Design insights on tubular skylights

Publisher’s version / Version de l'éditeur:

Lighting, Feb, pp. 38-41, 2005-02-01

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Design insights on tubular skylights

Laouadi, A.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site

LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

NRC Publications Record / Notice d'Archives des publications de CNRC:

https://nrc-publications.canada.ca/eng/view/object/?id=ab50cbc9-38ee-4ad5-bf63-97cc5268acfc https://publications-cnrc.canada.ca/fra/voir/objet/?id=ab50cbc9-38ee-4ad5-bf63-97cc5268acfc

Design insights on tubular skylights

Laouadi, A.

NRCC-47334

A version of this document is published in / Une version de ce document se trouve dans: Lighting, v. 25, no. 1, Feb. 2005, pp. 38-41

DESIGN INSIGHTS ON TUBULAR SKYLIGHTS

A. Laouadi

Indoor Environment Research Program

Institute for Research in Construction, National Research Council of Canada,

1200 Montreal Road Campus, Ottawa, Ontario, Canada, K1A 0R6

INTRODUCTION

Tubular skylights emerged as alternative products to conventional skylights to deliver daylight without the unwanted solar heat gains, and cover areas not usually covered by windows and skylights. Nowadays, tubular skylights compete with conventional skylights, particularly in commercial and residential buildings. Tubular skylights consist typically of three parts: collector to gather sunlight, pipe to channel sunlight downward, and ceiling diffuser to diffuse light to the indoor space. The collector is usually hemispheric and made up of clear glazing. The collector may also include some devices to enhance the lighting output of the skylight, especially at low sun altitude angles. The pipe is made up of an Aluminium sheet with highly reflective interior lining. Materials with reflectivity of 99% are commercially available. The diffuser is hemispheric or flat with translucent (opal) or clear glazing. Translucent glazing performs well in light diffusion, but is not efficient in light transmission. On the other hand, clear glazing is efficient in light transmission, but usually requires lenses for light diffusion. Due to this product complexity, prediction of the skylight performance has always been a difficult task. Although some tentative design guides have been proposed by researchers and some skylight manufacturers, tubular skylights still lack some design insights that would help building designers or architects to properly select and deploy skylight products to achieve the desired energy savings.

Laboratory and field measurements have been extensively used to predict the performance of tubular skylights (Jenkins and Muneer, 2003; Carter, 2002, Zhang and Muneer, 2002; Oakley et al., 2000; Salih et al., 2000; Shao et al., 1998; Allen, 1997). This work mainly addressed the measurement of the transmission efficiency of the skylight, in terms of the lumen output or intensity distribution, or the skylight daylight performance, in terms of indoor daylight availability and energy savings. However, this work is generally valid only for the conditions of the experiments and the considered skylight products, and therefore may not be generalized to other skylight products in real settings. Numerous theoretical models have also been attempted to address the transmission efficiency of the skylight (Swift and Smith, 1994; Edmonds et al., 1995; Zastrow and Wittwer, 1986). These models focused mainly on the pipe transmission efficiency, not including the collector and diffuser. Furthermore, these models did not address the solar radiation absorption by the skylight and its possible impact on the cooling and heating energy. Recently, the National Research Council of Canada has developed the SkyVision computer tool to address tubular skylights as well as conventional (or projecting) skylights with various shapes and glazing types. SkyVision employs analytical models, based on the ray-tracing method, to compute the optical characteristics and energy saving potential of skylights. SkyVision is available free of charge from the web site: http://irc.nrc.cnrc.gc.ca/ie/light/skyvision. Software validation reports can also be found in the web site.

The aim of this paper is to use SkyVision to provide some guidance for building designers to properly select a skylight product and achieve the desired energy saving without compromising indoor daylight uniformity. The specific objectives are:

• To quantify the daylight transmission and solar radiation absorption performance of the skylight; • To develop appropriate skylight spacing; and

• To predict the extent of the energy saving potential of the skylight.

S

KYLIGHT

T

RANSMISSION AND

A

BSORPTION

E

FFICIENCIES

Of the primary parameters affecting the daylighting and thermal performance of skylights are the transmission and absorption efficiencies of the skylight. The daylight transmission efficiency affects the indoor illumination levels, and the solar radiation transmission and absorption efficiencies affect the solar heat gains to the indoor space. The overall optical characteristics of the skylight depend not only on the optical properties of the glazing making up the skylight, but also on the physical dimensions of the skylight. Figure 1 shows the profiles of the transmission efficiency to the sun beam light, and absorption efficiency to solar radiation, as a function of the sun altitude angles for typical commercial 360 mm (14”) and 530 mm (21”) diameter tubular skylights. The pipe length is fixed at 1000 mm with a constant specular reflectivity of 0.95. The collector dome is made up of a single clear acrylic glazing. The diffuser is assumed to be perfect diffuser with a constant light transmission efficiency of 0.90. The skylight transmission efficiency decreases with the sun altitude angle to a minimum value at around 10o, then increases for lower angles. This trend was also experimentally observed. The minimum transmission efficiency is found to be 0.50 and 0.63 for the 360 mm and 530 mm diameter skylights, respectively. However, the solar radiation absorption efficiency of the skylight increases substantially with decreasing sun altitude angles. This means that in winter days at a sun altitude angle of 30 deg. (e.g., in Sydney, Australia), the skylight may absorb up to 190% more solar radiation than in summer days. The 360 mm diameter skylight may absorb about 16% more than the 530 mm diameter skylight. Some of this absorbed energy may end up in the indoor space, which is beneficial in winter and not desirable in summer. 0 0.2 0.4 0.6 0.8 1 0 10 20 30 40 50 60 70 80 9

Sun Altitude Angle (deg.)

Skylight Transmission or Absorption Efficiency

14" diameter : Transmission " : Absorption 21" diameter : Transmission " : Absorption

0

Figure 1: Profiles of the transmission efficiency to sun beam light and absorption efficiency to solar radiation of typical commercial tubular skylights.

S

KYLIGHT

S

PACING

Skylight spacing is a major design parameter. Skylight spacing affects not only the indoor illumination levels, but also the total cost of the skylights and energy savings achieved with a dimming or switching

lighting control system. The current practice for skylight spacing is based on the criterion of the illuminance uniformity on the work plane covered by the skylights. The IES (IESNA, 2000) method defines the spacing as the distance between any two adjacent skylights that yields an illuminance between the skylights equal to that directly under the skylight center. Therefore, this method demands for perfect illuminance uniformity. The CIBSE code for interior lighting (CIBSE, 1994) and the Australian Standard AS 1680.1 for interior lighting recommend that the ratio of the minimum to average illuminance over any task area and immediate surrounds should be not less than 0.8 and 0.7, respectively. Laouadi (2004) has recently proposed an alternative spacing method called the Surface Area Coverage (SAC) to guarantee the desired energy saving. The SAC is defined as the portion of the floor surface area, which is under an illuminance equal or greater than the recommended task illuminance (skylight spacing = SAC1/2). Contrary to the IES, CIBSE and AS 1680.1 methods, the SAC method is a function of the outdoor daylight availability and the task visual requirement.

To evaluate the forgoing spacing methods, the illuminance profile under the skylight has to be known. A typical profile for a 530 mm (21”) diameter tubular skylight calculated by SkyVision for three ceiling heights (3 m, 4.5 m, and 6 m) is presented in a dimensionless form in Figure 2. The surface reflectance values are fixed at 0.2, 0.5 and 0.8 for the floor, walls and ceiling surfaces, respectively. It should be noted that this profile does not depend on the skylight size if the building dimensions are sufficiently larger than the skylight so that the skylight can be approximated by a point source (i.e., ceiling height greater than five times the skylight diameter (IESNA, 2000)). For the three investigated ceiling heights, the skylight spacing to ceiling height ratio is found to be constant and equal to 0.966, 1.492 and 1.74 according to the IES, CIBSE and AS 1680.1 methods, respectively. However, according to the SAC method, the spacing ratio depends on the skylight size, sky condition and recommended task illuminance. Tables 1 and 2 summarizes the skylight spacing ratios, calculated for the 360 mm (14”) and 530 mm (21”) diameter tubular skylights, respectively, according to the SAC method under a typical clear summer day in Sydney (Latitude 33.86o South), Australia. Since these spacing ratios are calculated when the outdoor daylight is maximum, the SAC-based spacing values are regarded as the maximum spacing that should not be exceeded to achieve the expected energy savings. Contrary to the illuminance uniformity based spacing methods, the SAC-based spacing ratio decreases with increasing ceiling heights for buildings with visual requirement lower than 400 lux. This means that, to achieve the expected energy savings, skylights should be spaced closer together than suggested by the IESNA, CIBSE or AS 1680.1 method for buildings with high ceilings (greater than 4.5 m) or with quite high visual requirement (recommended illuminance higher than 200 lux), depending on the skylight size. However, for buildings with low ceiling heights (lower than 4.5 m) and low visual requirement (lower than 200 lux), skylights should be spaced further apart. While the latter case may result in lower initial expenses as the number of skylights is reduced, the illuminance uniformity may be an issue. Therefore a trade-off between the illuminance uniformity, energy saving (and cost) should be sought.

It should be noted that the foregoing skylight spacing results apply to tubular skylights with perfect ceiling diffusers and to large buildings requiring a quite high number of skylights (at least three skylights per row). For large buildings, the building wall reflectance may not significantly influence the amount of indoor illumination at points far away from the walls. However, for small buildings requiring several skylights (less than three skylights per row), the skylight spacing ratio should be calculated for each design case as the building wall reflectance may have a significant impact on the amount of indoor illumination, and therefore the spacing ratio.

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.5 1.0 1.5 2.0

Distance from Skylight Center to Ceiling Height Ratio

Illuminance Ratio Under Skylight

Figure 2: Profile of the floor illuminance distribution under a single 530 mm (21”) tubular skylight, calculated relative to the maximum illuminance directly under the skylight center.

Table 1 maximum spacing-to-ceiling-height ratio for 360 mm (14”) diameter tubular skylight based on the SAC method under a typical clear summer day in Sydney, Australia. N/A means that the

recommended illuminance cannot be achieved.

Recommended task illuminance (lux) Ceiling Height (m) 100 200 400 3.0 4.5 6.0 2.212 1.533 0.987 1.628 0.811 N/A 0.942 N/A N/A

Table 2 maximum spacing-to-ceiling-height ratio for 530 mm (21”) diameter tubular skylight based on the SAC method under a typical clear summer day in Sydney, Australia.

Recommended task illuminance (lux) Ceiling Height (m) 100 200 400 3.0 4.5 6.0 3.220 2.398 1.875 2.417 1.684 1.158 1.775 1.004 N/A

E

NERGY SAVINGS

A medium-size commercial building located in Sydney, Australia, is considered to calculate the potential lighting energy savings of 530 mm (21”) diameter tubular skylights. The building has a floor surface area of 400 m2 and ceiling height of 4.5 m. The building is occupied from 9 AM to 9 PM all days. The building lighting system is controlled to complement natural lighting, and maintains an illuminance higher than 400 lux throughout the floor space. Two control strategies are investigated: ON/OFF automatic and

continuous dimming with a base load of 10%. Figure 3 shows the monthly lighting energy savings for a skylight spacing ratio = 1.11, and Figure 4 shows the annual lighting energy savings as a function of the skylight spacing ratio. The energy savings are based on the local weather data imported from the US DOE international weather data web site and then converted into the SkyVision’s format (SkyVision provides the weather converter and a link to the USDOE web site under the Help menu). The lighting energy savings are calculated relative to when there is no lighting control (lights are on during building occupancy). The energy savings increase with decreasing spacing ratio. As expected, the SAC-based spacing method (spacing ratio around 1, see Table 2) results in significant energy savings. However, following the CIBSE or AS 1680.1 method (spacing ratio around or higher than 1.5), the energy savings are lower, particularly for the on/off automatic control.

0 5 10 15 20 25 30 35 40 45 50

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

M

onthly Energy Savings (%

) ON/OFF Automatic

Continuous Dimming

Spacing ratio = 1.11

Figure 3: Monthly lighting energy savings of 530 mm (21”) diameter tubular skylight, calculated for a medium-size commercial building located in Sydney, Australia.

0 10 20 30 40 50 1.48 1.11 0.90 Spacing-to-Ceiling-Height Ratio A nnua l Lighting E n e rgy S a v ings (%) ON/OFF Automatic Continuous Dimming

Figure 4: Annual lighting energy savings of 530 mm (21”) diameter tubular skylight, calculated for a medium-size commercial building located in Sydney, Australia.

C

ONCLUSION

The SkyVision tool was used to evaluate the daylight transmission and solar radiation absorption performance and energy saving potential of typical 360 mm (14”) and 530 mm (21”) diameter tubular skylights for commercial building applications. The following points are highlighted:

• Typical tubular skylights for commercial building applications (with aspect ratio L/D ≈ 1.8 to 2.8 ) may transmit more than 50% of sunlight, and may absorb up to 190% more solar radiation in winter than in summer.

• For buildings requiring large number of skylights (at least three skylights per row), the skylight spacing-to-ceiling height ratio based on the illuminance uniformity criterion was found to be 0.966, 1.492 and 1.74 according to the IES, CIBSE and AS1680.1 methods, respectively.

• Skylight spacing ratios based on the energy saving criterion (the SAC method) are dependent on the skylight product size, ceiling height, outdoor daylight availability and task visual requirement. A trade-off between energy (and cost) savings and illuminance uniformity should therefore be sought.

• For buildings requiring several skylights (less than three skylights per row), the skylight spacing should be calculated for each design case as the building wall reflectance may significantly influence the indoor illumination levels.

• Potential lighting energy saving of a medium-size commercial building located in Sydney, Australia, may reach up to 40%, particularly when a continuous dimming control system is used. While this lighting energy saving may not off-set the investment in skylights and lighting controls due a very low electricity rate in Australia (around 0.15 cent/kWh), non-energy benefits of skylights such as the quality of daylight and its impact on indoor surface appearance, and human performance and health are of great importance to the designer and building occupants.

ACKNOWLEDGEMENTS

This work was funded by the Institute for Research in Construction of the National Research Council Canada; the CETC Buildings Group of Natural Resources Canada; Panel on Energy Research and Development (PERD), Office of Energy Efficiency, Natural Resources Canada; and Public Works and Government Services Canada. The author is very thankful for their contribution.

REFERENCES

Allen T., Conventional and tubular skylights: An evaluation of the daylighting systems at two ACT commercial buildings, Proceedings of the 22nd National Passive Solar Conference, Washington DC, pp. 97-129, 1997.

Australian Standard, Interior lighting. Part 1: General principles and recommendations. Standard Association of Australia, 1990.

Edmonds I.R., Moore G.I., Smith G.B., and Swift P.D., Daylighting enhancement with light pipes coupled with laser-cut light-deflecting panels, Lighting Research and Technology, 27(1), pp. 27-35, 1995.

Carter D.J., The measured and predicted performance of passive solar light pipe systems. Lighting Research and Technology, 34(1), pp. 39-52, 2002.

CIBSE code for interior lighting, Chartered Institution for Building Services Engineers, London, 1994. IESNA, Lighting Handbook, Reference And Application Volume. New York: Illuminating Engineering Society of North America, 2000.

Jenkins D. and Muneer T., Modelling light-pipes performances- a natural daylighting solution. Building and Environment 38, pp. 965-972, 2003.

Laouadi, A. Design with SkyVision: a computer tool to predict daylighting performance of skylights. CIB World Building Conference, Toronto, Ontario, Canada, pp. 1-11, May 01, 2004.

Oakley G., Riffat S.B., and Shao L., Daylight performance of lightpipes. Solar Energy 69(2), pp. 89-98, 2000.

Salih A., Shao L. and Riffat S., Study of daylight and solar infrared transmitted through light pipes under UK climate, Proceedings of the CIBSE national conference, pp. 198-207, 2000.

Shao L., Elmualim A.A. and Yohannes I., Mirror lightpipes: Daylighting performance in real buildings, Lighting Research and Technology 30(1), pp. 37-44, 1998.

Swift P.D. and Smith G.B., Cylindrical mirror light pipes, Solar Energy Material and Solar Cells vol. 36, pp. 159-168, 1995.

Zastrow A. and Wittwer V., Daylight with mirror light pipes and with fluorescent planar concentrators, SPIE vol. 692 Materials and optics for solar energy conversion and advanced lighting technology, pp. 227-234, 1986.

Zhang X. and Muneer T., A design guide for performance assessment of solar light-pipes. Lighting Research and Technology, 34(2), pp. 149-169, 2002.