Internal Lighting by Solar Collectors and Optical Fibres

Abstract

Sunlight concentration on small surfaces is widely studied [1-3], experimented and mostly applied to photovoltaic power generation [4-6]. More rarely these solar collectors are coupled to optical fibres [7-9], with the advantage of always having a circular absorber shape. On the contrary the photovoltaic (PV) cell is typically squared and therefore it requires a secondary optical system to reshape the image and to improve the light distribution uniformity. The introduction of optical concentrators, especially high concentration systems, has two positive effects: it reduces the area of expensive solar cells and it increases their efficiency. The main reasons for this development are enhanced efficiency of CPV (concentrating photovoltaic) systems due to new solar cells, improved size of PV installations and increasing interest in alternative technologies, both due to government incentives and to the poor Silicon availability. In general it can be assumed that an improvement in the volume of the collection system reduces the costs, given that the system provides a higher production of energy. This chapter presents optical systems exploiting the sunlight using optical collectors and fibres to illuminate building interiors. In particular Sect. 5 describes in details a solar plant demonstrator installed to provide illumination of museum showcases [10]. This daytime lighting system was developed from design to production, installation and testing in working conditions. The light focused by the solar collector can be used either for direct illumination or to accumulate power [11] for lighting at times when there is no sunlight. The first function is obtained coupling an optical fibre to a solar collector. The second consists in focusing the solar light on a PV cell, which converts the light into electrical energy. This function has been suggested by the long closing times, typical of the museum. Due to the fact that the internal illumination was not required for hours or entire days (museum closure day), during these periods the PV cells can exploit the solar light. A solar collector with optimised features and collection performance was specifically designed for mass production to reduce costs. The evolution of solar concentrators for fibre coupling [12-13] is discussed in Sections 2-3, with theoretical and experimental comparisons based on optical tests [14] performed on the realised samples. The field tests, with direct exposition to the sun, required to design and built suitable mechanical systems to support and move the concentrators: examples of tracking systems [15-16] are reported in Sect. 4.