Experimental study on thermal performance of U-type evacuated glass tubular solar collector with low inlet temperature

The collector which consists of 16 U-type evacuated glass tubes is tested. Thermal efficiencies of such solar collector are obtained for the typical conditions with the solar irradiance of 375 W/m2, 435 W/m2, 535 W/m2, 675 W/m2 and 735 W/m2 generated by a solar simulator, and the mass flow rate of 0.02 kg/s, 0.04 kg/s, 0.06 kg/s, 0.08 kg/s, 0.1 kg/s. The results suggest that the thermal efficiency is higher at negative reduced temperature and it is positively correlated with mass flow rates and solar irradiances, as well as negatively correlated with inlet temperatures. Additionally, taking into account the error effects, the lower solar irradiance is, the higher sensitivity to changes of the mass flow rate and the solar irradiance efficiency changes will have. The results could be a guideline for the application of U-type evacuated glass tubular solar collector in solar-assisted heat pump system for space heating.