By Jennifer Li, P.E., LEED AP
From the April 2017 Issue
Lighting in commercial buildings has been the target of energy reduction strategies for many years. Historically, reductions in lighting energy consumption have been achieved by implementing new technology with improved luminaire efficiency, such as replacing T12 fluorescent lamps with T8 or T5 lamps or upgrading to LEDs. However, recent changes to applicable codes and standards have begun to change the baseline of installed lighting efficiency and are eroding the cost-effectiveness of this approach. Given these market changes, facility managers, architects, engineers, and lighting designers must now go beyond simplistic efficacy based improvements. Task tuning is one lighting controls solution for this challenge.
Task tuning involves dimming lights in a space so that the average illuminance at the working plane is appropriate for the type of use in that space. It is sometimes also called institutional tuning or high-end trim. This advanced lighting control strategy can be implemented in many commercial buildings to reduce operational lighting energy. Most commercial spaces, for a variety of reasons, are overlit. With more widespread adoption of dimmable ballasts and LED lighting, there are more opportunities to apply this efficiency measure that is relatively simple to implement.
Measured Savings Results
Task tuning has the potential to save up to 35%¹ of lighting energy without decreasing occupant satisfaction. But, as with any new technology, there is a knowledge barrier to overcome. To facilitate adoption of this energy saving strategy, a field study was conducted to quantify the magnitude of energy savings from task tuning and to identify best practices and barriers to further implementation. This goal was achieved through field monitoring of commercial lighting systems.
As part of this study, lighting energy consumption was measured in 17 spaces both before and after implementation of task tuning. The measured data shows that the average savings from task tuning was approximately 22% of the dimmable lighting energy. On a per square foot basis, for a typical commercial space lit with 1 watt/ft², this savings equates to 0.6 kWh/ft2. The distribution of energy savings across the measured spaces is illustrated in Figure 1.
Energy savings calculations highlighted the following characteristics that tended to correspond to high potential for task tuning savings:
- a lighting system that had not been commissioned or was commissioned by the owner;
- a lighting system that had been designed by a contractor, as opposed to a lighting designer or electrical engineer;
- a lighting system in an education, public assembly, or office building; and
- a lighting system with dimming controlling significant electric power in spaces with longer hours of operation, such as open offices with photocontrol or LEDs (dimmable systems for rooms with A/V equipment did not exhibit high levels of savings opportunity).
Economics Of Task Tuning
On an increasing number of projects, dimming controls are included due to design requirements such as daylighting, controllability needs, or occupant satisfaction (to eliminate the distraction of on/off switching). In this scenario, the only incremental cost to conduct task tuning is for the time taken to tune, which is estimated to range from $0.03 and $0.06 per square foot. This equates to a simple payback of between 0.5 and 1.1 years. Due to these short payback periods, it is recommended that task tuning be implemented in new construction projects or major renovations in which a dimming system is already planned as part of the design requirements. For the same reason, if a dimming system already exists in a facility, task tuning should be strongly considered as a method to achieve cost-effective energy savings.
The most cost-effective task tuning applications would focus on buildings with large areas of similarly controlled lighting, such as large open offices or a number of classrooms for which the same level of tuning could quickly be applied. Regardless of the building, it is likely not cost effective to measure the light levels in all spaces. Chapter 9, Section 15 of The Lighting Handbook2 identifies a methodology for identifying and measuring a sample of representative locations. The resulting lighting level reduction should then be applied to all similar spaces.
Higher cost-effectiveness can also be achieved when tuning is coupled with a lighting retrofit, depending on the existing system. There is a high fixed cost in merely getting into the building and understanding the spaces and associated lighting controls. Once this is done, the time associated with actually tuning the lights is small.
Occupant Comfort And Staff Training
Task tuning is essentially a trade-off between energy consumption of a lighting system and light levels in a space. When performing task tuning, it is important for facility management to balance energy savings with occupant visual comfort; aggressive tuning will result in high energy savings at the expense of occupant satisfaction. Because of this, it is recommended that occupant feedback be collected in conjunction with task tuning. Although this may result in lower immediate energy savings, it would increase energy savings persistence, as facility managers would be less likely to override tuned controls based on occupant complaints.
In order to implement task tuning, facility management staff or contractors should be trained and proficient in a variety of lighting related subjects.
- Use of a light meter
- Fundamentals of lighting: A course was developed by the Illuminating Engineering Society that covers a range of lighting specific subjects at an appropriate level for gaining proficiency in task tuning.
- Lighting controls: The Lighting Controls Association offers a variety of lighting control and dimming control classes.
- Basics of interfacing with major control systems on the market: The biggest variable in any task tuning effort is understanding the nuances of the lighting control systems serving a given space. Facility staff and contractors should work with control system manufacturers to develop training on the basics of their systems.
The field study discussed in this article was funded by Minnesota’s Conservation Applied Research and Development Grant Program. A copy of the study’s full report, as well as additional resources that include a video intended to assist in training, can be found here .
¹ Williams, A. et al. Lighting Controls in Commercial Buildings. Leukos, Vol. 8, No. 3, Pages 131-180, January 2012
² DiLaura, D. et al. 2011. The Lighting Handbook. Illuminating Engineering Society, New York, NY
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