Research on the universal model of filled-type evacuated tube with U-tube in uniform boundary condition

• Heat transfer model of UFET has been given with different structures of U-tube.
• The universal model of UFET in uniform boundary condition has been shown.
• Gives theoretical and experimental studies of three kinds of UFET.
• Presents comparative analysis of thermal performance of three kinds of UFET.

The heat transfer efficiency of solar collector tubes is a crucial factor for the application of solar heating and cooling systems (SHCS). How to enhance solar collector efficiency has become a hot topic. In this paper, a new type of collector tube – filled-type evacuated tube with U-tube (UFET), was presented to increase solar collector efficiency by eliminating the thermal resistance between the absorber tube and the copper fin of the conventional evacuated solar collector. First, a UFET heat transfer model was built with different U-tube structures and the universal UFET model in uniform boundary condition. Furthermore, the thermal performances of three types of UFET were compared theoretically and experimentally. The results show that the maximum solar collector efficiency of UFET with three U-tubes reaches 0.82 and is higher than other types with the same heat transfer surface area. In addition, the theoretical result is 4% higher than the experimental result, indicating that the developed model is validated by the experimental investigation in this study.