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RESEARCH SUMMARY DETAIL

A Cost-Effective Core Daylighting System

Author's Title: Cost-Effective Controlled Illumnination Using Daylighting and Electric Lighting in a Dual-Function Prism Light Guide
Author(s) Name: A. Rosemann, G. Cox, P. Friedel, M. Mossman, and L. Whitehead
Year of Publication: 2008
Search Related Keywords: Cost Estimating   Energy Efficiency  Lighting/Daylighting Design  Office  Sustainable/Green Design 

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Design Issue
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This study introduced a core daylighting system as a potentially more cost-effective and energy efficient alternative to existing systems in standard building construction.


  • Electric lighting used in office buildings represents a significant amount of energy consumption worldwide. Lighting is used during the day when sunlight is most available and when energy costs are at their highest. Utilizing daylight in place of electric lighting may reduce power consumption and associated pollution.
  • Core daylighting systems have been incorporated into buildings nationwide (Tsangrassoulis, et al., 2005; Greenup & Edmonds, 2004; Callow & Shao, 2003; Schlegel et al., 2004) but have been disproportionately costly compared to their actual energy savings.
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Design Criteria
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Author Identified:
  • Recognize that the core daylighting system developed in this study, utilizing a dual-function prism light guide to allow daylight into a build core, may help reduce power consumption while maintaining a pleasant, visually uninterrupted environment for occupants in multi-story buildings with open-plan layouts.
  • Recognize that other efficient light sources besides fluorescent lamps (e.g., light emitting diodes; LEDs) may be used with the daylighting system presented in this study.
     

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Key Concepts
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  • A dual-function prism light guide based on the prism light guide (Whitehead, Nodwell, & Curzon, 1982) was designed to appear similar to a fluorescent luminaire and fit flush with a standard (2 ft x 4 ft) T-bar ceiling. The luminaire had a rectangular cross-section capable of holding one or more fluorescent tubes (28 W lamp, 2900 lumens, 4100K, CRI 82, 20,000 h life). The light emitting surface (bottom) was lined with a prismatic film. The remaining three interior surfaces were covered with a reflective multilayer dielectric film. The design allowed sunlight to pass through the guide and into the room through an extraction panel and electric light from the fluorescent tube to strike the prismatic film and pass through it into the room. A dimmable ballast was used to maintain the desired level of illuminance.
  • The electric lamps provided at least 500 lx in the space, even when running at 50% power, indicating that the electric lighting offered sufficient illumination without natural light during times when sunlight was not available. Daylight through the window offered adequate lighting (minimum 500 lx) within 3 m from the window but not beyond that. Daylight through the light guide offered uniform illumination and exceeded (average illuminance 643 lx; sun position 43.4º altitude, zimuth 211.5º) minimum illumination standards throughout the space.
  • When using the dual-function prism light guide control settings, daylight and electric lighting adjusted to the lighting conditions. During cloud cover over the sun (when illuminance dropped below a certain level), ballast settings changed immediately and electric lighting was added as a supplement. During cloud cover dissipation, illuminance increased through the daylight component and electric lamp output decreased gradually over an approximate span of 40 seconds.
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Research Method
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  • A portable test facility (10 m long x 3 m wide x 3 m high) designed to simulate one section of a multi-storey office building was constructed. The daylight collection system was mounted above the south wall windows (30% luminous transmittance).  The dual-function prism light guides were mounted flush to the T-bar hanging ceiling.
  • illuminance was measured with a calibrated illuminance meter at a 1 m interval grid 0.8 m above the floor for electric lighting only, daylight through windows only, and daylight through the light guide only. More detailed measurements (0.5 m x 0.5 m grid) were taken where illumination conditions were stable.
  • A Digital Addressable Lighting Interface (DALI) system was installed at each guide section to monitor and react to outdoor daylight conditions. The system was designed to automatically adjust to changes in daylight to maintain a 500 lx interior illuminance. The interior illumination area was divided into zones (3; 3 m from the south wall; 3 to 7 m from the south wall; >7 m from the south wall) with a sensor mounted on the ceiling at each zone to monitor workplace illuminance 80 cm above floor.
  • Key parameters  of the light guide performance (e.g., ballast setting for each light guide section; illuminance at 9 m from south wall reference point) were measured .
  • descriptive statistics were used to analyze the data.
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Limitations
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  • The author did not identify any limitations.
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Commentary
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Graphs of results; a cross-sectional diagram of the dual-functional prism light guide; photographs inside the light guide; a section of the dual-function prism light guide; images of the portable test facility exterior and its interior with light guides mounted with and without ceiling tiles; and a floor plan of the test facility showing sensor locations were included. An additional discussion section was attached. Future studies investigating human preferences for the different types of lighting algorithms were recommended.



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Adapted From
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Author(s): A. Rosemann, G. Cox, P. Friedel, M. Mossman, Ph.D., and L. Whitehead, Ph.D., Department of Physics and Astronomy, University of British Columbia, Vancouver
Article Title: Cost-Effective Controlled Illumnination Using Daylighting and Electric Lighting in a Dual-Function Prism Light Guide
Publisher: Sage Publications
Publication: Lighting Research and Technology
Publication Type: Refereed Journal
Date of Publication: 2008
Funder/Sponsor: Natural Sciences and Engineering Research Council of Canada, 3M Company, BC Hydro Power Smart, British Columbia Institute of Technology
ISSN: 1477-1535
Volume: 40
Issue: 1
Pages: 77-88