Optical fiber daylighting: sizing the concentrating collector

The many advantages of daylighting, both in terms of human health and comfort ([1], [2] and [3]), and in term of energy saving and reduced environmental impact, are well known. A hybrid lighting system based on the integration of daylighting and artificial lighting is currently under investigation at the University Roma Tre, in Italy. Such a system is intended to provide adequate and almost constant lighting to a laboratory without windows located at ground level in a three-stories building, collecting solar radiation by means of concentrating parabolic dish collectors located on the roof, and transporting the light to the laboratory by means of a bundle of optical fibers. This system is composed of a sun-tracking parabolic dish collector (primary parabolic collector, or PPC), coated with a highly reflective film, which reflects the solar radiation to a secondary collector (SOE), whose surface is highly reflective in the visible range, while it is transparent in the near infra-red range (cold mirror), thus allowing the collection of this fraction by PV elements. The radiation reflected by the secondary collector, now reduced to the visible part, reaches the heads of the bundle of optical fibers. From there, after traveling through the fibers for approximately 10 meters, the light reaches tubular diffusing elements in the luminaries. Using theory and experience in the field of telecommunication antennae, a new Matlab procedure is presented, which allows the definition of the optimal collecting surface profile and of the optimal position of the heads of the optical fibers, and the evaluation of the effective collecting area, taking into account mutual shading between the different elements. This, in turns, together with the knowledge of local climatic conditions, enables the correct sizing of the collectors as a function of the required light output.