Efficient Dual-Function Solar/Fluorescent Light Guideto Enable Cost-Effective Core Daylighting

摘要

As a component of our cost-effective core daylighting research program, we have for the first time developed a hollow light guide that can be used to distribute sunlight collected by a heliostat device, and can also operate as an efficient fluorescent light fixture when the sunlight is insufficient to illuminate the workspace. This dual-function prism light guide structure has a rectangular cross-section that is similar to standard commercial fluorescent light fixtures, but with an increased depth in order to pipe the sunlight. The bottom surface of the guide is lined with a thin polycarbonate film with prismatic surface features, while the top and sides are lined with a highly-reflective multi-layer optical film having a luminous reflectance greater than 98%. The angular constraints of the sunlight that is directed into the guide by the external concentration system, combined with the high reflectance of the surfaces, enables sunlight to be efficiently transported along the 10 m length of the guide. As required, the guide can also illuminate the room using fluorescent lamps, since the light from the lamps strikes the prismatic bottom surface at angles for which it is an efficient transmitter. Photometric characterization of the new dual-function guide has been performed, and the result is a demonstrated illuminance, uniformity and efficiency well within recommended standards. The dual function design approach has yielded a daylighting system that can be cost-effectively incorporated into standard office building construction.