# An Innovative Parabolic Trough Collector Design with a Twin-Cavity Receiver
## 3 Results and Discussion
### Thermal Analysis Results  
This section is dedicated to the results concerning the thermal investigation of the studied collectors. The thermal efficiency and the total thermal losses are presented in Figures 8 and 9, respectively, for the whole operating range (20 \({}^{\circ}\)C-200 \({}^{\circ}\)C) of this study. Figure 8 comes to prove the superiority of the PTC-TC against the PTC-F in thermal efficiency. More particularly, the mean enhancement gained from the use of the cavity configuration reached 8%, which is a remarkable percentage for this kind of application. The specific enhancement ranged from 5.25% up to 9.85%, achieving its lower values in high operating temperatures due to the increase of heat losses. According to Figure 9, thermal losses seemed to take reasonably higher values in the PTC-TC geometry, since the receiver surface in the latter was much greater than in the PTC-F.  
Figure 7: PTC-TC optimization results: optical efficiency values for different combinations of the parameters (\(\varphi\)) and (\(\omega\)). In Figures 10 and 11, the mean values of the receiver and the glass temperature are presented for the whole operating range. As Figure 10 shows, the receiver temperature seemed to be greater in the PTC-TC case for almost the whole operating range. This is reasonable considering that the receiver of the PTC-TC absorbed 10.14% more solar irradiation than the PTC-F receiver. It is interesting to comment that for the inlet temperature of around 160 \({}^{\circ}\)C, the temperature of the receiver for the PTC-F was about 7 K higher compared to the PTC-TC case. This result happened due to the earlier transition of the PTC-TC to the turbulent state compared to the PTC-F. According to Figure 11, the glass temperature level was higher in the PTC-TC, which is a reasonable result because the receiver absorbed more solar power and it also had a greater size than that of the PTC-F geometry. Figure 8: Thermal efficiency of the PTC-TC and PTC-F.  
Figure 9: Total thermal losses of the PTC-TC and PTC-F. Figure 12 illustrates the receiver temperature allocation for the operating point of T\({}_{\rm i}\) = 100 \({}^{\circ}\)C for each PTC. As Figure 12 shows, the receiver temperature fields were higher in the PTC-TC scenario than expected, since in this case, the solar absorption was greater. It is also critical to say that the temperature in both receivers had, reasonably, higher values in the region where the solar irradiation was concentrated in every case. This proves that the ray tracing simulation is very close to the real ray tracing procedure. Figure 11: Mean glass temperature comparison. Figure 10: Mean receiver temperature comparison. Figure 13 gives an illustration of the temperature fields of the working medium and the receiver at a transversal section of both collectors in the middle of them. It is clear, as seen in Figure 13, that the temperature of the working fluid was higher in the cavity receiver case, as was expected. The same happened with the temperature of solid bodies. Both occurred because the receiver in the cavity case was greater, and it absorbed more solar irradiation with a much greater equivalent solar absorbance. Figure 14 depicts the temperature distribution in the receiver and the working fluid in each collector through several transversal sections along each absorber. It became obvious that the temperature allocations were becoming higher going from the inlet to the outlet of the collectors. In addition, the PTC-TC seemed to prevail against the PTC-F, appearing in higher temperature fields in all the compared positions along the collectors. At this point, it is useful to discuss the findings of the present work. It is clear that both examined designs were efficient choices for operation up to 200 \({}^{\circ}\)C. The PTC with the twin-cavity was the best choice, and this fact indicates that the use of the twin-cavity is a promising choice for the design of future PTCs that operate at medium temperatures.