Exploring the Sustainability of Direct Air Capture Technologies: An Integrated Analysis from Energy, Emergy and Environmental Perspectives

Direct air capture (DAC) technology is transitioning from the prototype stage into commercialization, yet its large-scale deployment still faces challenges related to high energy consumption and associated environmental and ecological impacts. In this study, we first develop a unified thermodynamic model for the two mainstream DAC routes, solid adsorption (S-DAC) and liquid absorption (L-DAC). Based on this consistent framework, we then conduct life cycle assessment (LCA) and emergy analysis to jointly construct a systematic sustainability evaluation framework integrating energy, emergy, and LCA methods. The Sustainability Composite Index (SCI) was proposed by integrating key indicators to provide a holistic measure for sustainability assessment across different DAC systems. Results indicate that overall, S-DAC demonstrates superior sustainability with a composite index (SCI) of 0.823, compared to 0.759 for L-DAC, although trade-offs exist across different indicators. Under the baseline scenario S-DAC performs well in emergy transformity (Tr), and second law efficiency, whereas L-DAC excels in energy consumption, net carbon removal efficiency, and total environmental impact. The choice of electricity and heat sources exerts considerable influence on the sustainability performance of both DAC systems. Among the 24 energy source scenarios, the photovoltaic power (PV) S-DAC integrated with biomass heat achieves the highest sustainability with an SCI of 0.907. Conversely, the coal-fired L-DAC system and natural gas-powered integrated solutions exhibit the poorest sustainability, with an SCI of 0.711. Notably, even when clean energy is employed, different systems still involve trade-offs across metrics. These findings underscore that comprehensive sustainability assessment is essential before scaling up DAC technologies.