The increasing future energy demands can be met with solar-based devices such as solar air heaters (SAHs) only if the efficiency of these devices is enhanced with suitable design changes. Flow channel design of a SAH is an essential aspect for enhancing its thermohydraulic performance for a wide range of Reynolds numbers. In this article, a systematic approach has been adopted to investigate various nonrectangular channel designs numerically, and results are compared with the conventional rectangular design. The energy input to all designs is kept constant. The channel design that gives the best performance was further investigated by incorporating a sinusoidal wavy absorber having variable wavy roughness parameters. The flow and heat transfer characteristics have been evaluated in terms of friction factor (f) and Nusselt number per unit friction factor (Nu/f), temperature factor (To–Ti)/I, thermal effectiveness (ɛ), and Nusselt number (Nu). The results show that the SAH duct having a semi-ellipse cross section offers the best thermohydraulic performance and has maximum augmentation in the temperature factor of about 10% compared to conventional SAH. Moreover, semi-ellipse SAH with sinusoidal wavy absorber has a maximum value of f/fs and Nu/Nus at A/Dh = 0.12 and λ/Dh = 0.8 for the range of Reynolds numbers used, respectively. Here, A is the amplitude and λ is the wavelength of the absorber plate. New empirical relationships for Nu and f are established as a function of flow and geometric parameters that agree well with numerical results.