Theoretical efficiency of an all-glass tubular solar collector using a chemically deposited SnS-Cu_xS absorber inside the inner tube

Theoretical efficiencies of an evacuated-tube solar collector using a chemically deposited SnS-Cu_xS absorber thin film ( alpha =0.91) in contact with the working fluid are presented. A mathematical model is developed and solved numerically to yield photothermal conversion efficiencies eta of the collector for the heat transfer coefficient h_b of 0.026 W m^-2 K^-1 (good vacuum). The analysis shows that the eta of such a collector would be unacceptably low, approximately=25%, for the operating parameter f approximately=0.1 degrees C m² W^-1 which represents the typical operating condition of an evacuated-tube solar collector. However, the use of a low emissivity ( epsilon ) transparent coating applied to the outer surface of the absorber tube (i.e. in the place of the spectrally selective solar absorber coating in the conventional all-glass evacuated-tube collector) can yield eta approximately=75% for f approximately=0.1 degrees C m² W^-1 for epsilon =0.1 with water as the heat transfer fluid and eta approximately=68% with a mineral oil as the heat transfer fluid.