An experimental study on the impacts of key parameters of all-glass evacuated tubes on the thermal performances of all-glass evacuated tube solar water heaters

In this paper, the effects of the stagnation parameter Y, the solar irradiance under stagnation H, and the average heat loss coefficient ULT of individual all-glass evacuated solar collector tubes, which were identified as the major parameters to characterize the thermal performance of the tubes, on the thermal performance of all-glass evacuated tube solar water heaters consisted of such evacuated tubes were investigated by experiments. The experiments were conducted to obtain the heat gain Qs produced by six all-glass evacuated tube solar water heaters with the similar specifications but manufactured by different producers. Each heater consisted of 20 all-glass evacuated solar collector tubes and one tube from each heater was selected randomly to measure its Y, H, and ULT . The experimental results show that both H and ULT have effects on Qs , with Qs decreasing when H or ULT increases, but H has more significant effects on Qs than ULT does. However, Y does not have significant effects on Qs . It is apparent that one effective way to increase the heat gain of an evacuated tube solar water heater is to reduce H and ULT of individual all-glass evacuated solar collector tubes. An error analysis was conducted which clearly shows that the errors caused by the uncertainties associated with the measured Qs , Y, H, and ULT have negligible effects on these results.