An enclosed-type evacuated U-tube solar collector (EEUSC) with high efficiency and low cost was designed and constructed. A copper fin was employed in the U-tube to assume a constant heat flux. The thermal performance of the EEUSC was evaluated under a wide range of operating conditions. Moreover, to increase the heat transfer efficiency in the U-tube over the thermal resistance of the air gap, a novel method was developed, which entailed filling the gap with high-thermal-conductivity liquid. Multi-walled carbon nanotube (MWCNT) nanofluid was used as the working fluid. Evaluation results showed that the efficiency of the EEUSC is influenced primarily by the air gap and that it increases by 4% with the use of the MWCNT nanoliquid. Calculations based on this improvement revealed that the annual CO2 and SO2 emissions will reduce by 1600 kg and 5.3 kg, respectively, when 50 solar collectors are employed. (c) 2015 Elsevier Ltd. All rights reserved.