# Performance Prediction and Optimization of Multi-Mirror Combined Solar Dish Collector
## 3 Model Validation and Analysis Optimization
### Influence of Design Parameters on the Performance of Combined Dish System  
In this section, the influence of design parameters is evaluated, including the effect of system focal length, receiver size, specular focal length, and optical errors on the performance of combined solar dish system. From Figure 5, it can be seen that the system intercept factor first increased with the increase in the focal length of the system and then decreased under the same optical error. This means that, when the receiver size is fixed, a system focal length that is too large or too small will result in a small interceptor. Because a large focal length reduces the maximum angle \(\theta\) at which the reflected ray is intercepted by the receiver, a small focal length will increase the incident angle of the solar ray to the receiver, which will also decrease the interceptor. Taking the optical error of 1 mrad as an example, when the system focal length was 4 m, the intercept factor of the system was 94.26%. When the system focal length increased to 6.5 m, the intercept factor reached a maximum of 99.30%, and then it gradually decreased with the increase in system focal length. Furthermore, as the optical error increased, the reflected light intensity was diffused and weakened, and the radiation intensity intercepted by the receiver was significantly reduced. Therefore, the system intercept factor at the same focal length decreased significantly with the optical error, and the fluctuation range affected by the focal length became larger. When the optical error was 5 mrad, the system intercept factor increased obviously from 87.55%, corresponding to 4 m focal length, to 92.82%, corresponding to 5.5 m focal length, and then decreased rapidly to 74.84%, corresponding to 10 m focal length; the fluctuation range was obviously larger than the case with a smaller optical error. Therefore, for the combined dish system, when the optical error was greater than 2 mrad, under the same receiver radius, the influence of the system focal length on optical performance was more obvious. The optimal focal length of the system under different optical errors was also different; thus, choosing a suitable focal length of the system is especially important. Figure 5: Variation of the intercept factor of the system with the focal length of the system under 1-5 mrad optical error, when the radius of the receiver is 0.1 m.  
From Figure 6, when the optical error was 2 mrad, the system intercept factor gradually increased with the increase in receiver radius. When the receiver radius was greater than 0.1 m, the system interceptor was close to 100%. When the receiver radius was 0.15 m, the intercept factor of combined dish system at different system focal lengths (6 m, 7 m, 8 m, 9 m, and 10 m) could reach 99.69%, 99.62%, 99.54%, 99.46%, and 99.38%, respectively. When the receiver radius was less than 0.08 m or greater than 0.1 m, the intercept factor was lower for the larger system focal length with the same receiver radius. However, when the receiver radius was between 0.08 and 0.1 m, the intercept factor of the combined system with the system focal length of 7 m was the largest under the same receiver radius. In the condition that the system intercept factor was equal, there were multiple sets of different receiver radii and system focal length parameter values. Therefore, these two factors should be considered at the same time when designing the optical system. Figure 7 shows how the intercept factor of the combined dish system varied with the receiver radius under different optical errors (1-5 mrad). It can be seen that the system intercept factor decreased with the increase in optical error at the same receiver size. Obviously, a greater optical error led to a smaller change in the intercept factor with the same radius of the receiver. Therefore, when the optical error is small, the system intercept factor is obviously affected by the receiver radius.