## 3 Results and Discussions
### Energy and Exergy Performance of NHCEs  
The energy and exergy performances of NHCEs with the HTF of solar molten salt are calculated in this study. The DNI, ambient temperature, and mass flow rate are 800 W/m\({}^{2}\), 20.0 \({}^{\circ}\)C, and 3.0 kg/s, respectively, and the inlet temperature of the PTC system is assigned from 300 to 580 \({}^{\circ}\)C. The energetic efficiencies of the PTC system with different HCEs are exhibited in Figure 7, where it is observed that the NHCE-IRR-II and NHCE-IRR-III possess superior performance compared with the CHCE, and the former is the best. Additionally, the gap between the energetic efficiency of the NHCE-IRR-II and that of the NHCE-IRR-III widens with the increasing inlet temperature. The reason for this phenomenon is that the effectiveness of IR-reflector of NHCE-IRR-II to block the heat loss is reduced at high absorber temperature; by contrast, the effectiveness of IR-reflector of NHCE-IRR-III is strongly maintained because its adaptive spectral selectivity characteristic by shifting the cutoff wavelength greatly contributes to the optimal reduction of heat loss. In contrast to the NHCE-IRR-II and NHCE-IRR-III, the energetic efficiency of the NHCE-IRR-I is lower than that of the CHCE at the inlet temperature below 350 \({}^{\circ}\)C. This is because, though the TCO film in NHCE-IRR-I can intercept a partial amount of heat loss, a certain amount of solar irradiance is blocked by the TCO film, and this part of solar irradiance loss would surpass the amount of heat loss intercepted by the glass envelope with TCO film at lower operating temperatures or higher solar irradiance. In the case of the solar irradiance of 800 W/m\({}^{2}\), the solar irradiance loss is larger than the blocked heat loss until the inlet temperature reaches approximately 350 \({}^{\circ}\)C, at which the energetic efficiency of NHCE-IRR-I is same with that of CHCE. Furthermore, with higher inlet temperature, the energetic efficiency of NHCE-IRR-I gets much superior compared with CHCE. As shown in Figure 8, the percentages of energetic efficiency enhancement (\(P_{\eta}\)) of three NHCEs obviously grow with the increasing operating temperature; the percentage values in NHCE-IRR-I, NHCE-IRR-II, and NHCE-IRR-III reach 6.5%, 12.2%, and 25.7% at the inlet temperature of 580 \({}^{\circ}\)C.  
Figure 7: Energetic efficiency of PTC system with different HCEs versus inlet temperature. Exergy is an important indicator to evaluate the maximum useful work possible during a process of heat collection of the PTC system. The exergetic efficiency of the PTC system versus different inlet temperatures is exhibited in Figure 9. As similar to the energy efficiency analyzed above, NHCE-IRR-III achieves the best performance among three kinds of NHCEs. The NHCE-IRR-II is inferior to NHCE-IRR-III, and the growing gap between the exergy efficiency of NHCE-IRR-III and that of NHCE-IRR-II appears at higher operating temperatures. This demonstrates that IRR-III achieves an exceptional ability in improving the useful work possible for the PTC system. For the NHCE-IRR-I, a reduced exergy efficiency compared with the CHCE occurs at lower inlet temperature, then its exergy efficiency is gradually enhanced and eventually surpasses that of the CHCE with the increasing operating temperature. The reason for this phenomenon is because that the solar irradiance loss incurred by TCO film in NHCE-IRR-I is larger than the blocked heat loss at lower operating temperature, thus causing the reduced exergy. With the increase of operating temperature, however, the blocked heat loss gradually gets higher and eventually exceeds the solar irradiance loss, which contributes to the enhanced exergy in NHCE-IRR-I compared with the CHCE. As shown in Figure 10, the percentages of exergy efficiency enhancement (\(P_{\chi}\)) of three NHCEs obviously grow with the increasing inlet temperature, the percentage values of NHCE-IRR-I,  
Figure 8: Percentage of energetic efficiency enhancement of NHCEs versus inlet temperature.