In this study, a 2-D CFD simulation was carried out using ANSYS Fluent to analyze the thermal and flow behavior of a single-pass rectangular solar air heater (SAH) duct fitted with semicircular roughness. Single, triple, and penta rib configurations were tested across Reynolds numbers (4,000 to 20,000) and rib pitches from 15 mm to 24 mm, corresponding to P/e ratios between 10.7 and 17.1. The objective was to evaluate how these geometrical variations influence heat transfer (Nusselt number), pressure loss, outlet temperature, and the overall thermo-hydraulic performance (THPP). The penta-15P configuration showed the highest heat transfer rate (Nu = 137.8) and pressure drop (54.14 Pa) at Re = 20,000, while also delivering the highest outlet temperature of 306.2 K at Re = 4,000. Despite the rise in flow resistance, the best balance between heat transfer and frictional loss was achieved by the penta-24P setup at Re = 4,000, which recorded the maximum thermo-hydraulic performance value of 2.59. These findings suggest that while more complex rib designs enhance thermal efficiency, they also increase flow resistance. Among all tested designs, the penta-24P configuration stands out as the most efficient option for improving solar air heater performance at lower flow rates.