Growing energy needs from a thriving global economy, and the damage done by burning fossil fuels, are two of the world's biggest problems. Renewable energy sources provide a solution to both of these issues. Over the last decade, there has been a surge of interest in this area from government officials, financiers, and academics. A solar air heater (SAH) is an appliance that makes use of solar thermal technology to lessen the load on electrical use for tasks like heating, cooling, and drying in homes, businesses, farms, factories, and other settings. Modifying the SAH-duct route and the absorber plate may increase thermal efficiency, which is the main issue with SAH. The SAH-domain has never seen the use of transverse vortex generators (TVGs), despite their widespread use in basic research, gas turbine blades, and micro-channels. This study optimises the smooth rectangular duct of a solar air heater (SAH) geometrically in order to improve the SAH's thermal-hydraulic performance using in-line circular thermo-viscous gradients (TVGs). This work paves the way for further investigations into the use of TVGs to improve heat transmission in the SAHdomain. Some potential directions for further study are listed below. There needs to be a 3D numerical study to see the effect of side-walls, a more advanced experimental study using high-speed cameras and thermal sensors, a fluid-structure-interaction study for vortexinducedvibrations to further improve heat transfer, and TVGs with a variety of cross-sections (square, rectangle, ellipse, etc.) arranged in a variety of configurations (side-by-side, staggered, etc.)