Image courtesy of the Energy Center of Wisconsin.
Daylight harvesting control, or the practice of using light sensors to automatically reduce electric light in a space when sufficient daylight is present, has been demonstrated to produce significant energy cost savings in buildings. As a proven energy-saving strategy, it has been implemented into many sustainable building projects. It was also incorporated into ASHRAE/IES 90.1-2010 and IECC 2009/2012, the major energy standards. As all states in the United States are required to adopt a commercial building energy code at least as stringent as ASHRAE/IES 90.1-2010 by October 2013, daylight harvesting will become a staple in new construction.
For a daylight harvesting control system to satisfy the owner, achieve the design intent and gain occupant acceptance, a chain of proficiency must be realized. The equipment must operate properly, the designer must articulate the design intent and properly place and orient devices, and the contractor must properly install them. Best practice encourages these proficiencies be formalized within The Commissioning Process, in which a Commissioning Authority, executing a Commissioning Plan approved by the owner, checks each step to ensure that the owner is delivered a fully functioning lighting control system that satisfies the design intent.
With a grant from the Minnesota Department of Commerce, Division of Energy Resources, the Energy Center of Wisconsin (ECW) put this notion to the test in a study of 20 office and public assembly spaces in Minnesota and Wisconsin, collecting sub-hourly measurements of light levels, lighting power, and heating/cooling data over six months (January 13, 2012 through July 10, 2012) for 1) controls as they were found and 2) controls after the system was recommissioned to realize operation as close to the ideal as possible.
Collected data included current of controlled lighting circuits, critical workplane light levels, open-loop light levels, HVAC supply air temperature, voltage and power factor, and window treatment position. The results were published in “Commissioning for Optimal Savings from Daylight Controls,” published in February 2013.
Spaces analyzed in the ECW study.
“When installed, commissioned and operated to perform as designed, daylighting controls can be an economically attractive solution,” the researchers note in the report. “With the levels of performance we measured, owners break even at a cost of $1,000-$2,000 per kW of controlled lighting for these systems, which is in line with current system costs. This opportunity is most promising in new construction or major renovation, where daylighting can be included as part of the design. But there are retrofit opportunities wherever daylight is abundant.”
However, they further note, “We have identified a significant amount of savings being ‘left on the table’ in systems that are designed for substantial energy savings but which fall short … A path to retaining these savings in projects is a more robust, formalized commissioning focus on daylighting control systems. This value needs to be communicated in discussions among owners, designers, contractors and utility program managers.”
Results for the As-Found Controls
For controls as they were found in the existing spaces, ECW measured median control system lighting energy savings of 20%. For every kW of lighting controlled, 809 kWh was saved. Incorporating associated HVAC energy savings increased median savings to 23%, or 915 kWh per controlled kW.
“If we look at the units of kWh/kW, this essentially simplifies to the number of equivalent hours in a year for which the lights would be fully OFF,” the researchers write. “If a typical commercial space operates for 3,500 hours per year, it seems reasonable that a typical daylighting control system could keep the lights at or near OFF for a quarter of that time … On a per square foot basis, for a typical commercial space lit with 1W/sq.ft., this savings equates to 0.9 kWh/sq.ft.”
Distribution of lighting energy savings for the 20 spaces prior to recommissioning. (Note “recommissioning” in this sense means commissioning applied to an existing building, as some of the projects were not initially commissioned.)
Distribution of lighting energy savings for the 20 spaces prior to recommissioning, presented graphically.
The study focused on the question: Could higher savings have been realized in these spaces?
ECW further evaluated each space based on a metric it created called “controls effectiveness,” calculated as Actual Savings/Ideal Achievable Savings. So if a control system achieved 25% energy savings but was found to have a potential of 50% energy savings, then its controls effectiveness was 0.50, meaning it was achieving only half of its potential savings.
The Actual Savings were measured. The Ideal Achievable Savings were estimated based on control operation and potential benefit from recommissioning, taking into account space limitations such as properties of the glazing, furniture and material finishes. For example, if the space had dark finishes that absorbed light, this would reduce the potential of the daylight harvesting control system, since more light would be needed to achieve the desired light level, and hence less energy savings could be realized.
ECW determined that the average controls effectiveness for the systems installed in the 20 studied spaces was 51% (median 66%), meaning these systems were achieving only half of their potential savings. Four of the systems, in fact, were not saving any energy at all.
Distribution of controls effectiveness for the 20 spaces prior to recommissioning.
Controls effectiveness was further expressed based on initial commissioning efforts that were undertaken for the space. The level of controls effectiveness was found to directly correlate to the extent of commissioning that had occurred.
Controls effectiveness based on level of commissioning undertaken prior to study.
Interestingly, there was no correlation between controls effectiveness and the age of the system. Nor were major equipment problems discovered. The problem, ECW discovered, primarily centered on execution—problems that could be addressed through recommissioning.
Major problems reflected a lack of:
• Defined light level targets
• Review of design documents for proper location, orientation and sequence of components
• Functional testing of controls (including tuning)
• Owner training on proper use of controls
More specific problems related to the controls included (in decreasing order of frequency):
• Improper calibration of sensors
• Improper zoning
• Heavy internal shading (due to glare problems)
• Improper relay connections
• Furniture selection (cubicle walls too high)
“Even after a thorough design process, best practices suggest that successful automatic daylighting controls require significant calibration and commissioning efforts during and after construction in order to function properly and reach their energy savings potential,” the researchers note in their report.
Primary problems leading to less than optimal daylighting control along with the frequency of each problem. The right column indicates whether the problem could typically be addressed by recommissioning. (Note some spaces had more than one problem; ECW tracked only the primary problem.)
Results After Recommissioning
In each space, ECW tuned/calibrated sensors, reoriented sensors, connected components that were disconnected, changed time settings, and made other adjustments, spending 1-2 hours per space.
Median lighting energy savings increased to 43%, or 1,725 kWh for each controlled lighting kW, including HVAC impacts. In other words, recommissioning saved an additional 690 kWh per controlled kW in the median case and up to 2,420 kWh per kW in the worst case.
Lighting energy savings after recommissioning.
Energy savings before and after commissioning. Note that the “after” case is represented by the sum of the two bar colors, and not only the red. Also note that in one space, WPPI W2, ECW found that recommissioning resulted in negative savings (additional energy used) as the original setpoints resulted in lighting levels that were, at times, below the prescribed design level.
Improvement was found even in spaces with a high controls effectiveness.
Graphic depicting increase in lighting energy savings for each system before and after recommissioning; data is plotted as a function of initial savings for each control system.
This equated to an additional 20% lighting energy savings above the median 23% already being realized, an 87% improvement.
Typical daylight harvesting control lighting energy savings.
Meanwhile, average controls effectiveness increased from 50% to 75+% solely through improving operation of the controls.
Controls effectiveness significantly increased after recommissioning.
The researchers concluded:
1. “First, when installed, commissioned and operated to perform as designed, daylighting controls can be an economically attractive solution for some building owners and managers. The systems we monitored typically exhibited substantial energy savings.”
2. “Secondly, we have identified a significant amount of savings being ‘left on the table’ in systems that are designed for substantial energy savings but fall considerably short of optimal performance.”
3. “The demonstrated savings indicates that there is value to be captured in the commissioning process for building projects. As a result, the real opportunity here comes from contractors, commissioning agents and utility program implementers to demonstrate this value to building designers and owners and ensure that these steps are completed.”
4. “Finally, there is a substantial number of daylighting control systems already implemented that have room for improvement due to incomplete execution.”
The lessons learned in this study go beyond daylight harvesting control, posing implications for all lighting control systems, particularly as they become more sophisticated to satisfy the latest generation of energy codes and standards. The Commissioning Process, properly executed, can ensure delivery of control solutions that satisfy the design intent and owner’s operational needs. Elements of this process are now featured in ASHRAE/IES 90.1-2010 and IECC 2012, and are expected to become staple activities in construction.
Click here to:
• Download the complete Energy Center of Wisconsin study
• View a recorded webcast presenting the study results
• Download a daylighting control commissioning tip sheet
• Download a daylighting control glossary
The Lighting Controls Association offers a course on The Commissioning Process Applied to Lighting Controls as part of its Education Express online education system, including recommendations for installation verification and functional testing of popular lighting control systems. The course is registered with NCQLP (LC) and AIA CES (LU/HSW). To register and take this course, click here.