Step 2: Load Minimization - Lighting and Daylighting

As previously shown in the "Building Energy Use Statistics" page, lighting accounts for 23% of the U.S. commercial sector energy use and 11% of Canadian commercial buildings energy use. Lighting is the second largest energy end-use in commercial buildings in heating dominated climates and possibly the largest end-use in commercial buildings worldwide.

1. Efficient Lighting Designs Via Reduced Lighting Power Densities (LPDs): From a design perspective, reducing lighting energy use is straightforward (compared to other end-uses), but requires an understanding of fluorescent ballast metrics such as ballast factor, lighting efficacy (expressed in Lumens/Watt), luminaire efficiency and more importantly, appropriate selection of illuminance levels. [1]

High illuminance levels can significantly affect the ability to achieve designs that exhibit low lighting power densities (LPDs). More critically though, high illuminance levels will create significant glare and occupant discomfort. As shown in the table at right, lighting for “Common Applications” including filing, conference rooms, reading and writing, lunchrooms, and corridors can be designed with horizontal illuminance levels of 10 to 30 footcandles (100 to 300 Lux) based on the latest IES Handbook.[2] There are a few exceptions that require higher illuminance levels such as reading small print, poor quality prints, drawings/blue prints and grainy photocopies, to name a few. Illuminance requirements for “Education” are also included in the table with most tasks requiring 10 to 50 footcandles (100 to 500 Lux) with the exception of specialized activities such as stage demonstrations, shop classrooms and demonstrations in science laboratories.[3]

The ASHRAE 90.1 series of new construction standards have progressively reduced the LPD requirements thanks to improvements in the efficacy of fluorescent lighting systems from approximately 65 Lumens/Watt for standard F34 T12 lamps with electromagnetic ballasts to 100 Lumens/Watt for high performance T8 lamps with extra efficient instant start ballasts.[4] As shown in the attached table, LPD levels have dropped by approximately 50% between ASHRAE 90.1-2001 and the Advanced Energy Design Guide AEDG50. This reduction has also been made possible because of improvements in lamp phosphors that produce better light quality and higher Colour Rendering Indexes (CRI), together with a better understanding of the physiology of the human eye and workplace lighting ergonomics.

Today, there is a large selection of T8 lamp-ballast combinations that allow lighting designers greater flexibility to easily attain LPD levels of 0.7 W/ft² (7.5 W/m²) or better using lighting designs with fixture densities of 50 to 70 ft²/fixture (4.6 to 6.5 m²/fixture) using best–in-class high efficiency fixtures. Low LPDs can be achieved even in small spaces and private offices as shown in the table of lamp-ballast metrics presented below by using low ballast factor (BF) ballasts in combination with reduced wattage lamps. As an example, designs based on a fixture density of 64 ft²/fixture using a typical 2 x 4 recessed fixture with a prismatic lens and a fixture efficiency of 70% can achieve low LPDs and maintained illuminance levels of 50 footcandles (500 Lux) in open offices and 30~ footcandles (~300 Lux) in cubicles using various combinations of ballast and reduced wattage lamps. Higher illuminance levels from those shown could be possible using higher efficiency premium fixtures. T5 fluorescent lamps are not included in the table because their performance is still slightly below the best-in-class T8 lamps when used in standard ceiling height applications.[5] [6]

Large numbers of new energy efficient construction projects are routinely attaining the lowest LPD levels recommended by the ASHRAE Standards and those indicated in the above table while providing proper illuminance levels at zero or minimal incremental costs. [7] [8] Low LPD levels are also being attained in schools as demonstrated in the Integrated Class Room Lighting System (ICLS) pilot program sponsored by the New York State Energy & Development Authority (NYSERDA). Under the program, seven schools were retrofitted in 2008 that resulted in “In Use” LPDs of less than 0.7 W/ft² (7.5 W/m²). [9]

An illustration of potential energy savings and HVAC load reduction that can be achieved in new commercial construction with best-in-class low LPD lighting designs are shown below for the same hypothetical 5-storey 36,000 ft² (3,345 m²) building used previously. The savings were derived with eQUEST and are based on a design with an LPD of 0.7 W/ft² (7.5 W/m²) against a reference LPD of 0.92 W/ ft² (9.9 W/m²) which, as shown in the table of lamp-ballast metrics, represents an entry level lighting design. Four geographical locations are shown that include two heating dominated locations, a mild climate region and a cooling dominated climate. As shown in the first bar graph, the efficient lighting design on its own achieves overall energy savings of 2% in heating dominated climates, 5% in a mild climate and approximately 10% in cooling only climates. The smaller savings in heating dominated climates are primarily due to the interactive effect associated with reducing the internal heat from the lighting equipment during the winter period in perimeter zones which increases the space heating energy use. The space peak cooling load and required air flow are reduced by approximately 6% while peak electrical demand is reduced by a range of 6 to 12%, depending on climate.

Cumulative energy savings and HVAC load reduction from the efficient lighting together with the architectural optimization are included in the next set of results and estimated to be in the range of 35 to 36% in heating dominated climates, 29% in a mild climate and just under 16% in a cooling only climate. Similarly, the overall space peak cooling load and required air flow are reduced by 20 to 40% and peak electrical demand by a range of 15% to 35%.

2. Daylighting: As previously cited in the ”Architectural Elements & Building Envelope" page, good levels of natural lighting can be attained with a WWR of 30 to 40% according to the guidelines from the Lawrence Berkeley National Laboratories (LBNL) report Tips for Daylighting with Windows. [10] Acceptable levels are possible even with a WWR of 25%, provided windows have a high visible transmittance (VT) of > 0.5 and are positioned high to mimic a clerestory and help achieve a deeper daylit zone that can extend up to 15 feet (4.6 meters) from the window. [11] It is important to note though that a WWR of 25% is the minimum amount of window area deemed acceptable by occupants based on satisfaction surveys. [12]

Results of daylighting simulations performed with DOE2.1E are shown below to demonstrate that acceptable levels of daylighting are possible even with low WWR designs. For this purpose, two building models were created depicting a curtain wall design with a continuous strip window that exhibits a WWR of 50% and a punched window design with a smaller WWR of 25%. Hourly reports of the variable “daylight illuminance at Pt1” were specified and the DOE 2.1E models parametrically run at various distances from the window. [13] [14] Predicted illuminance levels at desk height on March 21 at solar noon (equinox) for a Toronto location are shown below in tabular form plus a series of daily illuminance profiles and a 10-day profile extending from March 15 to March 25. The results only include diffuse light (sky-related light) and ignore direct light from the sun (sun-related light).

The punched window layout consisting of 6 ft high by 5 ft wide (1.8 x 1.5 m) windows, spaced at 5ft (0.75 m) intervals and positioned above a 2.5 ft (0.75 m) sill, receives ample daylighting with average illuminance levels of 44 ftcandles (440 Lux) at 10 ft (3 meters) from the window despite its low WWR. This illuminance is achieved both in front of the window, as well as, in front of the opaque wall. At 15 ft (4.5 m) from the window the predicted illuminance is still a respectable 27 ftcandles (270 Lux) thanks to the high position of the window.

The curtain wall with a continuous strip window also assumes a 6 ft high window, positioned above a 2.5 ft (0.75 m) sill. This layout receives excessive natural light, translating into high illuminance levels of 80 ftcandles (800 Lux) at 10 ft (3 meters) from the window. This level of illuminance will produce significant glare, especially for occupants working with computer screens, triggering the need to close window shades; a problem often seen in a large number of high rise office buildings with high WWRs.

From an energy use standpoint, daylighting controls can help achieve lighting energy savings that are reported to be in the range of 25% to 30% of total lighting energy use. These savings can be difficult and costly to achieve, especially if based on continuous dimming control strategies. In addition, some lamp-ballast dimming systems have a lower efficacy and higher input power compared to non-dimming systems. As a result, care is needed when selecting a dimming lamp-ballast system to ensure that it exhibits a similar input power and efficacy to that of non-dimming lamp-ballast systems.

The overall energy savings and HVAC load reduction that can be achieved from daylighting controls are shown below in two separate sets of tables and graphs for the same hypothetical 5-storey building used in the previous section, based on a conservative on-off control strategy of 50% of perimeter zone fixtures that are closest to the window. The results were derived with eQUEST.

The first bar graph shows the net energy savings from the daylighting controls on their own which are estimated to be in the range of 0.5% to 1% in heating and mild climates and approximately 4% in cooling only climates. From an HVAC load reduction standpoint, the space peak cooling load and required air flow are reduced by approximately 3 to 4% while peak electrical demand is reduced by approximately 5%. These savings are small because the daylight controls are applied to an already efficient lighting design with a low LPD of 0.7 W/ft² (7.5 W/m²). In addition, daylighting controls reduce lighting loads in perimeter zones only translating into much smaller net electrical savings in heating dominated climates compared to an efficient lighting design that also affects interior zones which are not penalized by an increase in space heating energy use.

Cumulative energy savings from all the measures (daylighting controls + low LPD lighting design + architectural optimization) are shown in the next set of results and estimated to be in the range of 35 to 37% in heating dominated climates, 30% in a mild climate and just under 20% in a cooling only climate. Similarly, the overall space peak cooling load and required air flow are reduced by 23 to 42% depending on the climate and peak electrical demand by a range of 12% to 28%.

Although the reduction in energy use and equipment size from daylighting controls is small, there is a well established direct correlation between spaces that are only lit with natural lighting and increased productivity. Numerous studies have frequently reported a higher degree of occupant satisfaction and the perception of a more pleasant work environment when working in naturally lit spaces.

[1] Ballast Factor (BF) is defined as the ratio of the lumen output of a ballast compared to a reference ballast. Standard or normal ballasts have a BF of 0.87 to 0.88. Low BF ballasts have a BF that can range from 0.71 to 0.78.

[2] IESNA Lighting Handbook – Reference & Application. 10^th Edition, 2011. Chapters 22, Lighting for Common Applications.

[3] IESNA Lighting Handbook – Reference & Application. 10^th Edition, 2011. Chapter 24, Lighting for Education.

[4] Liebel, B., Brodrick J. 2005. “Squeezing the Watts Out of Fluorescent Lighting” ASHRAE Journal. Vol.47, No. 11, November. pp. 52-54.

[5] Liebel, B., Brodrick J. 2005. “Squeezing the Watts Out of Fluorescent Lighting” ASHRAE Journal. Vol.47, No. 11, November. pp. 52-54.

[6] Advanced Energy Design Guide for Small and Medium Office Buildings: Achieving 50% Energy Savings Toward a Net Zero Energy Building. Chapter 5, Electric Lighting – T5 Lamps and Ballasts, page 144.

[7] Platinum Record: Case Study of the Exelon Headquarters LEED Platinum Certification. Lighting Design and Application (LD+A). October 2007. pp. 81-83. The project achieved a low “In Use” LPD of 0.6 W/ft² (6.4 W/m²).

[8] Hanson, M et al. 2006. “Halfway to Zero Energy in a Large Office Building” ACEEE Summer Study on Energy Efficiency in Buildings. The project achieved an LPD of 0.64 W/ft² (6.9 W/m²), including task lighting while maintaining a general illuminance of 30 to 35 footcandles (300 to 350 Lux).

[9] Classroom Lighting, Field Test DELTA Snapshots. Issue 3, Lighting Research Center (LRC) Rensselaer Polytechnic Institute. January 2008. accessed at

http://www.lrc.rpi.edu/publicationDetails.asp?id=918

[10] O’Connor, J., et al. 1997. Tips for Daylighting with Windows: The integrated Approach. Berkeley: Lawrence Berkeley National Laboratory, pages 3-3 and 3-10.

[11] O’Connor, J., et al. 1997. Tips for Daylighting with Windows: The integrated Approach. Berkeley: Lawrence Berkeley National Laboratory, page 3-2.

[12] IESNA Lighting Handbook – Reference & Application. 10^th Edition, 2011. Chapter 14, Designing Daylighting, page 14.26.

[13] See DOE 2 Supplement, Version 2.1E, Daylighting, pages 2.37 – 2.50. The DOE 2.1E daylighting calculation uses sky-related light and sun-related light that enters the window, as well as, light reflection from walls, floor and ceiling.

[14] Winkelman, F., Selkowitz, S. Daylighting Simulation in DOE-2: Theory, Validation and Applications. Lawrence Berkeley Laboratory (LBL). To calculate interior illuminance DOE separates the daylight incident on a window into “sky-related light” and a “sun-related light”. Both are accounted for directly or by reflection. The interior illuminance levels predicted by DOE 2 are reported to be within 15% of SUPERLITE, a detailed illuminance calculation program.