RP-1415--Thermal and Lighting Performance Metrics of Tubular Daylighting Devices PDF

Tubular Daylighting Devices "TDD's " have emerged as alternative products to conventional skylights to deliver daylight without the unwanted solar heat gains, and cover areas not usually covered by windows and skylights. Nowadays, TDD's compete with conventional skylights, particularly in commercial and residential buildings. TDD's consist typically of three parts: a collector on the roof to gather sunbeam and sky diffuse light, a hollow pipe guide in the plenum/attic space to channel light downwards, and a light diffuser at ceiling level to spread light indoors. The collector is usually single or double transparent glass or plastic dome projecting above the plane of the roof. The collector may also include geometry or some optical devices to enhance the lighting output of the device, especially at low sun altitude angles. The pipe can be straight rigid, elbowed rigid or flexible, and is typically constructed from an aluminum sheet with highly reflective interior lining mirror. The pipe may be fully or partially insulated at ceiling or roof level to suit a particular building type. The diffuser is hemispheric or flat with multi-pane translucent "opal " or clear prismatic glazing. TDD technologies have been rapidly and continuously evolving to meet high standards of energy efficiency in buildings and a glare-free indoor environment. Energy-saving potential of TDD's is well recognized, and TDD's are encouraged or mandated, particularly in commercial buildings. However, due to this product's complexity and rapid technology development, prediction of the thermal and lighting performance metrics of TDD's has always been a difficult task. Building designers and engineers, and TDD manufacturers, lack reliable and accurate calculation methods and design tools that would allow them to predict the energy performance of installed TDD's, to show compliance with building energy codes, and to rate existing and/or innovative products.

This research project addressed the development of computational algorithms and metrics for the lighting and thermal performance of TDD's.