Thermal performance of direct-flow coaxial evacuated-tube solar collectors with and without a heat shield

Experimental performance evaluation and comparative analyses based on heat extraction of direct-flow coaxial evacuated-tube solar collectors with and without heat shields are presented in this paper. A test system to evaluate the thermal performance of medium-temperature solar collectors was developed. The experimental analysis shows the evacuated-tube solar collector performed better with a heat shield, especially at higher working temperatures. The collection efficiency of the solar collector with a heat shield was 54.70% at the highest inlet temperature of 123.9 °C during the test period, which is 31.49% higher than for the solar collector without a heat shield. The calculated heat-loss coefficient for the evacuated-tube solar collector with a heat shield was 1.86 W/(m2 K), which is an improvement of 50.80% compared with that of the collector without a heat shield.