Concentrating characteristics of Fresnel lens with prism secondary concentrator and optimization of high concentrating photovoltaic module with triple-junction cell

Abstract

At present, Fresnel lens is commonly used as a concentrator in high concentrating photovoltaic (HCPV) module, and the triple-junction cell is currently one of the most common multi-junction cells used in it. The triple-junction cell is composed of three p-n junctions in series. Each sub-cell in it absorbs different-wavelength light. The solar cell efficiency of Ⅲ-V multi-junction high concentrating photovoltaic increases up to 46%, which the corresponding module efficiency is quite different from. The output power of the solar cell is related to not only the illumination energy, but also the spectral distribution and the uniformity of the illumination. The loss caused by the non-ideal concentration of the concentrator in the module is as high as 20%. After sunlight enters the lens, the direction of transmission of a monochromatic light is different, because a lens has different refractive index for different-frequency light. So the light disperses when leaving the lens, and thus the colors are arranged in a certain order to form a spectrum. Owing to the dispersion and the differences in refractive index among different spectral bands, the illumination distributions of the three spectral bands are different and non-uniform on the focal plane of lens. The divergence of light will obviously weaken the non-uniformity of the illumination on the solar cell surface. So the divergence angle of the light source has a greater influence on the cell performance because the non-uniformity of illumination has a negative influence on the performance of the cell.In this paper, according to the establishment of optical model and three-dimensional cell circuit network model under non-uniform illumination, taking Ⅲ-V triple-junction cells for example, we study the concentrating characteristics and photovoltaic characteristics of HCPV module with Fresnel lens concentrator and prism secondary concentrator. The results show that due to the non-parallel incident light and dispersion of the Fresnel lens, the concentrating spots of short-wave light, medium-wave light and long-wave light are divergent and their illuminations are non-uniform, resulting in the spectral response mismatch loss of the three sub-cells in the triple-junction cell, and the photovoltaic performance of the HCPV module also declines. The results show that the secondary optics element is obviously effective in reducing the non-uniformity of the illumination and the temperature which the Fresnel lens creates. However, each waveband of light has a different spot size at the same position, similar to the Fresnel lens without the secondary optics element. So the varieties of cell performance at different positions are similar too. And, by optimizing the focusing characteristics of the three wave bands along the optical axis, the power output of the HCPV module can increase more than 10%. The simulation results are verified experimentally.