This paper theoretically analyses the thermal performance of extended surface of absorber tubing in a flat plate solar collector by using a passive technique to reduce the thermal barrier in a conventional flat plate solar collector. This technique changes the geometry of the flow path of the fluid. The objective is to increase the convective heat transfer coefficient with minimum pressure drop while enhancing the effective heat transfer area between the absorber fluid and the surface it is in contact with. The two types of velocity enhancers used and compared in the work are rod and tube. The velocity enhancers are kept frictionally engaged within the inner side of the absorber tube wall, with the tubes and rod being parallel to fluid flow path. It was found that, while increasing pumping power minimally, the rod velocity enhancer provides greater heat transfer rate than the tube velocity enhancer configuration. Compared to the plain tube, the increase in efficiency of the rod and tube velocity enhancers are 15% and 10%, respectively. (C) 2017 Elsevier Ltd. All rights reserved.