The thermo-hydraulic characteristics of a solar air heater with quarter-circular ribs on the absorber plate are studied numerically. A comparison of the three different arrangements of the quarter-circular ribs is performed. The ribs are aligned periodically in a transverse manner to the flow. The numerical simulation of the mass, momentum, and energy conservation equations are carried out for a two-dimensional computational domain using the finite volume multi-grid solver of ANSYS-Fluent-18. A constant and uniform heat flux of 1000 W/m2 is supplied to the aluminum collector plate. Many pertinent input parameters, viz. flow parameters like Reynolds number and roughness parameters like relative roughness pitch and relative roughness height are varied. Reynolds number is varied from 3800 to 18000, whereas the relative roughness pitch is varied from 7.14 to 17.86, and the value of relative roughness height is kept fixed, i.e., 0.042. The impact of relative roughness pitch and Reynolds number on the thermo-hydraulic performance of the ribbed solar air heater is analyzed here. The optimized rib geometry in the investigated range of parameters is obtained. Pumping power calculations are done. The contours of temperature, pressure, and velocity are illustrated pictorially to understand the flow physics with clarity. It is found that the solar air heater with quarter-circular ribs having a relative roughness pitch of 7.14 delivers a maximal thermal enhancement ratio (representing the overall energy performance) of 1.88 in the studied parametric range. The Nusselt number and friction factor correlations are proposed by performing a non-linear regression analysis.