Enhancement of Thermal and Exergetic Performance of Double-Pass Solar Air Heater Using Activated Carbon Nanocoating Derived from Waste Coconut Shells

The study investigates thermal improvements in a double-pass solar air heater equipped with an absorber plate treated with activated carbon nanoparticles derived from coconut shell waste at varying air flow rates. A comparison of experimental results from a black paint-coated absorber is also examined to determine the thermal performance augmentation over the modified coated absorber plate. The nanoparticle concentration on black paint is limited to 6% as the thermal conductivity of the nanofluid has no significant effect on its improvement beyond 6%, and the nanofluid becomes unstable. In the current study, airflow rates for the rectangular duct of the double-pass solar air heater (SAH) were increased for both the conventional black paint and the activated carbon nanoparticle (ACNP) absorbers. As the mass flow rates were increased from 0.029 kg/s to 0.11 kg/s, the thermal efficiency for the black paint-coated plate ranged from 23.6% to 42.43%, and for the ACNP-coated plate from 29.14% to 57.26%, respectively. With increasing flow rates, the temperature difference between the outlet and inlet air streams also decreased, from 30.5 ∘ C to 13 ∘ C for the black paint absorber and from 35.1 ∘ C to 19.1 ∘ C for the nanoparticle-coated absorber. This indicates that although at higher flow rates the convective heat transfer increases and thermal losses decrease, the lower air residence time within the collector limits the temperature rise achievable at the outlet. Integrating ACNPs led to quicker energy recovery, as evidenced by a 25% lower energy payback time (EPBT). This study validates that biomass-derived nanocoating improves thermal and second-law efficiency and serves as an economically viable, green alternative for solar air heating systems. This study demonstrates, for the first time, that waste-derived activated carbon nanoparticles from coconut shells — a circular-economy, low-cost absorber coating — can deliver thermal and second-law efficiency improvements in a double-pass SAH that are comparable to those of synthetic nanoparticle coatings, while reducing energy payback time by 25% and achieving CO 2 mitigation of up to 532 kg/m 2 ·yr, establishing biomass-derived nanocoating as a technically viable and economically competitive strategy for solar air heating applications.