Investigation of adjustable lightshelf for daylighting classrooms considering different sky conditions of Dhaka

Abstract

In a classroom the students near the windows may get enough daylight for their learning activities but individuals far from the windows often do not get sufficient daylight. Most of the schools experience the same problem in dense Dhaka city. Increasing the size of windows to penetrate daylight into the deep of the classrooms can create glare problem near the windows. Windows are usually shut down to avoid glare near the windows and depend on the artificial light for visual works, which increase the load of electrical energy. The situation also varies time to time for the different sky conditions and seasons. Light shelf can be an effective solution in this condition as light shelf is supposed to reflect daylight to the inner part of a room. Studies show, in a tropical location, such as Bangladesh, the introduction of light shelf at any height produces an overall reduction of illumination on the work plane throughout the interior space. However, light shelf can be an effective element to enhance the quality of daylight in tropical buildings, if designed and located properly. The objective of this research is to study the impact of the width of light shelf on indoor day lighting quality considering different seasons and sky conditions in context of Dhaka, at the same time consider the performance of adjustable light shelf for better results. Three case classrooms of different depths were selected after field survey. One is prototype Govt. primary school classroom designed by Local Government Engineering Department (LGED). Another is a high school classroom and third one is a university classroom. 3D Modeling was done by ECOTECT software and daylight simulation was performed by a dynamic annual Climate-Based Daylight Modeling (CBDM) method considering all weather sky luminance model (i.e. DAYSIM). As in Dhaka the sky condition differs in seasons it has been tried to get the effective width of light shelf through computer simulation study in four different dates of a year: 20 March, 21 June, 22 September and 21 December; which are the equinox and solstice dates for Dhaka. Primary analysis shows 300mm wide light shelf is effective to increase illuminance level at deeper parts of the classroom throughout the year in context of Dhaka. Due to different conditions of sky and seasons of a tropical city, a fixed width does not perform effectively throughout the year. Detail study shows that it need different width in different times. For February, March and April 450 mm wide external light shelf works skillfully. For May, June and July 300 mm wide external light shelf works best. For August, September and October 600 mm wide external light shelf can work effectively. For November, December and January 750 mm is effective. An adjustable light shelf is feasible as it not only fulfill the need all over the year but also can serve in a day if necessary as illuminace varies in a day in different times. It is expected that the outcome of this research will help architects and designers to generate some guidelines for light shelf configurations to improve the luminous environment of school buildings in Dhaka as well as in the other cities of Bangladesh.