Design of Non-Imaging Condenser Based on Critical Interception to Eliminate Multiple Reflections and Performance Analysis
Objective Solar energy is highly valued as a renewable energy source due to its clean,sustainable,and inexhaustible nature.Solar concentrating technology has attracted significant attention because of the wide distribution of solar radiation and relatively low energy flux density.Currently,this technology finds extensive applications in various fields such as photothermal conversion,photoelectric conversion,and photochemical conversion.Based on the principle of non-imaging optics,the compound parabolic concentrator(CPC)offers numerous advantages including the absence of mechanical tracking devices,simultaneous collection of direct and diffuse radiation,and flexible operational timeframes.Consequently,it has gained wide acceptance in engineering applications.To address the problems of standard compound parabolic concentrator(S-CPC),such as limited concentrating ability at the end of the reflector,uneven energy flow density distribution,and reduced solar radiation collection,we construct a high-critical interception CPC(H-CPC)and a low-critical interception CPC(L-CPC)optimization model based on the principle of critical interception.Methods By intercepting the S-CPC at appropriate positions,it is possible to enhance optical efficiency effectively.However,truncation at excessively low positions may compromise the uniformity of surface energy flux density on the absorber.We address issues such as inadequate concentration effect on the condenser's end surface,multiple incident light reflections,and uneven energy flux density distribution on the absorber's surface by employing different interception positions on the S-CPC for improved optical performance.In addition,we conduct a comparative analysis between truncated CPCs and the S-CPC in terms of optical efficiency,energy flux density,and radiation collection amount.To verify the reliability of the truncated CPC model we constructed,a high-precision three-dimensional(3D)printer is used to print the truncated CPC model,and the pinhole imaging experiment is carried out outdoors.Results and Discussions The experimental results indicate that the maximum absolute error between the theoretical and experimental values of the position of the solar rays arriving at the surface of the absorber is 1.08 mm,the minimum absolute error is 0.03 mm,and the average absolute error is 0.32 mm.The maximum relative difference between the theoretical and experimental values is 0.77％,the minimum relative difference is 0.02％,and the average relative difference is 0.23％(Fig.5).This consistency between the theoretical and actual ray paths confirms the reliability of the truncated CPC model.By comparing the three different CPCs,we find that the average optical efficiency of S-CPC is 26.7％,while the optical efficiencies of the H-CPC and L-CPC are 38.4％and 46.3％,respectively.Therefore,in terms of optical efficiency,L-CPC is superior to the other two types of concentrators and has a larger range of incident light-receiving angles(Fig.6).When the light incidence angle is within the receiving half-angle range,the overall trend of the surface energy flow distribution of the three different surface CPC absorbers is consistent.However,as the incidence angle increases,the lower half of the absorber receives less and less light,leading to a more uneven energy flow density distribution.The uniformity of surface energy flux density of the S-CPC absorber is lower than that of the two truncated CPCs when the light incidence angles are 15° and 30°.Notably when the ray incidence angle is 15°,the peak energy flux density of S-CPC can be as high as 1.54× 104 W/m2,while the peak energy flux densities of H-CPC and L-CPC are 1.20 × 104 W/m2and 0.74×104 W/m2,respectively,which are 77.92％and 48.05％of that of S-CPC(Fig.7).The monthly radiation amounts of the three CPCs show a trend of first increasing and then decreasing on an annual time scale.From January to April,the monthly radiation of the concentrators gradually increases,peaking in April when S-CPC,H-CPC,and L-CPC receive 234.36,333.24,and 395.16 MJ/m2,respectively.Notably,H-CPC and L-CPC increased their monthly daylighting by 42.19％and 68.61％,respectively,compared to the standard CPC.The month with the lowest monthly light intake throughout the year is November,where the monthly light intake of the three CPCs is 112.79,166.89,and 201.50 MJ/m2,respectively.Even in this period,H-CPC and L-CPC still obtain 47.97％and 78.65％more solar radiation than standard CPC(Fig.9).Conclusions In this paper,we address the issues with the standard CPC,such as the suboptimal solar radiation collection effect and the uneven distribution of the energy flow density on the absorber surface,by designing two optimized CPC models based on the critical interception method.The reliability of the critical interception method is verified by outdoor small-hole imaging experiments,showing maximum,minimum,and average relative differences between theoretical and experimental values of 0.77％,0.02％,and 0.23％,respectively.Compared with S-CPC,the light receivable angle ranges of H-CPC and L-CPC designed by the critical interception method are significantly larger,and their average optical efficiencies are 38.4％and 46.3％,respectively,compared to 26.7％for S-CPC.The CPC absorber designed using the critical interception method has significantly improved the energy flow density distribution,making it more uniform.At the same time,the energy consumption ratio is higher,and the annual radiation collection of the two CPCs are higher than that of the S-CPC by 887.24 MJ/m2 and 1429.89 MJ/m2,respectively,demonstrating their practical value for engineering applications.
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