The effects of using copper metal foam as a volumetric absorber as well as heat transfer enhancement technique on the thermal characteristics of a direct absorption parabolic trough solar collector are examined experimentally. The thermal performance, temperature difference and friction factor for PTC in three cases (full porous, semi-porous and without porous) are investigated and compared with each other. The experiments were performed in different flow rates and inlet temperatures. The thermal performance of solar collector is calculated based on ASHRAE Standard 93-2010. Obtained results indicate that using porous foam increase the friction factor dramatically. Moreover, the maximum temperature differences for full-porous, semi-porous and free porous are 12.2 degrees C, 8.8 degrees C and 3.3 degrees C, respectively. It is found that by increasing the flow rate and decreasing the inlet temperature, the thermal efficiency of the collector can be enhanced. In addition, the maximum thermal efficiency improvements due to using full metal foam and semi metal foam compared to that of free metal foam are 171.2% and 119.6%, respectively. (C) 2019 Elsevier Ltd. All rights reserved.