Predicting Land Usage, Optimal Design and Performance of Large-Scale Deployments of Direct Air Capture Devices

Abstract:

Recent research on direct air capture (DAC) has focused primarily on fundamental aspects including chemical reactions and heat transfer processes, in order to optimize the performance of individual CO₂ absorbers. However, deploying CO₂ absorbers in large arrays will introduce a key additional issue: after the air is scrubbed of CO₂ through the first absorber, the following ones must work with inlet air that has even lower CO₂ concentration, thereby increasing their energy requirement for capture and separation. This constraint is analogous to that found in the context of power extraction in large wind farms. Minimizing this adverse effect requires increasing the spacing between absorbers, leading to increased land requirements and deployment costs. However, there appear to be no published models for predicting the performance of a large array of DAC units. To address this issue, we introduce a multi-scale theory for the aerodynamics of large scale direct air capture. We include effects of wind speed, atmospheric turbulence intensity, absorber geometry, capture processes, and carbon fluxes. For definiteness, we consider the well-documented individual absorber design proposed by Carbon Engineering, although our theory can also be extended to other designs. We obtain an explicit expression for CO₂ capture by a large array, which combines single-absorber performance with effects of windspeed, atmospheric turbulence, and layout geometry. We find that absorbers optimized for array operation may need to be significantly thinner than individually-optimized ones, and that this redesign may enable operation without fan assistance in strong winds, thereby reducing energy usage. In addition, overall capture is greatly improved by siting the array in high-wind locations, due to the enhancement in overhead carbon transport and in turbulent mixing. Our findings provide a quantitative link between atmospheric dynamics and the performance of large-scale DAC deployments.