@article{fd0391b4427047e68f9e058045b0457c,
title = "A hybrid parabolic trough solar collector system integrated with photovoltaics",
abstract = "It is challenging to reduce the massive radiation heat loss from the parabolic trough solar receiver and enhance the solar utilization efficiency of the parabolic trough collector (PTC) system. On the basis of the negative thermal-flux phenomenon discovered in the PTC system, a novel PTC system integrated with photovoltaic (PTC-PV) panels is proposed in this study to effectively enhance the thermal performance and solar utilization efficiency of the PTC system. Four modes of the hybrid PTC-PV system are put forward for catching the optimum configuration. The mathematical models of the hybrid PTC-PV system in terms of photothermal and photoelectrical conversions are established. Additionally, a test rig of the hybrid system is built, and the experiments are carried out in an indoor laboratory with a solar simulator. Comparisons between the experimental and simulated results show that the model has an excellent prediction ability. Furthermore, a 12 m-length PTC-PV system using molten salt as heat transfer fluid is studied to validate its thermal, exergy, and overall performance. The results show that the hybrid PTC-PV system exhibits dramatic superiority in overall performance over the PTC prototype system. The heat loss of the solar receiver in the hybrid PTC-PV system is significantly reduced by 44.0 %, and the photoelectrical efficiency of the PV panel in the PTC-PV system is drastically enhanced by 118.3 % compared to that installed on the ground. Moreover, the overall and exergy efficiencies of the PTC-PV system can be improved by 14.3 and 13.7 %, respectively, at the inlet fluid temperature of 580 °C.",
keywords = "Hybrid solar system, Parabolic trough collector (PTC), Photo-electrical/thermal, Photovoltaic (PV), Thermal and exergy",
author = "Qiliang Wang and Yao Yao and Zhicheng Shen and Hongxing Yang",
note = "Funding Information: This study was sponsored by the RGC Postdoctoral Fellowship Scheme 2020/2021 (3-RA59) of the University Grants Committee and the Postdoctoral Hub program (PiH/160/19) of the Innovation and Technology Fund of the Innovation and Technology Commission, the Hong Kong SAR Government. Funding Information: For a long time, many endeavors have been made to reduce radiation heat loss and improve the thermal performance of the PTC system. The development of the advanced solar selective-absorbing coating (SSC), which is deposited on the absorber tube in the PTR, is the most direct way to address the above problem by lowering its thermal emittance [20,21]. However, on the premise of ensuring a high solar absorptance, the thermal emittance of currently state-of-the-art SSCs only drops to 0.09 @400 °C [22], which will still cause a large amount of radiation heat loss. In this context, the author discovered the negative thermal-flux phenomenon at the upper part of the PTR [23], at which only one solar radiation absorbed by the absorber tube is lower than the radiation heat loss. Based on this discovery, some partial structure and material optimization methods have been proposed. Yang et al. [24] put forward a newly designed PTR by changing the original SSC at the upper part of the absorber into a metallic film that has ultra-low emittance. The results showed that though the upper one solar radiation was not absorbed by the PTR, the radiation heat loss from the absorber tube's upper part was significantly reduced, and eventually, the thermal performance of the PTR was also effectively improved. Qiu et al. [4] and Wang et al. [25] introduced a hot mirror and metal radiation shield into the upper part of the vacuum annular to block the radiation heat loss, respectively. All of the above studies achieved enhanced thermal performance for the PTR but simultaneously generated a potential drawback, that is, the underutilization and waste of whole or part of one solar radiation at the upper part of the PTR.where σ is the Stefan-Boltzmann constant, 5.67 × 10-8 W/(m2·K4); Ts, Tgu, and Tgl are the temperatures of the absorber tube, upper-half and lower-half glass temperature, K; Ds and Dg represent the diameters of the absorber tube and glass envelope, m; Fs-gu and Fs-gl are the view factors that the radiation heat flux leaves from the absorber tube and strikes surface gu and gl, their values equal to 0.5. εs, εc, and εg refer to the emittances of the SSC, high-reflective aluminum coating, and glass envelop. The values of the latter two are 0.03 and 0.89. In this study, the PTR employs the commercial Schott's 2008 PTR70 receiver [38]. According to the test record from the National Renewable Energy Laboratory [27], the value of εs can be calculated by the equation as follows.Besides the PV performance, the thermal performance of the PTR is also evaluated. The heat losses of prototype PTR and the proposed PTRs in four modes are simulated based on the established model. In order to verify the model, the experimental results carried out by the National Renewable Energy Laboratory [27] are adopted to make comparisons with the simulated heat loss of the prototype PTR. As shown in Fig. 9, the simulation data yields a good consistency with the experimental results, exhibiting the heat transfer model of the PTR has good calculation precision. It is also observed that the PTR in Mode A has almost the same heat loss as the prototype PTR, and the PTR in Mode B has a slightly decreased heat loss. As shown in Fig. 10, the heat loss reduction ratios of the PTRs in Modes A and B are approximately 0.5 and 6.0 % at the absorber temperature of 600 °C, respectively. Different from Modes A and B, the heat losses of the PTR in both Modes C and D are significantly reduced by above 36.0 %. And with the increasing absorber temperature, the heat loss reduction ratio is further improved. This is because the emissive heat from the absorber tube increases with the absorber temperature to the fourth power, the aluminum coating on the glass envelope is thereby capable of blocking much more emissive heat at high absorber temperatures compared with that at low absorber temperatures. At the absorber temperature of 600 °C, the heat loss reduction ratio reaches approximately 43.0 and 44.0 % for Modes C and D, respectively. All of these demonstrate that the aluminum coatings covered on the glass envelopes in Modes C and D exert significant roles in reducing the heat loss of the PTRs. Publisher Copyright: {\textcopyright} 2022 Elsevier Ltd",
year = "2023",
month = jan,
day = "1",
doi = "10.1016/j.apenergy.2022.120336",
language = "English",
volume = "329",
journal = "Applied Energy",
issn = "0306-2619",
publisher = "Elsevier BV",
}