Abstract
Heat collecting elements (HCEs) are the core components in the parabolic trough collector (PTC) system because photothermal conversion of the whole system occurs in the HCEs. However, considerable heat loss from the HCEs at high operating temperature exerts seriously negative impact on the photothermal conversion efficiency of the PTC system and subsequent application systems. To effectively reduce the heat loss and thus enhance the overall performance of the PTC system, in our previous work, we proposed three kinds of novel HCEs by partially depositing different IR-reflector coatings on the inner and outer surfaces of the glass envelope. The infrared (IR)-reflector of actual transparent conductive oxide (TCO) film, IR-reflector with a fixed cutoff wavelength of 2.5 μm, and the IR-reflector with optimal cutoff wavelength showed extremely effective roles in the reduction of heat loss in HCEs. In this paper, the comprehensive energy and exergy performances of these three novel HCEs in a real 72 m small-scale PTC system are further investigated by the mathematical models established. Additionally, the comparisons among overall performances of the proposed HCEs under different direct solar irradiances are also carried out. The results show that the simulated data yields good consistence with the experimental results, and that all three of the novel HCEs achieve superior overall performance compared with the conventional HCEs. The PTC system installing the novel HCEs with the IR-reflector coating which possesses the optimal cutoff wavelength has the best energetic and exergetic efficiencies, which are significantly improved by 25.2% and 28.1% compared with the conventional HCEs at the solar irradiance of 800 W/m2 and inlet temperature of 580 °C. Moreover, the proposed novel HCEs have a much superior performance at lower solar irradiance. The performance-enhanced PTC system will play a significantly positive role in the performance improvement of the heating and cooling of buildings in the future.
Highlights
At present, high-temperature solar-thermal conversion processing is accomplished mainly by the concentrated solar collection technologies [1,2], with the implementation forms including the parabolic trough collector system, tower collector system, dish collector system, and linear Fresnel collector system [3,4,5]
System, solar rays projected on the trough mirrors are reflected and concentrated onto the heat collecting elements (HCEs), the most of rays would pass through the glass envelope, and are blocked and absorbed by the absorber tube deposited with solar selective-absorbing coating (SSC) [10], the solar energy absorbed is eventually converted into the heat energy of heat transfer fluid (HTF) flowing inside of absorber tube
It is calculated that the root-mean-square deviation (RMSD) value remains within 6.0% and the mean bias error (MBE) value is about −0.8%, which demonstrates that the simulated result yields satisfactory consistence with the experimental data
Summary
High-temperature solar-thermal conversion processing is accomplished mainly by the concentrated solar collection technologies [1,2], with the implementation forms including the parabolic trough collector system, tower collector system, dish collector system, and linear Fresnel collector system [3,4,5]. In the case of high service temperature such as 400~550 ◦ C in HCEs, a conventional ideal IR-reflector with the cutoff wavelength of 2.5 μm 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 μm from the absorber tube to the surroundings. The performance-enhanced PTC system with the NHCEs could contribute to the efficient building heating and cooling systems
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