Heat transfer analysis and thermal performance investigation on an evacuated tube solar collector with inner concentrating by reflective coating

Abstract Evacuated tube solar collector (ETSC) is widely used for domestic and industrial applications. Using reflective coating in an ETSC to realize concentration is a method to heighten the thermal efficiency due to solar irradiation could be reflected by the coating on the lower half of the outer glass tube and concentrated to the eccentric inner absorber. In this paper, an evacuated tube solar collector with inner concentrating (ETSC-IC) by reflective coating was proposed and fabricated. A series of experimental tests of the ETSC-IC was conducted to investigate the thermal performance. In addition, the mathematical model of the novel collector with inner concentrating was developed, and the structural parameters of the inner reflective concentrating evacuated tube, including the coating angel and the eccentric distance, have been analyzed. The optical and thermal analysis revealed that the best coating angle of the evacuated tube was 180°, and the thermal performance of the ETSC-IC enhanced with the increase of the eccentric distance under the best coating angle. The thermal efficiency of the ETSC-IC could reach 0.65 to 0.72 on the condition that the solar irradiation was around 950 W/m2 and the temperature of inlet water varied between 45 °C and 70 °C, and the ambient temperature was 31.2 °C. The thermal efficiency of the ETSC-IC was improved by approximately 10% compared with the conventional evacuated tube due to the enhanced irradiation concentrating and the reduction of heat loss caused by the coating on the outer glass tube.