define light level targets and to review design documents to ensure proper installation of controls, functional testing and owner training. Specific problems included improper sensor calibration, zoning, and relay connections coupled with furniture selection (e.g., cubicle walls too high) and heavy internal shading (typically due to solar glare problems). The researchers spent one to two hours per space tuning/calibrating and reorienting sensors, connecting disconnected components, changing time settings and making other adjustments as needed. Median lighting energy savings increased to 43 percent, or 1,725 kWh for each controlled kilowatt, including HVAC impacts.
In other words, recommissioning saved an additional 690 kWh per kilowatt in the median case and up to 2,420 kWh. ECW achieved energy savings even in spaces already realizing a high level of controls effectiveness. Meanwhile, average controls effectiveness increased from 50 to 75 percent solely through improving operation of the controls.
The researchers concluded that properly designed, installed and commissioned daylight harvesting control systems can generate substantial energy savings and be economically attractive to owners. However, systems that fall short of optimal performance still leave significant energy savings on the table. Commissioning can be an effective way to optimize performance and energy savings in new and existing buildings. The lessons learned go beyond daylight harvesting control, posing implications for optimizing all lighting control systems, particularly as these systems become more sophisticated to satisfy the latest generation of energy codes.
To download the study, visit www.ecw.org/mndaylighting.