A multifunctional rooftop unit for direct air capture†‡

Direct air capture (DAC), which captures CO₂ from ambient air, is a critical technology to reduce greenhouse gases in the atmosphere in order to avoid climate disasters. Due to the relatively low concentration of CO₂ (400 ppm), a large amount of air needs to be moved through DAC devices, which requires lots of energy. Currently, DAC technologies are deployed mainly in centralized systems and require extensive infrastructure and initial capital cost. A potential solution is to utilize existing infrastructure for DAC. In this study, we propose a distributed DAC system that utilizes existing commercial rooftop heating and air conditioning (HVAC) units to capture CO₂ from the air. There are approximately 15 million such units already installed on commercial buildings in the United States, and they move a large amount of air every day. Adding DAC functionality to these units will significantly reduce the cost of infrastructure and operation. A modular approach was used to introduce DAC into a rooftop unit. Modules filled with triethylenetetramine-functionalized polyacrylonitrile sheets were developed and installed on the condenser coil side of the rooftop unit. The rooftop unit with DAC functions effectively captured CO₂ from the air, and the addition of the DAC modules had little effect on the unit's original functionality. A preliminary techno-economic analysis was also conducted, and the results potentially suggest that utilizing existing commercial rooftop units for carbon capture is a feasible approach to reducing greenhouse gases.