Daylighting by optical fiber

Even on a grey day with an overcast sky there is normally an excess of light outdoors in comparison with what is required at most work places indoors. This excess of light can be harvested, concentrated and distributed indoors by fiber optics to replace most of the electrical lighting that is used today.

A system suggested in this report, 1-axial turning troughs, is predicted to have an efficiency of between 33 and 16% in utilising the collected light. It tracks the sun merely for its altitude and not in the east-west direction. In doing so it could have an operation period of five hours each day with its peak efficiency at noon, if the system is south oriented.

This system is dependent on unblocked sunlight and would have to be combined with an alternative light source to provide continuous lighting. Provided clear weather a collector area of less than 5 m2 is predicted to be sufficient for an office of 100 m2 located in Copenhagen. With this location the system could deliver at least 500 lux of illumination five hours a day between the vernal and autumnal equinoxes, when the sun is visible. This level of illumination meets the recommendations for several situations.

The system would also include the possibility to produce hot water by utilising the infrared portion of the sunrays.

At least three fiber optic daylighting systems exist already. It is the Japanese Himawari, the German SOLUX and the American Hybrid Lighting. They are all 2-axial tracking systems that depend on sunlight. The first two utilises Fresnel lenses to concentrate the light and the third uses a reflecting parabola.

There is also a wide variety of other daylighting systems that are more or less actively light collecting and that utilises either sunlight or diffuse light from the sky.

There are several benefits of using daylight for lighting purposes, energy savings being one of them. Not only is electric lighting replaced, but also unwanted heating produced by this lighting is reduced. Correctly designed a daylighting system can both filter away unwanted heat in the light and supply heat to the building depending on the season.

Other benefits of daylighting include health advantages and psychological benefits that have been shown in studies. Some concrete examples are less absenteeism at work places and better performance by students in daylighted schools.

Author: Andre, Erik; Schade, Jutta

Source: LuleƄ University of Technology

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