BUILDING SCIENCE
Performance of Tubular Daylighting Devices
By Aziz Laouadi and Hamed Saber
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Figure 1. Typical tubular |
Tubular daylighting devices (TDD) are used to collect and channel daylight through the roof of a house or building into interior spaces. Compared to conventional skylights, they have the advantages of energy savings (small area relative to the amount of useful light they can admit), lower solar heat gains, and relative ease of installation. Straight TDDs are common, although non-linear light guides with bended sections are sometimes needed to suit the geometry of a building.
The light output of a TDD varies according to time of day, outdoor daylight availability, house/building location, roof orientation and exposure, and tube length-to-diameter ratio. A typical device can illuminate an area of 150 to 300 ft2. The area of coverage is dependent on the height of the ceiling-the higher the ceiling the more widely the light will be uniformly distributed.
TDDs collect solar heat and carry it indoors. The solar heat gain is desirable in winter for reducing the heating load, but in summer it increases the cooling load. In addition, TDDs may lose indoor heat to the outdoors through their roof openings. These aspects have an influence on the energy performance of a TDD and hence on the ability to comply with energy codes and standards. However, TDD technologies have been evolving rapidly to meet high standards of energy efficiency in houses and buildings and the need for glare-free lighting.
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Figure 2. Typical collector shapes. |
TDDs typically consist of three parts: a collector on the roof to gather sunbeam light and diffuse sky light (Figure 2), a hollow pipe guide in the plenum or attic space to channel light downwards, and a diffuser at ceiling level to spread light indoors (Figure 1).
Tubular Light Guide
The tubular light guide can be straight-rigid, elbow-rigid, or flexible, and is usually made from an aluminum sheet. The interior surface of the aluminum guide is usually coated with special materials to highly reflect the visible component of sunlight and absorb the infrared component so that mainly visible light gets into the indoor space. Coating materials with a light reflectivity up to 99% are commercially available.
Depending on the building type and building (fire and energy) code requirements, the exterior surface of the pipe may have a section covered by insulation at the ceiling level (e.g., attic space of residential buildings) or roof level (e.g., plenum space of commercial buildings), or the entire pipe surface may be covered by insulation.
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Figure 3. Visible (light) transmittance of a straight pipe (without any elbow) |
Figure 3 illustrates how the amount of sunlight is transmitted through a straight pipe (without the roof collector, ceiling diffuser and any elbow) as influenced by the pipe aspect ratio, reflectivity of the interior pipe surface, and the angle at which sunlight strikes the pipe opening surface (the angle of incidence).
It should be noted that the results of Figure 3 are indicative for the vertical pipe transmittance only. Adding a complex collector (with prismatic structures) on the top of the pipe, or multiple elbows will modify the profile of the pipe transmittance of Figure 3, especially at high incidence angles (or low sun altitude angles).
Installation
Installation methods for TDDs vary depending on roof type (asphalt shingles, roof tiles, membranes, metal roofing, etc.). Assemblies must be installed to resist air and water infiltration, and damage due to wind and snow load. Manufacturers' instructions provide specific details. Careful attention to detail is required when re-roofing takes place.
Conclusion
TDDs are an excellent means for admitting daylight to buildings. Their optical performance affects the illumination they provide. Their thermal performance affects heat gain and loss and a building's ability to meet energy code requirements. NRC Construction has completed research, on behalf of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) that will greatly enhance the ability of manufacturers to design and rate TDDs in terms of their performance characteristics. Adoption of the models by industry will allow specifiers to select products that will give the best optical and thermal performance for given locations and TDD orientations.
Dr. Aziz Laouadi and Dr. Hamed Saber are research officers at NRC Construction.
Excerpted from Construction Technology Update #82: Performance of Tubular Daylighting Devices found at http://www.nrc-cnrc.gc.ca/ctu-sc
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