Ambient wind conditions impact on energy requirements of an offshore direct air capture plant

Abstract

This study proposes an off-grid direct air (carbon) capture (DAC) plant installed on the deck of an offshore floating wind turbine. The main objective is to understand detailed flow characteristics and CO2 dispersion around air contactors when placed in close proximity to one another. A solid sorbent DAC design is implemented using a commercially deployed air contactor configuration and sorbent. The sorbent is assumed to undergo a temperature vacuum swing adsorption cycle. Computational fluid dynamics (CFD) is used to determine the local conditions entering each unit based on varying wind speed and angle. Two-dimensional (2D) simulations were used to determine the pressure drop through a detailed air contactor design considering the sorbents APDES-NFC, Tri-PE-MCM, MIL-101(Cr)-PEI-800, and Lewatit VP OC 106. Only APDES-NFC was explored for further analysis in the three dimensional (3D) CFD dispersion model. 3D simulations were used to model flow patterns and CO2 dispersion using passive scalars. A worst case scenario is analyzed for all DAC units in adsorption mode with fans running simultaneously. 2D simulations show an under utilization of contactor length, and quantify pressure loss curves for four common sorbents. One commercially deployed sorbent is considered for further analysis; a pressure drop of 390.62 Pa is experienced for a flow velocity of 0.73 m s−1 through a 1.5m×1.5m×1.5m contactor. Using 3D simulations, fan energy demands are computed based on flow velocities and applied pressure gradients. There is found to be a decrease in overall fan power demand as wind speed increases. High wind speeds can passively drive the adsorption process with fans shut off at certain wind directions. This occurs at an average contactor inlet velocity of 17.5 m s−1, correlating to a hub height (150 m) wind speed of 24 m s−1. Thermal energy demands are computed based on inlet CO2 concentrations entering downstream units. Thermodynamic work for desorption based on an assumed second law efficiency is compared to thermal energy for desorption computed using a simplified isotherm method, taking into account the impacts of humidity on CO2 adsorption. Going from 414.72 ppm to 300 ppm CO2 inlet concentration requires an additional 63.9 kWh (t- CO2)−1 using the 2nd law thermodynamic efficiency method and 25.9 kWh (t- CO2)−1 using the isotherm method. Contactor arrangement, wind angles, and wind speeds have a significant impact on flow patterns experienced, and resulting CO2 dispersion. High wind speeds assist in CO2 dispersion, resulting in higher inlet concentrations to downstream DAC units and decreased thermal energy requirement.