<p>The paper deals with natural and mixed convection in a vertical rectangular duct exposed to solar radiation. The physical problem under consideration describes flow and heat transfer processes in a water-filled glazing chamber irradiated from the side. Due to the absorption properties of water, the near-infrared irradiance initiates a non-uniform volumetric heat source described by the Beer-Lambert law. Based on the Navier-Stokes equations with Boussinesq approximation and the energy equation, a mathematical problem for well-developed viscous flows in a rectangular duct is formulated. New exact analytical solutions in series are obtained, which include terms accounting for the effect of the heat source. The influence of the duct aspect ratio on the temperature and velocity fields is evaluated. While natural convection flow is always reversible, in the case of combined free and forced convection, a condition for the appearance of flow reversal is derived. Hydrodynamic and thermal characteristics such as Fanning friction factor, bulk liquid temperature, and Nusselt numbers at the walls are determined. The influence of the lateral walls diminishes with the increase of the aspect ratio, and when the duct is sufficiently narrow, the thermal characteristics of the flow approach the corresponding values for a plane-parallel channel.</p>