Rays-concentrating And Heat Transfer Mechanism Of The Parabolic Trough Solar Collector And Its Thermal Performance

In recent years,concentrating solar power(CSP)technology has developed rapidly and become an important branch in the field of solar utilization.The parabolic trough solar collector(PTC)-based technology is one of the most widely used CSP technologies,having attracting extensive attention worldwide.In our country,the PTCbased power generation technology started late,and both theoretical research and engineering practice experience are lacking.Therefore,the research work in this field needs to be further deepened.In this thesis,the optical and thermal performance of the PTC were investigated comprehensively using simulation study coupled with theoretical analysis,aiming to reveal its rays-concentrating and heat transfer mechanism and to propose a new structure of receiver tube for improving its thermal performance.Firstly,the optical performance of the PTC under ideal conditions was studied in detail based on Monte Carlo Ray Tracing(MCRT)method and theoretical analysis.The mathematical principle and program implementation of the MCRT were introduced,and the optical models were established and verified.Some important parameters,such as the effective angle range receiving reflected rays,the angle range that cannot receive reflected rays and the width at the top of the parabolic reflector that cannot receive incident rays,were deduced,taking into account the rays-spillage,the distribution range variarion of local concentrating ratio(LCR)and the shadowing effect of the absorber tube.The influences of three main structural parameters,including aperture width,focal length and absorber outer diameter,on the PTC's optical performance were explored in detail,and the variation of optical properties was theoretically expounded.It was revealed that the peak LCR was increased with increasing the aperture width,while the the uniformity of the heat flux distribution was improved.Larger focal length shrank the high LCR distribution range,posing threat of local overheat on the absorber wall.When aperture width,focal length and absorber outer diameter exceeded a certain range(Wc>12.93 m or fc<0.21 m or fc>7.31 m or da,o<25 mm),the rays-spillage was caused,resulting in huge optical loss.Secondly,based on individually theoretical characterization of all the non-ideal optical factors,including sunshape,optical errors and incident angle,their influences on the optical performance of the PTC were investigated comprehensively.Coordinate transformation was performed for optical modeling,and an effective sunshape model was established for rays sampling.The problem that the inverse function of the sunshape model cannot be found was solved by using the acceptance-rejection sampling method.Results showed that larger circunsolar ratio(CSR)improved the uniformity of heat flux distribution,while reduced the optical efficiency.The advantage of high quality reflector in improving optical efficiency was only apparent on clear days.The optical efficiency was more sensitive to slope error than to tracking error.The absorber alignment error along X-axis caused the greatest optical loss,and that along the positive Y-axis posed threat of overheat on the absorber wall.When the absorber alignment error and the tracking error were in the opposite and the same direction respectively,the compensation effect and the weakening effect were caused respectively on optical efficiency.The non-zero incident angle resulted in cosine loss and end loss,which,respectively,weakened the effective incident solar irradiance and caused rays-spillage at one end of the collector,reducing significantly the optical efficiency.The formulas of critical absorber diameter under non-ideal conditions were derived theoretically.A new theoretical algorithm was developed for calculating the optical efficiency,which has advantages of high accuracy and saving time.Based on the proposed algorithm,the effective sunshape size was discussed for purpose of engineering calculation,and the changing properties of optical efficiency were well expounded theoretically,realizing successfully the mutual validation between the MCRT and the proposed algorithm.Based on clear identification of all the heat transfer forms in the receiver tube,the thermal performance of the receiver tube was studied numerically.The heat flux distribution obtained by optical simulation was loaded onto the absorber outer surface by User Defined Functions(UDF)to simulate the heat transfer under actual heat flux condition.The numerical models were validated by typical test data and classical empirical formulas.The thermal performance of the receiver tube,such as distribution of heat flux,temperature,variation of fluid velocity,heat loss and thermal efficiency,were analyzed and discussed.In addition,the influences of various parameters and conditions,such as operating parameters,ambient conditions and non-ideal optical factors,on the thermal performance of the receiver tube were also examined comprehensively.Findings showed that when the mass flow rate increased or the inlet fluid temperature decreased,the thermal efficiency of the receiver tube was improved.When applied in sites with abundant solar resources,the receiver tube worked more efficiently,while suffered greater safty challenge.The influences of wind speed and ambient temperature on the receiver's thermal performance were negligible.The effects of non-ideal optical factors on the overall performance of the PTC were mainly reflected in affecting its optical performance.Finally,based on the idea of enhancing the heat transfer between the high heat flux area of the absorber wall and the fluid,a novel unilateral spiral ribbed absorber tube was proposed for improving the thermal performance of the receiver tube.A detailed comparison between the new receiver tube with the conventional smooth receiver tube and the full spiral ribbed receiver tube were conducted in terms of both thermal and hydraulic performance.Furthermore,the heat transfer enhancement mechanism of the new receiver tube was revealed based on the field synergy theory.The influences of five main structural parameters,including rib pitch interval,rib height,crest radius,corner radius and spiral angle,on the thermal performance of the new receiver tube were also discussed.It was found that the performance evaluation criteria(PEC)of the new receiver tube was improved by increasing the rib pitch interval or reducing the rib height or increasing the crest radius or the corner radius.The maximum PEC by adjusting individually the four parameters was 1.125,1.098,1.096 and 1.108,respectively.When the spiral angle was 30°,PEC reached the maximum value of 1.301.This thesis conducted a detailed study on the optical and thermal performance of the PTC,revealing the mechanism of its rays-concentrating and heat transfer.The findings in this study will enrich the basic research theory in the field of PTC,and provide important theoretical guidance and technical support for the application and promotion of PTCs in China.