Study On The Performance Of CPC Collector Based On Round Tube

Compound Parabolic Concentrator(CPCs) designed based on tubular absorber is regarded as to be a potential collector in solar thermal systems operating at medium temperature. In order to evaluate the optical performance of such CPCs and optimize its design, it is essential to theoretically calculate the optical efficiency for solar ray incident at any angle. Theoretical calculation of optical efficiency of such CPC is extremely complex due to complex radation transfer within CPCs and ray-tracing analysis is usually employed in the past. However, ray-tracing analysis is time expensing and sometimes even impossible for the design optimization of CPCs due to a large number of potential designs. In this work, an attempt was made to theoretically investigate the angular dependence of optical efficiency of CPCs with tubular absorber with the aim to be convenient for performance evaluation of such CPCs.Firstly, one-, two, three- and four-reflection models of symmetric CPC(S-CPC)have been developed for calculating the optical efficiency factors and camparing with those obtained by ray-tracing analysis. Results show that more than 40%radiation incident at the angle of zero will arrive on tublar absorber after more than two reflections, three-reflection model, i.e. radiation arriving on the absorber after more than three times is regarded to be that arriving on the absorber after just three reflections, can accurately predict the optical performance of S-CPCs.Secomdly, one-, two-, three- and four-reflection models are suugested to calculate optical efficiency of asymetrci CPC(A-CPC), and found that two-reflection model can accurately predict the optical performance of A-CPC.Thirdly, six A-CPCs were dsigned for concentrating radiation on all-glass evacuated solar tubes(EST) with the inner tube of 47 mm and cover tube of 58 mm in diameter. From the viewpoint of optical efficiency averged over the acceptance angle,A-CPC designed based on the hat-shaped virtual absorber with single V-groove atthe bottom of reflectors is the best solution, the one designed based on cover tube is the most inferior solution. It is also found that, in terms of annual radiation collection on unit length of solar tubes, the design based on cover tubes annually concentrates the most radition due to the large geometric concentration factor and the one designed based on hat-shaped absorber without single V-groove at the bottom annually collects the least radition. From the point of radiation concentration per unit aperture area, it is found that, A-CPC designed based on hat-shaped absorber with single V-groove at the bottom is the best, others are almost identical in the performance.Finally, an A-CPC solar collector with EST as the receiver and collecting area of 2.7m2 was fabricated and tested where the A-CPC is designed based hat-shaped absorber without V-groove at the bottom, site latitude of Yuxi, China, and requirement of the least daily operation period of 6 hours, and reflectors was made of stainless steel with a reflectivity of 0.69. Ray-tracing showed that, the optical efficiency of the A-CPC over the acceptance angle was 52.55%, and experimental measurements indicated that thermal efficiemcy of the collector was in the range of38-40%.