This paper deals with an experimental investigation of mixed convection effects of flat plate solar collector with copper rod and tubes thermal performance enhancers in absorber tube and comparison was made between them for the same operating condition. The highest value of Rayleigh, Grashof, and Richardson number based on wall heat flux showed the heat exchange mode in the flat plate solar collector as of a mixed convection mode with the free convection predominant mode. The thermal performance enhancer collector significantly reduced the Rayleigh, Grashof and Richardson numbers compared to smooth pipe flat plate solar collector at all the operating condition. The experimental heat transfer coefficient for the flat plate solar collector was analyzed, and the rod thermal performance enhancer collector was found providing a higher heat transfer than tube thermal performance enhancer collector and smooth copper pipe flat plate solar collector. The highest performance evaluation factors were 1.38 and 1.29 for rod and tube thermal performance enhancers respectively. © 2019 Elsevier Ltd