LIGHT GUIDING SHADES FOR SHADING AND DAYLIGHTING SCHOOLS

As part of the Victorian Schools Lighting Demonstration sponsored by Sustainability Victoria  light guiding shades (LGS) were installed at Huntly Primary School, Huntly, Victoria as illustrated below.

                            

Fig 1 Exterior view of light guiding shades.                       Fig 2. Interior view looking towards shades        Fig 3. Distribution of light on the ceiling.

ADVANTAGES IN SCHOOLS :     The heating load in school classrooms in summer is high so, generally, classroom windows must be shaded to reduce radiant heat gain. Usually external or internal shading drastically reduces the natural lighting and electrical lighting is necessary. However, light guiding shades accept a fraction of the incident sunlight and guide it through the shade itself into the room so that the sunlight is diffusely distributed over the ceiling. If the ceiling is white most of this natural light is reflected down to illuminate the interior. As indicated in Fig 1 above the input area at the leading edge of the shade is about one tenth of the window area. This relatively small area, in direct sunlight, admits plenty of light. However, it does so in an optimal way so that it is distributed deep into the room rather than falling on the floor near the window. The light is emitted upwards, so that the luminance of the light source as seen from inside is low - about the same as the luminance of the view through the window under the shade.  Thus the light guiding shade provides optimal shading AND optimal natural illumination for school buildings in a single device.

COMPARISON WITH LIGHTING PROVIDED VIA PHOTOVOLTAIC PANELS :   Most of the electrical load in conventional schools is due to fluorescent lighting. Some of this electrical load can be generated by photovoltaic panels installed on the roof. This electricity is then converted back to illumination in fluorescent lamps. However, photovoltaic panels convert only about 10% of the incident sunlight to electrical power and of the 36 Watts of electrical power applied to a fluorescent lamp only 10 Watts is emitted as visible light. The other 26 Watts is emitted as heat and IR radiation. Thus the conversion : sunlight to electricity and back to fluorescent light is very inefficient - about 3% efficiency overall. In comparison a light guiding shade distributes sunlight directly into the space to be illuminated. Of the sunlight incident on the aperture about 60% passes through the shade and the window and into the room. As a result of these marked efficiency differences it can be shown that spending money on light guiding shades is about five times more effective in reducing Greenhouse emissions in schools than spending the same money on photovoltaic panels. Given that money has to be invested in some form of shading for windows the cost benefit of light guiding shades is probably much higher.

Comment on school application by David Tallis, Sustainability Victoria, Dec 13 2006.

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