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.