Author: Zou, Bin
Title: Rays-concentrating mechanism and thermal performance of parabolic trough solar collectors
Advisors: Yang, Hongxing (BSE)
Degree: Ph.D.
Year: 2020
Subject: Solar collectors
Solar energy
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Services Engineering
Pages: xxxvi, 265 pages : color illustrations
Language: English
Abstract: The parabolic trough concentrator (PTC) is currently the most cost-effective and widely used solar collector in concentrating solar power (CSP) area, showing great development prospect. The performance of the PTC is the determinant of the whole PTC-based thermal system, which depends on the rays-concentrating of the reflector and the heat transfer in the receiver tube. In previous studies, attentions were mainly focused on simulating the overall optical performance, while the theoretically quantitative analyses of the rays-concentrating of the reflector were seldom discussed. The optical performance of the PTC under non-ideal conditions were never examined based on the individual characterization of non-ideal optical factors, and related theoretical analyses were also scarcely performed. Furthermore, thermal improvement of the receiver tube based on the idea of enhancing the heat transfer between the fluid and the high heat flux area of the absorber wall was seldom explored as well in the past. Therefore, this thesis is committed to investigating comprehensively the optical and thermal performance of the PTC, aiming to reveal its mechanism of photo-thermal conversion and to propose an effective thermal improvement method. The Monte Carlo Rays Tracing (MCRT) coupled with theoretical analysis is used for investigating the rays-concentrating process, and a computational fluid dynamics (CFD) tool is adopted for simulation of the thermal and hydraulic performance of the parabolic trough receiver (PTR). Firstly, the optical performance of the PTC under ideal optical conditions was explored. The MCRT models were established and validated by other proven methods presented in literatures. Detailed geometrical analyses of the ideal rays-concentrating process were conducted. Several important parameters reflecting the rays-spillage, the variation of heat flux distribution range and the shadowing effect of the absorber tube were derived theoretically. Based on the MCRT and geometrical analyses, the effects of structural parameters on the PTC optical performance were investigated. It was revealed that there was a critical diameter, within which the absorber could only receive partially reflected rays, resulting in rays-spillage and consequently causing huge optical loss. Both the aperture width and the focal length should be kept in a certain range to avoid rays-spillage. The distribution range of high local concentration ratio (LCR) on the absorber outer surface increased with increasing aperture width, while decreased with increasing focal length. The peak LCR increased constantly with increasing aperture width, while dropped firstly and then increased with increasing focal length. Larger absorber diameter reduced both the peak LCR and the high LCR distribution range, and improved the optical efficiency. As the focal length was small or the absorber diameter was larger enough, the apex area of parabolic reflector and the bottom part of the absorber tube could not receive any solar rays due to the shadowing effect of the absorber itself. All the critical parameters corresponding to above optical phenomena were deduced through geometrical analyses and could be used to explain the changing optical properties of the PTC under the ideal condition. Then, an investigation on the optical performance of the PTC under non-ideal optical conditions was implemented. All the non-ideal optical factors including the sunshape, all the optical errors and non-zero incident angle were characterized separately based on their generation principles. Coordinate transformation was performed and the effective sunshape model was established for optical modeling under non-ideal optical conditions. Results showed that larger circumsolar ratio (CSR) and specularity error produced more uniform heat flux distribution. The advantage of the high optical quality reflector in improving optical efficiency was obvious only in clear days. When tracking error and slope error were maintained within a certain range (less than 4 mrad and 2 mrad respectively), the drop of optical efficiency was limited. The downtrend of the optical efficiency caused by tracking error became gentler under larger slope errors, and the optical loss was more sensitive to slope error than to tracking error. The offset direction along X-axis caused the greatest optical loss, and that along positive Y-axis posed threat of overheat to the absorber. When absorber alignment error and tracking error were in the opposite direction, the optical loss could be compensated, defined as compensation effect, whereas that in the same direction enlarged the optical loss, defined as weakening effect. The slope error weakened the compensation effect and aggravated the weakening effect. Succeeding to the above MCRT simulations, detailed theoretical analyses on the rays-concentrating process of the PTC under non-ideal optical conditions were conducted. Based on the theory of spatial analytic geometry, the critical absorber diameter under any optical error conditions was derived theoretically. And then, a new simple algorithm based on the idea of viewing the sun as consisting of countless line light sources was developed for quick calculation of optical efficiency. The proposed algorithm, compared with the MCRT, had a great advantage of time saving, which was suitable for engineering calculation. Finally, based on the derived formulas and the proposed algorithm, the effective sunshape size was further discussed for the sake of engineering application. In addition, the changing properties of optical efficiency achieved by the MCRT in the preceding chapter were also expounded using the theoretical results. Finally, the thermal performance improvement of the parabolic trough receiver (PTR) was explored. The basic thermal performance of the conventional straight-smooth PTR (CSS-PTR) was analyzed, and a unilateral spiral ribbed PTR (USR-PTR) was proposed based on the idea of enhancing the heat transfer between the fluid and the high heat flux area (i.e. bottom part) of the absorber to improve its thermal performance. In order to realize the simulation of the heat transfer under actual heat flux conditions, the heat flux distribution obtained by MCRT in preceding chapters was loaded as the boundary condition on the absorber outer surface by User Defined Functions (UDF). It was revealed that the temperature distribution of the absorber is completely dependent on the heat flux distribution, and the circumferential temperature difference remained constant in the longitudinal direction. As the flow rate grew, the distribution of fluid temperature on the cross section changed from annular stratification to vertical stratification. When the annulus space of the receiver was filled with air or the glass envelope was broken, the collector efficiency was reduced by 4.31% and 26.1% respectively, indicating that ensuring high vacuum degree in the annulus is critical to achieving high performance. Comparisons of the USR-PTR and the CSS-PTR showed that, in most cases of the discussed flow rates (0.5~3.5 kg/s), the overall performance of the USR-PTR was better than that of the CSS-PTR. The circumferential temperature difference of the USR-PTR could be reduced by 8.5%~27.4% to the CSS-PTR. The thermal improvement mechanism of the USR-PTR was also analyzed according to field synergy theory, which indicates that the synergy between the velocity field and the temperature gradient field of the fluid in the USR-PTR was much better than that in the CSS-PTR. Finally, based on the performance evaluation criteria (PEC), the influences of the rib's structural parameters, including pitch interval, rib height, corner radius, crest radius and spiral angle, on the overall performance of the USR-PTR were investigated comprehensively. The PEC of the USR-PTR by adjusting individually the above five rib structural parameters was 1.125, 1.098, 1.108, 1.096 and 1.301, respectively. In summary, this thesis conducted a detailed study on the optical and thermal performance of the PTC, aiming to reveal its rays-concentrating mechanism and to seek effective thermal improvement measures. The findings in this study enrich the basic research theory in the field of PTC, and provide important theoretical guidance for the application and promotion of PTCs. The developed algorithm for optical efficiency is suitable for engineering application, and the proposed thermal improvement method provides a new idea for engineers and designers to optimize the structure of the PTR.
Rights: All rights reserved
Access: open access

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