Integrated DAC-HVAC systems for CO₂ capture and sustainable hydrogen production from condensed water

Carbon dioxide removal is essential to meet net-zero targets and stay within the 1.5 °C climate goal, with Direct Air Capture (DAC) technologies playing a critical role in removing both current and historical emissions. In parallel, low/zero-carbon fuels such as green hydrogen play a crucial role in decarbonizing hard-to-abate sectors and enabling a clean energy transition. This study introduces a novel integration of DAC technology within building Heating, Ventilation, and Air Conditioning (HVAC) systems, designed to recover water from the DAC cycle for sustainable hydrogen production. In the Temperature-Vacuum Swing Adsorption (TVSA) process, water condenses as a byproduct during the desorption phase. This condensed water can be repurposed as a feedstock for green hydrogen production via electrolysis, offering a sustainable water input for proton exchange membrane (PEM). By coupling DAC with hydrogen generation, the system supports both negative emissions and clean fuel production. The HVAC-integrated DAC system reduces energy demand by leveraging the building's exhaust air stream, which exhibits relatively stable temperature and humidity levels. This integration not only lowers the thermal requirements of the DAC process but also reduces HVAC energy consumption through increased air recirculation. In addition to energy benefits, the system contributes to improved indoor air quality by removing carbon dioxide from indoor environments, where concentrations often exceed 1000 ppm. The system uses a solid sorbent of SBA-15 functionalized with tetraethylenepentamine (TEPA) in a TVSA cycle. A thermodynamic analysis was conducted, which was used in the economic analysis to size equipment, estimate their base costs, and quantify operational costs. The economic assessment showed a levelized cost of CO2 capture (LCOC) of 221 $/ton, and a levelized cost of hydrogen production (LCOH) of 5.95 $/kg H₂ with Qatar's grid electricity and 14.71 $/kg H₂ with solar photovoltaic energy. These results suggest a financially viable pathway for carbon removal, with the potential for further improvements by integrating a hydrogen production unit using the recovered water.