## 1 Introduction
In this framework, an optimization method by depositing IR-reflector coatings on the glass envelope of the HCE in the NTR to reduce the heat loss in the NTR was proposed. IR-reflector coating, which has high reflectance in the infrared wavelength band and high transmittance in the solar irradiance wavelength band, was employed and deposited on the inner and outer surfaces of glass envelope in the (NTR as shown in Figure 1[19]. IR-reflector coating on the inner surface of glass envelope can effectively block the infrared emissive heat loss from the absorber tube in the NTR, and the IR-reflector coating on the outer surface of glass envelope can reduce the emissive heat loss from the glass envelope due to its low infrared emittance. However, the IR-reflector coatings would inevitably incur the solar irradiance loss due to the reduced transmittance of the glass envelope with IR-reflector coatings. Taking the actual transparent conductive oxide (TCO) film [20] as an example, the solar irradiance transmittance of this IR-reflector is 69.9%, which means approximately 30% of the incoming solar irradiance cannot pass through the TCO film, thus forming additional solar irradiance loss. Generally, an ideal IR-reflector is designed with a determined cutoff wavelength (\(\lambda_{c}\)) at which the spectral property of material, such as spectral transmittance, spectral reflectance,changes sharply (Figure 2). The value of \(\lambda_{c}\) is usually selected as the fixed value of 2.5 \(\upmu\)m to obtain the maximum solar transmittance in the case of lower service temperature. This is because the blackbody's emissive power from the surroundings under lower temperature mainly locates in infrared wavelength band above 2.5 \(\upmu\)m (such as 373 K exhibited in Figure 2), that coincides well with the spectral selectivity of IR-reflector with a fixed cutoff wavelength of 2.5 \(\upmu\)m. In the case of high service temperature such as 400-550 \({}^{\circ}\)C in HCEs, a conventional ideal IR-reflector with the cutoff wavelength of 2.5 \(\upmu\)m still enables the vast majority of solar irradiance to pass through, but the effectiveness of blockage of emissive heat loss of the HCE in the NTR is definitely reduced. This is because the emissive heat loss of the HCE at high temperatures explosively increases, and the main wavelength band at which blackbody's emissive power from the absorber tube locates would shift to lower wavelength (such as 673 K exhibited in Figure 2) [21], these two factors result in the escape of a fair amount of heat loss below 2.5 \(\upmu\)m from the absorber tube to the surroundings. In this framework, the cutoff wavelength of ideal IR-reflector coating was optimized and shifted to lower optimal cutoff wavelength (\(\lambda_{c,opt}\)) as shown in Figure 2. The optimized ideal IR-reflector coating optimally weighs the amount of transmitted solar irradiance and that of blocked emissive heat loss in the NTR, and thus maximizing the net heat gain in the NTR. The novel HCEs (NHCEs) with three kinds of IR-reflector (IRR) coatings, namely, TCO coating (NHCE-IRR-I), ideal IRR with a fixed cutoff wavelength of 2.5 \(\upmu\)m (NHCE-IRR-II), and ideal IRR with optimal cutoff wavelength (NHCE-IRR-III), were proposed, and the heat loss performance of three kinds of NHCEs was preliminarily analyzed in the previous work [19]. To further verify their overall performances in a real PTC system and observe the maximum potential of IR-reflector coating on the performance enhancement of PTC system, in this study, three kinds of NHCEs (NHCE-IRR-I, NHCE-IRR-II, NHCE-IRR-III) are employed into a small-scale PTC field with a 72 m loop. The energetic and energetic performances of different NHCEs are numerically investigated and compared by the established mathematical models. Additionally, the effects of two key parameters, namely, operating temperature and DNI, on the overall performance of the NHCE are also explored.