Multifunctional solar tower for CO2 reduction: A CFD analysis of photocatalytic methanol synthesis enhanced by natural convection dynamics

This study presents a solar tower photoreactor for CO 2 reduction to methanol that operates without any external energy input. The reactor combines transparent upper walls, which admit solar radiation to a bed of photocatalyst-coated spheres, with an absorber floor that converts unused radiation to heat and drives natural convection through the bed. A computational model coupling Monte Carlo ray tracing, surface reaction kinetics, heat transfer, and buoyancy-driven flow was validated in two parts against independent experiments, giving a mean absolute percentage error of 6.4% for the photocatalytic kinetics and 2.7% for the thermal convection behaviour. The validated model was then applied over a full diurnal cycle (240–1050 W/m 2 , 7:00–19:00) using measured solar irradiation and ambient temperature. The kinetics are treated as temperature-independent, since the source experiments were isothermal, so all temperature effects act through the flow field. Inlet water vapour content was the controlling operating variable. Tripling its mole fraction from 0.02 to 0.06 tripled both the reaction rate, from 25.1 to 75.2 μmol/(m 3 ·s), and the outlet methanol concentration, from 57.4 to 172.6 mmol/m 3 . Raising the inlet gas temperature from 30 to 50 °C lowered the outlet concentration from 57.4 to 52.4 mmol/m 3 , a loss of 8.4%, because a warmer feed weakens the draft. Raising the porous ratio from 0.25 to 0.75 increased the outlet concentration from 5.14 to 57.4 mmol/m 3 and the chemical energy efficiency from 0.88% to 2.38%, through a larger catalyst inventory and a longer residence time. Efficiency reached 7.10% at the highest water vapour fraction. From an aperture of 1963 m 2 , the tower produced about 62 kg/day at baseline conditions and 186 kg/day at the highest water vapour fraction, while sustaining gas velocities of 0.3–0.8 m/s and a stoichiometric oxygen-to-methanol ratio of 1.50 with no pumping.