Visual and energy performance of lightshelf daylighting systems for office buildings in a hot and arid climate

Abstract

The purpose of the research is to investigate how lightshelf daylighting delivery systems can manipulate sunlight and daylight both in terms of its light and heat by shading view apertures below the lightshelf to reduce solar heat gain and glare and by reflecting fight deep into the space through clerestory apertures above the lightshelf. It also investigates how to provide view with good interior lighting of deep daylight penetration, uniform distribution, and acceptable Daylight Glare Index (DGI) besides good energy performance of reduced electric lighting and cooling loads, and shaved peak loads for multi-story office buildings located in hot and climates. The procedure of this study is based on, first, daylighting evaluation in a daylighting laboratory using physical scale-models to compare 15 different alternatives of shading and daylighting systems applied to south fenestration of office space and to determine an optimum lightshelf depth. They consisted of a base, overhangs (3), internal lightshelves (3), external lightshelves (3) and combined lightshelves (9). Second, an energy evaluation of the lightshelf system with optimum lightshelf depth was conducted using a, developed computer software based on the daylighting evaluation data. Field measurement experiments were conducted in El Paso, Texas which confined the reliability of the laboratory daylighting evaluation. Results of the daylighting evaluation showed that the best lightshelf system in terms of deep daylight penetration (target illuminance= 46.5 fc), uniform illuminance distribution (target range= 46.5 to 140 fc) and control of glare discomfort (target DGI<-20) is the combined type, and its optimum lightshelf depth is twice the view aperture height for external lightshelf and twice the clerestory aperture height for internal lightshelf. The energy evaluations are performed on the optimum combined lightshelf, overhang and base cases for internal and external load-dominated buildings. Results showed that optimum lightshelf reduced up to 38% of total annual energy consumption, 42% of lighting load, 41% of cooling load, 34% of peak load demand and 20% of present worth of both first cost and operating and maintenance costs as compared with the base case. Finally, the study concludes with design guidelines of the lightshelf daylighting systems.