A sub tropical building facade combining light redirection, shading and ventilation; St Pauls School, Brisbane, Australia.
By Geoff Bleney, Heathwood Cardillo Wilson Architects, Brisbane, and Ian Edmonds, Queensland University of Technology, Brisbane.
Introduction.
The facades of sub-tropical buildings provide a challenging problem of energy conservation. For a school building designed to operate without air conditioning and with minimal electrical lighting it is necessary that the facade eliminate radiant heat gain and glare and provide excellent ventilation in summer. It is desirable that the facade provide good daylighting with minimal glare accompanied by radiant heat gain in winter. A possible approach is to severely shade all windows and provide daylight, ventilation and radiant heat gain via skylights. However this requires the use of specialised skylights (Edmonds et al., 1996). In Australia glazing the Northern façade provides the best balance between reducing radiant heat gain and increasing daylight. This note presents the design and preliminary performance for an energy conservative school building at latitude 27 degrees South, (Brisbane).
Design:
Fig. 1 below. Photograph giving a view of the Northern façade of the building. Fig. 2 below. A schematic of the Northern façade.
The facade comprises:
(1) A roof overhang extending approximately 1.2 metres to provide shading from summer sunlight.
(2) A clear acrylic glazing about 0.5 m high set out from the window frame to provide a fixed opening for ventilation and for deep penetration of diffuse daylight.
(3) A double glazing about 1.3 m high comprising a external 3mm clear glass panel and an internal laser cut panel, (Edmonds, 1993), 10 mm thick with laser cut spacing 3.6 mm and laser cut depth 6 mm to provide redirection of winter sunlight to the ceiling of the building.
(4) An external horizontal shade extending approximately 1.2 m from the façade to provide shading of the lower window from summer and winter sunlight.
(5) A louvered clear glazing about 1.3 m high for summer ventilation and ground reflected daylighting.
(6) The lower wall of the building.
Performance :
Figure 3 left shows a photograph inside one of the classrooms obtained in direct sunlight at about 10 am on May 12 2004 and providing an oblique view towards the Northern façade. The principal feature illustrated is the redirection of winter sunlight from the work areas of the classroom to the ceiling of the classroom. Note that a small amount of sunlight passes directly through the glazed door that is unshaded at this time of year. At the time this photograph was taken the sun elevation was such that 75% redirection of sunlight to the ceiling occurred. The laser cut panels reduced the horizontal illuminance on the floor and on the work table from about 24 klux to about 6 klux. As illustrated in the photograph the redirection reduces the reflected glare from surfaces to acceptable levels while providing a useful and essentially glare free source of reflected illumination and radiant heat from the ceiling.
Discussion:
The design of this building required East /West alignment and a glazed Northern façade. The design of North facing facades in the sub tropics is problematic due to the potential for very high radiant heat gain during the dry winter season (Reppel and Edmonds 1998). Much emphasis is currently being placed on using controllable or “smart facades” to address the problem of balancing radiant heat gain and daylight (Raicu et al. 2002, Alther and Gay, 2002; Assimakopoulus et al 2004 and Lee et al 2002). The early results from the building described here suggest that the careful combination of fixed components for shading, light redirection and ventilation can provide acceptable energy performance.
References.
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Edmonds I. R., Jardine P. A. and Rutledge G. Daylighting with angular-selective skylights : predicted performance. Lighting Research and Technology, 28(3), 122-130 (1996).
Raicu, A. Wilson H. R., Nitz P., Platzer W, Wittwer V, Jahns E., Façade systems with variable solar control using thermotrophic polymer blends. Solar Energy, 72 (1) 31 – 42 (2002).
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