# Performance Prediction and Optimization of Multi-Mirror Combined Solar Dish Collector
## 5 Conclusions  
This work presented a new optical model that can be used to analyze and optimize the optical performance of a combined solar dish system. Using the calculation method for direct efficiency of a reflecting point we previously proposed [28], the optical efficiency of a single mirror in the combined dish system is first calculated, and then the optical performance of the entire dish system is obtained through integrating the whole mirror. It is the first model to directly calculate the optical efficiency and the first optimizing method of a combined solar dish system. Compared with the results calculated by SolTrace, the proposed method showed good consistency. Moreover, using the Intel i7-6700 CPU 3.40 GHz computer as an example, only one single processor was used to calculate the intercept factor of a 3 m \(\times\) 3 m combined dish system when the receiver radius was 0.1 m. The proposed method took 4.3 s, whereas SolTrace needed 142.1 s. Therefore, this indicates that the results calculated using the proposed method are reliable, and the calculation speed is faster. In addition, optimization design requires simultaneously considering multiple impact factors; thus,using the proposed method is more computationally advantageous. Therefore, the optical model proposed in this paper is effective to study the optical performance of a combined dish system. Taking a combined solar dish system of 62.25 m\({}^{2}\) as an example, using this method, the optical performance of a multi-mirror combined solar dish system was analyzed with the change in system focal length, receiver radius, and optical error. The results can be summarized as follows:  
1. The intercept factor of the combined dish system decreased with the increase in optical error. Comparing to a typical single parabolic dish system, a lower optical error of the combined dish system with a spherical mirror can be achieved at the same cost, and its performance is better than the ordinary solar dish as the spherical mirror is more easily manufactured than the parabolic mirror under the ordinary manufacture process. 2. A better performance system can be achieved when each spherical mirror focal length is equal to the distance from the mirror center to the receiver center. However, considering the cost, a uniform focal length may be applied for the entire mirror system. The optimal results show that their performance difference is very small. 3. The system intercept factor increased first with increasing system focal length and then decreased. The optimal system focal length can be calculated for the optimized design. A larger optical error results in the intercept factor of the system being more significantly affected by the changes in system focal length. 4. The intercept factor gradually increased as the radius of the receiver increased. When the receiver radius was greater than 0.1 m, the intercept factor was close to 100%. However, a larger receiver will lose more heat, thereby decreasing the net efficiency of the system; hence, an optimized receiver is recommended. Using the proposed method, the system design parameters including receiver radius and system focal length were optimized to obtain the optimal net thermal efficiency of the combined dish system. Without considering the cost, the focal length of each mirror at different positions was further optimized. When the optical error was 2 mrad, the net thermal efficiency of the system reached a maximum of 86.05% when the radius of the receiver was 0.1 m and the focal length of the system was 7 m. At this time, the optimal system intercept factor was 98.81%. If the mirrors used had the same focal length, thereby reducing the cost, the optimal fixed mirror focal length was 8 m, and the net thermal efficiency could reach 85.87%. The optimal design parameters under different optical error conditions were calculated, which has a certain significance for the establishment of a combined solar dish system in the future. Conceptualization, W.H. ; methodology, W.H., C.Z. ; software, L.S.