Optical design of a Fresnel concentrating solar system for direct transmission of radiation through an optical fibre bundle

• Optical fibre bundle was used for direct transmission of concentrated solar radiation.
• We analysed the optical performance of a Fresnel lens with an optical fibre bundle.
• We conducted ray tracing modelling and direct measurement on the optics of the system.
• Two thirds of the loss is due to the absorption in the gaps between the individual fibres.

A challenge in high-temperature solar thermal applications is efficiently transferring the concentrated solar energy to the load. The use of a solar concentrator in conjunction with bundled optical fibres has advantages in terms of transmission efficiency compared to a conventional heat transfer system employing heat exchangers and a heat transfer fluid. In this paper, the transmission from a solar concentrator into and through an optical fibre bundle over distances of up to 100 m is estimated using simulations and experiments. A point focus Fresnel lens is employed to produce a concentrated beam with a smaller incident angle than the acceptance angle of the individual optical fibres in a bundle. Since the sunlight has a broad band spectrum and a very limited spatial coherence, interference effects can be neglected. Hence a ray tracing model covering absorption within the fibres and losses due to non-ideal internal reflections is employed. The intensity of the radiation across the exit plane of the fibre is found by integrating across all directions and wavelengths at each point. For a 15 mm diameter bundle comprising some 27,000 individual low OH fibres, the average transmission per metre length of fibre is found to be 52% with the Fresnel lens. Around two thirds of the loss occurred at the entrance plane to the bundle, due to absorption in the packing material between the individual fibres.