We report on an experimental study conducted to investigate the impact of heat enhancement devices on the thermal performance of a flat-plate solar collector. Different passive heat enhancement devices that include twisted strip, coil-spring wire and conical ridges were studied. The comparison showed no appreciable difference in the heat flux to the collector fluid. A detailed investigation of the observed trend showed significantly high values of Grashof, Richardson and Rayleigh numbers indicating that the heat transfer mode in the solar collector is of mixed convection type with free convection as the predominant mode. It is concluded that due to the significant damping of shear-produced turbulence by buoyancy forces, the applied passive methods based on the enhancement of shear-produced turbulence are ineffective in augmenting heat transfer to the collector fluid in flat-plate solar collectors. (C) 2009 Elsevier Ltd. All rights reserved.