Coupling of sunlight into optical fibres and spectral dependence for solar energy applications

• A novel two-stage solar concentrator and optical fibre is analysed.
• Solar concentration is exceptionally obtained as a function of wavelength.
• Higher focal ratios and narrower acceptance angles improve the coupling efficiency.
• Maximum concentration ratio of 2000 suns at the end of a single fibre is attained.
• Spectral optimisation is shown by controlling the chromatic aberration.

Optical fibres can add interesting possibilities in solar concentrator systems, such as transport of light for remote illumination as well as solar energy conversion. In order to effectively couple light from the sun into optical fibres, the key parameters that control the coupling efficiency should be identified. In this paper, the results of ray-tracing simulations of a novel two-stage solar concentrator and optical fibre are compared to experimental measurements. In particular, the coupling efficiency is optimised by analysing focal ratio and acceptance angle of the primary and secondary concentrators respectively, while solar concentration is exceptionally obtained as a function of wavelength to examine the spectral dependence of the system. The trend towards narrower acceptance angles resulted in improvement of the coupling efficiency and a maximum concentration ratio of 2000 suns (1 sun = 1 KW/m2) at the end of a single fibre, spectrally adjusted by controlling the chromatic aberration of the primary concentrator. Any mismatch between the primary and secondary concentrators significantly reduces the coupling efficiency, especially the angle of the secondary concentrator, which should be kept below the 50% of the numerical aperture of the fibre for maximum performance. By optimising these parameters and achieving efficient coupling, utilisation of solar energy applications via optical fibres, can be brought one step closer to useful commercial applications.