Nowadays, consumption of renewable energies such as solar energies is increasing. Electricity in solar systems is produced using either photovoltaics or concentrated solar power (CSP) systems. Therefore, concentrating technologies are key parts of CSP systems. The lower cost of linear Fresnel collectors makes them attractive for electricity production and thermal energy applications. However, due to partly losing the reflected rays, their optical efficiency is low. By installing a secondary reflector above the absorber tube of Fresnel collectors, lost rays are mostly redirected toward the absorber tube and a higher optical efficiency is achieved. The geometry and configuration of secondary reflectors have an intense influence on the optical efficiency. Therefore, there is a need to determine the most appropriate shape and configuration of secondary reflectors. In this study, four different types and orientations of secondary reflectors including circular, flat, few segmented, and parabolic were investigated and compared optically. All the geometric parameters were optimized for each case individually. To calculate the solar flux intensity and uniformity on the absorber tube, a ray-tracing method was implemented using a mathematical code that was developed based on MATLAB software. All the secondary reflectors were simulated in the code, with variable geometrical parameters. A wide range of geometrical parameters were investigated to obtain the most efficient geometry of every secondary reflector design in flux intensity and flux uniformity in comparison with the other cases. The results showed that few flat segmented collectors increase the flux to 87.9%. In addition, the highest uniformity of solar flux was obtained by using the circular and parabolic secondary reflectors. Considering the manufacturing cost as an important parameter, the flat reflector was the favorable one from the flux quantity point of view. Finally, if all the three criteria, i.e., flux quantity, its distribution, and manufacturing cost, were essential to be considered, the flat reflector might be chosen as the most appropriate secondary reflector.

中文翻译：

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二次反射器对菲涅耳聚光器太阳光通量强度和均匀性的影响

如今，太阳能等可再生能源的消耗量正在增加。太阳能系统中的电力是使用光伏或聚光太阳能 (CSP) 系统产生的。因此，聚光技术是CSP系统的关键部分。线性菲涅耳集热器的较低成本使其对电力生产和热能应用具有吸引力。然而，由于部分损失了反射光线，它们的光学效率较低。通过在菲涅耳收集器的吸收管上方安装二次反射器，损失的光线大部分被重定向到吸收管，并实现更高的光学效率。二次反射器的几何形状和配置对光学效率有很大影响。所以，需要确定次级反射器的最合适的形状和配置。在这项研究中，对四种不同类型和方向的二次反射器进行了研究和光学比较，包括圆形、平面、少量分段和抛物线。所有几何参数都针对每种情况单独优化。为了计算吸收管上的太阳通量强度和均匀性，使用基于 MATLAB 软件开发的数学代码实现了光线追踪方法。所有二次反射器都在代码中模拟，具有可变的几何参数。研究了广泛的几何参数，以获得与其他情况相比在通量强度和通量均匀性方面的每个二级反射器设计的最有效几何形状。结果表明，很少有扁平分段集热器将通量提高到 87.9%。此外，通过使用圆形和抛物面二次反射器获得了最高的太阳通量均匀性。考虑到制造成本是一个重要参数，从通量的角度来看，平面反射器是有利的。最后，如果必须考虑所有三个标准，即通量数量、其分布和制造成本，则可以选择平面反射器作为最合适的二次反射器。从通量的角度来看，平面反射器是有利的。最后，如果必须考虑所有三个标准，即通量数量、其分布和制造成本，则可以选择平面反射器作为最合适的二次反射器。从通量的角度来看，平面反射器是有利的。最后，如果必须考虑所有三个标准，即通量数量、其分布和制造成本，则可以选择平面反射器作为最合适的二次反射器。