At present, the Fresnel lens are commonly used as the condenser in high-concentrating photovoltaic (HCPV) modules. It is ideally believed that the output power of a III-V triple-junction solar cell which is placed on the focal plane of a Fresnel lens is the largest, because the intensity of the sunlight on the focal plane is the largest. Actually, according to our work, the dispersion of sunlight through a Fresnel lens and the nonparallelism and divergence of the incident light will lead to changes in the spectrum and the homogeneity of illumination, and cause a drop of the solar cell output. In this paper, the influence of the dispersion and nonparallel incidence of the light on the output of a triple-junction solar cell at different positions near the focal plane were theoretically studied, combined with the light-tracing simulation method and triple-junction solar cell circuit network model. The results show that the III-V triple-junction solar cell has the highest output power in both sides of the focal plane positions. The output power can be increased by about 15% after being optimized. The simulation results were verified by the experiments.

The cells in high concentrated photovoltaic module are usually high efficiency triple-junction solar cells. Due to the non-ideal concentrators, the light intensity distribution on a solar cell is highly non-uniform, so the appropriate increase of the ratio between light spot size and cell area is a method to reduce the influence of non-uniform illumination on the electrical performance of the solar cell. The circuit network model is used to calculate the influences of light spot intensity distribution and size on a triple-junction solar cell. The light spot intensities and sizes, the cell efficiencies, and the temperature distributions of the cell under four design schemes (uniform illumination, non-uniform illumination, maximum cell efficiency, and maximum module efficiency) are compared. The results show that the cell efficiency in the maximum module efficiency design is not the maximum cell efficiency under the standard testing condition. The design to make the cell achieve the maximum efficiency obtains the minimum module efficiency. The design to achieve maximum module efficiency has a smaller size of concentrator, so the cost of the module goes up. The design to achieve the maximum cell efficiency has a bigger size of concentrator and a lowest cell temperature, so the cost of the module will reduce and the reliability will improve. Above all, the requirement of electricity quantity should be fully considered in the module design, in which an appropriate geometric concentration ratio and light spot coverage to solar cells should be chosen.

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High concentrating photovoltaic (HCPV) technology plays a more and more important role in solar power generation due to its extremely high efficiency. However, the efficiency of the HCPV module can be reduced by many factors. Especially, there are not enough researches and knowledge on the light intensity distribution and non-uniform illumination of different wavelengths of light concentrated by Fresnel lens. It is generally considered that the maximum power of multi-junction solar cell is achieved when the cell is placed on the focal plane of Fresnel lens. But it is proved to be incorrect by our research. When light beams of different wavelengths go through the Fresnel lens, their light spot distributions on the optical axis are not the same as those when they have different refractive indexes in Fresnel lens. At the same time, the triple-junction solar cell consists of three sub-cells which absorb light beams of different wavelengths respectively. Therefore, the performance of triple-junction cells would be influenced by the light distribution along the optical axis, this is exactly what we want to study in this work. The method of simulating the light tracing is used to calculate and analyze the light intensity distribution and non-uniform characteristics of different wavelengths of light concentrated by Fresnel lens. Combined with them from the circuit network model of a triple-junction solar cell, the electrical performances of triple-junction solar cell at different positions along the optical axis are studied. It is found from the simulation that the performance of cell does not reach the best state when cell is placed on the focal plane. The power of cell on the focal plane reaches only 0.41 W while the maximum point arrives at 0.69 W. The high non-uniformity of light on cell surface when cell is placed on the focal plane causes the decline of power. And an outdoor HCPV testing system with the ability to change the distance between Fresnel lens and the cell is conducted. The experimental results and the simulation results match well, therefore our simulation approach is verified. It shows that the module achieves the maximum power on either side of the focal plane, and the output power can increase more than 20% after optimization. It is a result after equilibrium between light intensity and uniformity on cell surface.