An analysis of daylighting performances in office buildings and applications of light-guide system coupled with redirecting device
Student thesis: Doctoral Thesis
Daylight is a valuable natural resource and plays a crucial role in passive solar building design. Appropriate daylighting design not only provides a connection with outside, ensuring an adequate illuminance level with high uniformity and a glare-free environment but also provides an opportunity for energy saving and carbon reduction. However, in metropolitan Hong Kong, vast high-rise buildings are built closely together resulting in a severely indoor daylight environment. To acquire in-depth understanding of the daylight environment of current buildings in Hong Kong, a survey on the daylighting performance of local buildings was conducted. In total, 35 air-conditioned non-residential buildings built from 1962 to 2004 spreading over different districts were selected for the study. The findings suggest that the rear part of rooms rarely receives adequate amount of daylight and the average obstruction angle for buildings in Hong Kong is between 30° and 60°. It is envisaged that traditional daylighting designs are not able to fulfil the needs of occupants and there is a potential to use novel daylighting devices to transmit light from the zenith of a sky dome to interior part of the buildings. And therefore, the performance of several daylighting systems was investigated. The first one was a vertical light-pipe system. It was studied through field measurements and its performance was promising. For nearly half of the operating hours, the internal illuminance design level can be achieved by daylight alone. However, vertical light pipes are usually installed in floors near the roof and reduces its usability in high-rise non-residential buildings. The second type of system being studied was the laser cut panel (LCP). Its installation is simple and redirects daylight into deeper parts of a room. LCP can improve the indoor daylight environment under most sky conditions. Both uniformity and the illuminance of the rear part of rooms have significant improvement. The performance of LCP is highly dependent on the internal layout of the room. Any obstruction which intercepts redirected daylight seriously affects the performance of LCPs. Subsequently, horizontal light pipe (HLP) seems to be able to remedy the situation. However, the HLP cannot transmit light from high elevations effectively, and hence, another system integrating the LCP with a HLP (LLP) is of interest. To investigate the performance of this system, experimental works were conducted to identify the transmission characteristics of this system. The LCP can reduce the number of reflection with light pipes and hence it can increase the overall transmittance for light from high elevations. Through a simulation study, it is found that for most sky conditions, the LLP have a larger transmittance than conventional the HLP. Annual performance of a daylighting system is an important parameter to assess a design. However, even though computational power is improving, conducting an annual assessment of daylighting systems is still not feasible. Fortunately, employing daylight coefficient concepts can greatly reduce the simulation time for long-term system evaluation. In this study, a two-step daylight coefficient simulation approach is proposed. Its performance was validated against measured data. And finally, a simple daylighting assessment tool was developed. Its major algorithms are discussed. It is believed that this study can provide a better understanding and more accurate design method of daylighting systems for architects and engineers.
- Daylighting, Office buildings, Lighting