A concentrated photovoltaic (CPV) unit is generally composed of a primary optical element (POE), a secondary optical element (SOE) and a solar cell. The function of the POE is to concentrate a large-area solar energy into a small III-V solar cell. Furthermore, the SOE is applied to redirect the sun light into the solar cell and to distribute the energy uniformly on the solar cell [1
]. The most common type of refractive POE is a flat Fresnel lens. In addition, there are two major materials for Fresnel lens, i.e.
, PMMA and Silicone-on-Glass, and the latter is believed to have a better outdoor life and reliability under solar radiation and humidity [4
If only a POE is applied in a photovoltaic system, nonuniformity of the concentrated distribution on the solar cell surface is observed, resulting in localized hot spot or even damage of the solar cell [1
]. Localized hot spots and poor uniformity will reduce efficiency of the photovoltaic system and service life of the solar cell. Keeping the power production of a photovoltaic system in real operation conditions under influences of wind and vibration is important. Therefore, precise orientation to the sun of the photovoltaic system is required. One of the ways for lowering the effect of the inaccurate orientation on the concentrator system power efficiency and for improving the irradiation uniformity on the solar cell surface is the application of the SOE located before the solar cell [8
Secondary optical elements are specular or refracting optical elements of various forms. The parameters of SOE used in a CPV unit are usually tailored according to the design parameters of the POE, performance requirements and the size of the solar cell [8
]. Using a SOE with a Fresnel lens in a CPV unit will achieve high optical efficiency, lower the sensitivity to the sun tracking error, and improve the uniformity of irradiance distribution on a solar cell [8
The three major factors used in evaluating a CPV system include the optical efficiency, the acceptance angle and the uniformity of irradiance distribution on the solar cell. The optical efficiency of a photovoltaic system is defined as the ratio of the radiant flux on the solar cell and the POE surface. The acceptance angle θ90% is commonly defined as the incidence angle corresponding to 90% of the maximum optical efficiency at normal incidence.
In other words, the optical characteristic of a CPV unit is highly affected by the design of the SOE. Therefore, an optimum SOE tailored for a specific CPV unit is essential to achieve the highest optical performance. Andree et al.
proposed optimization of SOE for CPV unit with a flat Fresnel lens [8
]. Recently, Zhuang and Yu applied simplex algorithm to attain optimum prism displacements of a Fresnel lens, leading to a uniformity irradiance on the solar cell with broad solar spectrum [11
]. Buljan et al.
achieved high uniformity and chromatic balance for CPV system by using a dielectric solid RXI-type SOE in a CPV system comprising a flat Fresnel lens [12
]. Lv et al.
analyzed the performance of a CPV module considering the temperature-dependent model of the optical elements and the solar cell. [13
In this study, the size of the square Fresnel lens is 40 mm × 40 mm, the size the solar cell is 1.25 mm × 1.25 mm, and the theoretical geometric concentration ratio is 1024. The square solar cell is placed on the focal plane of the POE, of which the focal length is 130.1 mm. The Fresnel lens is made by silicone-on-glass with urethane prism grooves of pitch 0.2 mm on a 2 mm thick flat BK7 glass, and the internal transmittance of BK7 is assumed as 0.99 in the simulation [12
]. The commercial package, ASAP (Breault Research Organization, Inc., Tucson, AZ, USA), was applied to perform the optical simulations in this research.
In addition, light source with a divergence angle of 0.265 degrees and AM1.5D were assumed in the simulation. The Fresnel loss occurring at all interfaces/surfaces was also considered in the ray-tracing process. In the preliminary design steps of SOE, a single wavelength of 550 nm of solar spectrum was considered to determine the basic optical characteristic of the CPV unit with various SOE designs. The indexes of refraction of BK7 and urethane at 550 nm are 1.518 and 1.510, respectively [14
]. Finally, the optical characteristics of the CPV unit with the optimum SOE were evaluated in broadband solar spectrum in the range of 400–1300 nm.
This study presented the optimization process of tailoring an SOE for a silicone-on-glass Fresnel lens in a CPV unit based on single-wavelength simulation. The goal of the optimization was to achieve high values of optical efficiency and acceptance angles. The irradiance uniformity on the solar cell was also evaluated for the CPV unit with each type of SOE. Firstly, three types of SOE, KOD, KFTS and SP, were considered, and the optical characteristics of each type of SOE were evaluated by ray-tracing simulation under single wavelength of 550 nm. For each type of SOE, preliminary design and optimum design were determined and the resulting optical performances were simulated. Finally, a best KOD II-type SOE was achieved, which exhibits a highest acceptance angle of 1.7°, as well as a good optical efficiency. Nevertheless, the SP II-type SOE exhibits a best irradiance distribution on the solar cell.
Moreover, the simulation results of the CPV unit with the best KOD II-type SOE under broadband solar spectrum were also performed and the results were compared with those from single wavelength simulation. The acceptance angles were the same (1.7°), and the optical efficiency under normal incidence from broadband simulation (87%) was higher than that from single wavelength simulation (85%). Meanwhile, the irradiance uniformity under broadband solar spectrum simulation was not as good as that under single wavelength simulation. Therefore, the irradiance uniformity on the solar cell under broadband simulation can be considered along with optical efficiency and acceptance angle in the future optimization process of SOE.