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Energy Wise News, March 89, p. 39, 2005-03-01
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Skylights can save up to 45% of monthly electricity costs
Laouadi, A.
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Skylights can save up to 45% of monthly electricity
costs
Laouadi, A.
NRCC-48134
A version of this document is published in / Une version de ce document se trouve dans :
Energy Wise News, no. 89, March 2005, p. 39
Skylights can save up to 45% of monthly electricity costs
A Canadian researcher has found a way to calculate the energy savings from tubular skylights, and a way to work out the optimum distance they should be spaced apart.
Using Sydney weather data imported into SkyVision, a free software design tool, Canadian researcher Dr Aziz Laouadi found the combined system would use from 10% (in June) to 45% (in January) less
electrical energy than an electric-only system. The average monthly saving would be 29%. The single-storey building, with 400 square metres of floor area and a ceiling height of 4.5 metres, occupied from 9am to 9pm seven days a week, would have skylights spaced 5 metres apart
complemented by a building lighting system to maintain an illuminance higher than 400 lux throughout the floor space. The lights would be controlled by a continuous dimming system with a base load of 10%. SkyVision software was developed at the National Research Council of Canada to help building
designers select skylights and achieve satisfactory energy savings without compromising the quality and uniformity of indoor lighting.
To find out the optimum distance apart the skylights should be placed, Laouadi developed a spacing method called Surface Area Coverage (SAC), which he defines as the portion of a floor surface area that receives illuminance equal to or greater than the recommended task illuminance. The skylight spacing in metres is calculated as this SAC figure to the power of 0.5.
He found that using the SAC method, the spacing ratio (spacing divided by ceiling height) depended on the skylight size, sky condition and recommended task illuminance. He found that to achieve the
expected energy savings, skylights should be spaced closer together than suggested by the Illuminating Engineering Society of North America, CIBSE or Australian/New Zealand Lighting Standard 1680.1 methods.
The design is a trade-off between illuminance uniformity, energy saving and cost.
The payback for the Sydney building would be long, but other benefits such as the quality of the daylight, its impact on the way the interiors and people look and the occupants' health and performance, are important.
SkyVision software for designing installations of tubular skylights and other types of skylights is available free from http://irc.nrc-cnrc.gc.ca/ie/light/skyvision. It is 40MB.
The weather and sunlight data from New Zealand can be obtained from the US Department of Energy web site as a link from the Help menu within the program.
