A study on the evacuated-tubes integral-collector-storage solar water heater with two different coated absorbers

Based on measuring data of solar irradiance ratio on absorbers with cylindric shape in evacuated collector tube (ECT) as well as that solar absorptivity changing with the incident angle on the coating, this paper has analyzed and investigated regular patterns of the absorbed energy factor changing with the incident angle at different center-to-center distances of integral collector storage evacuated tube (ICSET) with full-circular and semi-circular selective coated absorbers. According to the clear-day model, the daily and annual absorbed energy for different center-to-center distances of ICSET with full-and semi-circular coated cylindrical absorbers, put southward at different northern latitudes and various tilted angles, have been calculated and compared. The results show that the annual absorbed energy of ICSET with full-circular coated absorber is 9% higher than with semi-circular coated absorber, in case no sunshade between tubes is considered. When the center-to-center distances of ICSET full- and semi-circular coated absorbers were set to 2.0 and 1.3 times the outer diameter of tube respectively, the annul absorbed energy of both absorber types is 95% of the absorbed energy with a shadeless configuration.