Flat-plate solar collectors (FPSCs) are widely used in solar thermal applications, yet their efficiency is often hindered by conventional circular tube geometries and the limited thermal properties of water. This study introduces a novel approach by utilizing triangular-shaped riser tubes and nanofluids (Al2O3-water and CuO-water at 2% volume concentration). The simulation, conducted via Computational Fluid Dynamics (CFD), focuses on the climatic conditions of Baghdad, Iraq, during summer (June, July, August) and winter (January). The results demonstrate that triangular tubes enhance the effective heat transfer area and promote improved convective heat transfer characteristics. The CuO-water nanofluid achieved a maximum outlet temperature of approximately 423 K at a low flow velocity of 0.005 m/s. Furthermore, the combined use of triangular tube geometry and CuO-water nanofluid resulted in thermal efficiency improvements of up to 30–35% compared to conventional circular tubes operating with water under identical conditions.