Displacing electric lighting with optical daylighting systems
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abstract
This paper presents the findings of a study that evaluates the overall daylight performance of two optical daylighting systems in two large physical scale models representing a multistory deep office building (30ft) with open-plan configuration. These two optical systems are: a passive optical light shelf, and a light pipe. The systems were designed to introduce daylight at the back of deep-plan spaces (15-30ft) using optimized geometries and highly reflective materials. The systems were designed for a south-facing faade located in a hot climate with predominantly sunny and clear sky conditions at latitude 30N. The main objectives of this experimental work are: to compare these daylighting systems with a reference case (typical office layout with partitions, blinds and shading devices); to take illuminance measurements and collecting detailed data under different sky conditions for extended periods of time; to analyze these data using the following metrics: Daylight Autonomy (DA), Useful Daylight Illuminance (UDI), Illuminance Contrast Gradient (ICG), High Dynamic Range (HDR) photographic evaluation, Unified Glare Rating (UGR) Coefficient of Variation (CV) of luminance, and Luminance Ratios (LR). Results have shown that both daylighting systems achieved a higher daylight performance than the reference case in terms of adequate light levels for office tasks, uniform light distribution throughout the room, and a visually comfortable space for occupants. The light pipe system had demonstrated to be an effective system that provides high illuminance levels, 300-1,500lux at the back of the space (15-30ft) for more than 7 hours (between 8:30am to 4:30pm) under clear skies. The light shelf increases the illuminance level along the space by about 190lux, as well as the uniformity across the space. The combined workplane illuminance values measured on clear days (500lux) are satisfactory without the need to turn on the electric lights, for more than 6 hours between 9:00am and 4:00pm. When the two daylighting systems are integrated in the space, their lighting performance is extremely well mostly under sunny skies; and under overcast skies they provide useful illuminance levels, 1% of EXHG (exterior horizontal global illuminance) at the back of the space. As for light uniformity and visual comfort, the two integrated systems demonstrated to have lower ICG, LR, and lower CV than the reference case, and with UGR values within the recommended lighting standards.
