Performance of Daylighting Systems for Sidelighted Spaces at High Latitudes
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This thesis is within the field of daylight in buildings. The purpose of the work has been to investigate how daylighting systems could improve the daylight uniformity and increase the daylight levels in the inner part of sidelighted offices. This is studied through full-scale and scale model experiments together with computer simulations. In sidelighted interiors the available daylight decreases rapidly with the distance from the window. The main parameters influencing the daylight distribution are daylight availability, building and window location, surface photometric properties and room geometry. The first part of the thesis shows which effect these parameters have for the daylight distribution. Daylighting systems are modifications in the windows with the intention of redirecting incoming daylight into the darker zones of deep rooms. It can be special types of glass, reflectors or other optical systems that reject, scatter or redirect the daylight in controlled directions. A lot of different daylighting systems are available, and those suitable for windows are presented shortly. A more detailed presentation is given for those systems investigated for quantitative performances; light shelves, venetian blind, prismatic panels and laser-cut panels. These daylighting systems were investigated in full-scale measurements under natural sky conditions and under an artificial overcast sky. The full-scale experiments were carried out under the natural sky in six daily occupied office rooms situated in Sandvika, near Oslo. Five rooms were adapted for experimental measurements of daylighting systems and one room was used as reference without daylighting system. In total four daylighting systems were examined; two light shelves with different surface properties, one prismatic panel and one laser-cut panel. The daylighting systems were investigated under both overcast sky and clear sky conditions. The results show that frequently changes in the sky luminance distribution make reproducibility of results difficult, and it was found that further daylighting experiments should be performed in more controllable surroundings. As a consequence, calibrations and fundamental tests were carried out for a newly built artificial sky in the daylight laboratory at the Department of Building technology, NTNU. The artificial sky, an octagonal box consisting of a luminous ceiling with mirrored walls which produce an infinite series of reflections. The distance between corresponding pairs of parallel mirrors is approximately 3.5 m, and the height of the box is 1.5 m. The investigations showed good reproducibility for quantitative daylighting measurements. The results also showed good agreement with corresponding experiments under both natural and artificial overcast sky performed by other researchers. A sample group of daylighting systems were evaluated with the objectives to increase the daylight level in the intermediate and rear part of a sidelighted room, and to level out the daylight distribution throughout the working plane. A corresponding model situation without any window modifications served as reference. Only a few of the investigated daylighting systems showed satisfactory performance data with respect to the evaluation parameters. • Exterior light shelves level out the daylight distribution throughout the room depth. Depending on the shelf and ceiling surface properties a positive angle of inclination can increase the daylight level in the rear wall zone up to 55%. • Laser-cut panels vertically fixed on the facade give a more uniform daylight distribution. A positive cut angle of 10° can increase the daylight level in the inner part of the room with 25% to 50%. • Tilted laser-cut panels above the daylight window produce somewhat better daylight uniformity in the working area. Depending of tilt angle and ceiling properties the daylight evel in the rear wall zone can be increased up to 25%. The strategy of these daylighting systems is to utilize daylight from the bright zenith of the sky vault to increase the daylight levels in the inner part of the rooms. The investigations show that the relative effect increases as a function of the height of the horizon obstruction. Daylighting systems can thus be an effective solution to increase the daylight levels in situations where obstructions reduce the daylight to unacceptable levels. To prepare for adequate daylight conditions in the interior there is a need for instructions related to the site planning stage. By computer simulations, a parameter study evaluated how the obstruction height and surface reflectances influence the daylight level in a sidelighted office room. The results showed that: • A horizon obstruction angle should be kept smaller than 25° to give potential for good daylighting in the interior of a sidelighted room. • A critical obstruction angle of 40° should not be exceeded. • For obstruction angles between these two values more detailed analyses should be carried out. Factors to be considered for each specific building are the window design (area and location), the room depth and the surface reflectances within and outside the building. This work has focused on quantitative performance data for daylighting systems under overcast sky conditions. To obtain a complete picture for the selected systems, the following investigations should also be performed: • mapping of data under sun conditions • evaluation of other performance parameters (luminance distribution, glare etc) Attention should also be paid to daylighting systems not considered in this work.