## 1 Introduction
The main idea of the design was to concentrate the incident solar radiation through Kohler integrator pairs divided in four or nine channels, and each channel consists of two optical elements, a Fresnel lens as the POE, and a free-form surface as the SOE. The design improved the uniformity of irradiance distribution on the solar cell. [PERSON] and [PERSON] [23] proposed the design of three types of SOE for Fresnel lens-based CPV units to achieve high optical efficiency and improve the irradiance uniformity. Some concentration systems have been also proposed for daylighting systems [23; 24; 25; 26]. [PERSON] and [PERSON] [24; 25] presented two light concentration systems to capture sunlight and then focus it over a small area. The first approach used two concave and convex parabolic reflectors: the concave parabolic reflector captured sunlight and directed the sunlight toward the convex parabolic reflector, which illuminates a bundle of optical fibers. In the second approach, a Fresnel lens was used to focus direct sunlight on a collimating lens, which illuminates a bundle of optical fibers. Furthermore, the authors presented the development of these two systems using a parabolic trough and a linear Fresnel lens [26]. [PERSON] and [PERSON] [27] proposed a combination of linear Fresnel lenses and a stepped thickness waveguide to concentrate the solar energy for a daylighting system. Additionally, CPV systems cannot capture diffuse solar radiation because of the narrow acceptance angle of the concentrators, so the use of the systems in regions of medium direct normal irradiation (DNI) is not cost effective [28]. In other words, the CPV systems are not suitable for medium DNI regions, such as Korea and Japan. [PERSON] et al. [29] proposed and invented a CPV system that usesauxiliary cells to collect diffuse solar radiation. The invention comprises a combination of two types of solar cells in a single module. The main cell is located at the focal spot of the concentrator and a low-cost secondary solar cell is added to the concentrator, surrounding the main cell. Direct solar radiation is concentrated upon the main cell, while diffuse solar radiation is collected by the low-cost secondary solar cell. This concept was implemented in [28; 30] using a prototype CPV module with an additional cell. In the implementation, a silicon solar cell was installed onto a CPV module. In this experiment, direct solar radiation is concentrated by a Fresnel lens onto a triple-junction solar cell, whereas diffuse solar radiation is collected by the additional solar cell. The experimental results showed that the electricity generated by the experimented CPV module with the additional crystalline silicon solar cell is greater than that for a conventional CPV module by an improvement factor of 1.44 when the mean ratio of diffuse normal irradiation to global normal irradiation is 0.4. In other words, the experimental results showed that the experimented CPV module provided a conversion efficiency increase of 44% compared to the conventional CPV module. Therefore, the CPV system with the additional solar cell improves the system efficiency compared to the conventional CPV systems, especially in medium DNI regions. However, there is no CPV system with irradiance uniformity that can capture diffuse solar radiation and no CPV system with diffuse solar radiation collection that uses an SOE to improve the irradiance uniformity. In other words, there is no study that integrates both the improvement of irradiance uniformity and the collection of diffuse solar radiation into a CPV system. In this paper, we propose a novel CPV system that combines both features of the abovementioned CPV systems, including the improvement of irradiation uniformity and the collection of diffuse solar radiation. The proposed system includes several CPV units and an additional low-cost solar cell. Each CPV unit consists of a Fresnel lens as the POE, a plano-concave lens as the SOE, and a multi-junction solar cell.