Revisiting TSA and TVSA for DAC: Operating Windows, Thermodynamic Bounds, and Exergy Efficiency

Direct air capture (DAC) using solid sorbents is increasingly implemented through continuous temperature–vacuum swing adsorption (TVSA), yet the thermodynamic performance of such systems has not been systematically characterized under realistic operating conditions. This study reproduces previously published post-combustion capture and DAC cases using an independent MATLAB implementation of an equilibrium-stage temperature swing adsorption (TSA) model, confirming consistency with the original formulation. The model is then extended to DAC-relevant conditions, including ambient-temperature adsorption and vacuum-enabled regeneration, to evaluate the interdependencies between working capacity, solids circulation, regeneration heat, and blower and vacuum work. Reduced desorber pressure is identified as a strong lever for lowering sensible heat demand, increasing regeneration exergy efficiency, and enabling stable operation─with vacuum-enabled regeneration achieving net efficiencies exceeding 30%. An uncertainty analysis incorporating published Toth parametrizations and a humidity-dependent WADST coadsorption model shows that structural variability in dry isotherms dominates adsorption-stage uncertainty, whereas humidity becomes a first-order driver of absolute residual loading under TVSA regeneration conditions. At the same time, the relative spread among Toth parametrizations remains larger than the relative humidity-induced variation across the full operating range. Overall, even under idealized equilibrium-stage assumptions, the regeneration heat requirement sets a thermodynamic lower bound on TVSA DAC energy performance, while blower work continues to dominate the remaining system-level energy demand. These insights provide guidance for sorbent development, contactor design, and reactor-scale analyses of vacuum-enabled DAC systems.