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An Experimental Evaluation of Daylight Systems and Lighting Control
Jens Christoffersen, M.Sc., Ph.D., Erwin Petersen, M.Sc., Ph.D., Kjeld Johnsen, M.Sc.
Danish Building Research Institute (SBI), P O Box 119, 2970 Hørsholm, Denmark
Tel: +45 4586 5533, Fax: +45 4586 7535, Homepage: http://www.sbi.dk
Abstract
Daylight has received increased interest in the last decade because of its aesthetic possibilities and its ability to satisfy human, biological, and global ecological needs. Daylight received in the interior, through the windows, serves the building interior with a variability in light intensity, colour and direction, constantly changing from sunrise to sunset, from day to day and season to season. More daylight conscious architectural design and the introduction of innovative daylighting systems and efficient lighting controls could displace a considerable part of the electricity consumption in non-residential buildings by utilising the natural resources offered by daylight. Daylight energy saving potentials are realised by dimming or switching off electric lights when there is sufficient natural light. In non-residential buildings, daylight availability coincides with normal working hours and therefore could displace a considerable part of the electricity consumption for artificial lighting. The paper describes an assessment of different designs of two simple daylighting systems, aiming at improvement of the utilisation of daylight in order to supplement and replace artificial lighting. It also evaluates a daylight responsive lighting control system and documents the system's ability to control the artificial lighting in response to interior daylight levels. The main results of the experimental study were:
- Daylight measurements on overcast days, showed that all tested daylighting systems caused an overall reduced work plane illuminance level throughout the room compared to the reference room with an unobstructed window of equal size. Subjective evaluations on overcast days showed, occasionally, that the perception of the room with daylight systems was experienced as being brighter than the reference room.
- On a fully overcast day, only daylight systems which increase the exposure to the high sky luminance area near the zenith can raise the daylight received at the back of the interior compared to traditional windows.
- Dimming the artificial lighting system caused the power consumption to be reduced from 100% to 38%, while the lighting output where reduced from 100% to 10%.
- The lighting control system tested could not discriminate the direction of light from daylight and artificial lighting, which caused the light level on the work plane to be below the desired light level even though the daylight level in the room where insufficient.
- Adjustment of the tested lighting control system to a desired light level on the work plane was complicated and very sensitive, e.g., to the work plane surface.
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