Optical analysis of flux uniformity and efficiency in low concentrating PV systems

Solar concentration has the potential to replace the expensive PV cells with cheaper optical elements and enhancing the overall output. Two main types of concentrators are used in CPV systems which are either reflective or refractive (Fresnel lens). One of the advantages can be achieved from using refractive concentrators is compact height which is not available in reflective concentrators especially in high concentration ratios. But the main problem with refractive concentrators is the hot spot generated at the centre of the PV cell. In this paper, different Fresnel lens parameters affecting the optical efficiency and flux distribution have been investigated, including groove width, draft angle, focal length, f/number, lens thickness and PV cell position beneath the Fresnel lens. Then, a Square Fresnel Lens (SFL) of Concentration Ratio (CR = 10) and design focal length (f = 145 mm), was compared with a reflective Square miniature Concentrator (SMC) with the same CR in terms of uniformity, optical efficiency and compactness. Results showed that optical efficiency increases with the decrease of groove width and draft angle. Simultaneously, optical efficiency is fixed with different lens concentration ratios of the same f / number, and decreases if the focal length is fixed to different concentration ratios. The best position of the PV cell for the square Fresnel lens of a focal length of 145 mm that gives good uniformity (with Standard Deviation = 3.38) is 185mm beneath the Fresnel lens with optical efficiency of 76.46% compared to the 79.47% achieved at the focal distance of 145mm but with point concentration. As for the reflective concentration with CR = 10, the optical efficiency achieved is 76.56% which is comparable to that of the Fresnel but with larger height of 230mm.