A U-type natural circulation heat pipe system is designed and applied to a parabolic trough solar collector for generating mid-temperature steam. Thermal performance of the heat pipe system is investigated experimentally. A detailed heat transfer analysis is performed on thermal behaviors of the system, especially the solar collector. The results show that the system can generate mid-temperature steam of a pressure up to 0.75 MPa. The thermal efficiency is found to be 38.52% at discharging pressure of 0.5 MPa during summer time. (C) 2012 Elsevier Ltd. All rights reserved.