Global performance analysis of a solar-driven indoor CO₂/H₂O capture system for air quality enhancement and cooling energy saving

A human-eco-friendly building requires high indoor air quality (IAQ) and low carbon emissions. Existing natural/mechanical ventilation methods rely on frequent exchange with fresh ambient air to remove excess respiratory CO₂ for high IAQ. However, in tropical or subtropical regions, the introduced ambient air features higher humidity and temperature over indoor air, which inevitably increases the energy burden of air conditioning (AC) accompanied with severe CO₂ emission. To circumvent this paradox, this study developed a dynamic solar-driven indoor CO₂/H₂O capture system with selective air recirculation. Rather than simply introducing ambient air to reduce indoor CO₂ concentration, this scheme actively captures CO₂/H₂O and selectively returns post-captured air with sub-ambient temperature, relative humidity and CO₂ concentration, thus sustaining high IAQ with reduced AC load and CO₂ emission. The system's feasibility and stability are demonstrated through a 20-city global study and a 10-variable sensitivity analysis. Compared to traditional methods, this new system could save up to 64 % of cooling energy while maintaining the indoor CO₂ concentration below 1000 ppm and relative humidity within 25 %-55 %. The energy-saving potential of the proposed system increases in regions with larger AC demands, as evidenced by Dakar achieving the highest energy-saving potential and thus decreasing CO₂ emission. This system could further reduce CO₂ emission by direct capture and storage, whose amount is positively correlated to the amount of solar irradiance in cities. Furthermore, the system demonstrates robustness under off-design conditions and is flexible to be scaled up/down. Overall, this study provides effective-green ventilation strategies and paves the way for developing human-eco-friendly buildings.