The use of flat solar air heaters is extensively worked out in low-temperature solar energy utilization. The efficiency of such heaters has to be low because of the low convective heat transfer coefficient between absorber plate and flowing air which increases the absorber plate temperature, leading to higher heat losses to the ambient. Several designs of solar air heaters having different roughness geometries have been done by a number of investigators. The present paper deals with a comprehensive review of different technologies and methodologies applied for different designs to attain an appropriate enhancement in heat flux. Analytical and experimental result as well as worked out values of result with respect to heat transfer data in terms of Nusselt number, utilizing the equations or correlations developed by various researchers and investigators have been represented with respect to the roughness and flow parameters (p/e, e/D and Re), for the comparison of the rate of heat transfer. The experimental values of the average Nusselt number for multi V roughness have been found to be the highest. However, the addition of a booster mirror to increase the rate of radiant energy and the use of computational methodology has been found even better as compared to the previous one.