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.
To design a fiber optic daylighting system several aspects have to be
considered. The collected light has to be concentrated to pass through the
aperture made up by the fiber end. The fiber will only accept and transmit
light within its acceptance angle, which can range from less than 20° and
up to over 80°. This makes it desirable to utilise a high power light
source supplying light with uniform direction. The sun is such a light
source, the sunrays incident on the earth are close to parallel. However,
the availability of sunlight is unpredictable in most climates: the sun can
disappear behind clouds for a second or for days....
