Assessment of uncertainty in mean heat loss coefficient of all glass evacuated solar collector tube testing

The mean heat loss coefficient of an all glass evacuated solar collector tube (referred to as solar tube in this paper) is a key indication rating its thermal performance and quality. In order to improve the comparability of the testing results from different testing laboratories and give a reliable result on the basis of a specific standard, it is essential to evaluate and analyze the uncertainties related to measuring devices errors and test procedure. In this study, the measurement uncertainty in the mean heat loss coefficient, UL, of a solar tube, a dominant solar product used in China, was statistically obtained by 22 repeated measurements based on GB/T 17049-1997, a test standard issued by the National Technology Supervision Bureau of China in 1997. The uncertainty in the other parameters in the characteristic equation of UL of a solar tube were analyzed and evaluated in detail. The results showed that the measurement uncertainty in UL was mainly caused by the unsuitability of the test procedure, and the measuring errors of the temperature sensors and instruments contributed a little to the measurement uncertainty of UL. This finding implies that a further revision on the test procedure described by GB/T 17049-1997 is necessary in order to reduce the uncertainty in UL of solar tube testing.