Comparative study on two novel intermediate temperature CPC solar collectors with the U-shape evacuated tubular absorber

Two truncated compound parabolic concentrating (CPC) solar collectors, which combine the external CPC and the U-shape evacuated tube together, have been developed and tested in this paper. The CPC solar collectors are edge-ray and gap-lossless collectors with different concentration ratios of 3.06 (3 x) and 6.03 (6x) and half-acceptance angles of 10 degrees and 3 degrees, respectively. A theoretical model of the CPC collector has been created and the detailed numerical simulation for optical, thermal and heat transfer fluid (HTF) behaviors of the CPC collector is carried out. In addition, the theoretical model has been validated using experimental results obtained by the testing system. The main objective of this paper is to report test results of the 3 x and the 6x CPC solar collectors with the U-shape evacuated tube, and to evaluate thermal performance which includes the overall heat loss as well as thermal efficiency, thereby providing useful data for practical solar process heat applications at the intermediate temperature within 80 degrees C and 250 degrees C, such as industrial process heat and solar cooling. Experimental results indicate that the tilt angle of the 3 x CPC collector is unnecessary to be adjusted in 1 day, however, it should be adjusted during the year. Correspondingly, the 6x CPC collector does need to be adjusted five times in 1 day due to its small half-acceptance angle. Under given test conditions, the daily thermal efficiencies of the 3 x and the 6 x CPC collectors can reach 40% and 46% at the collecting temperature of 200 degrees C, respectively. These verify that these kinds of CPC solar collectors are feasible for a wide range of intermediate temperature applications. (C) 2013 Elsevier Ltd. All rights reserved.