Prospects For Direct Air Capture Onboard Floating Offshore Wind Turbines

Abstract

Direct air capture (DAC) is a method for removing CO2 directly from air. To date, no studies have considered installing DAC offshore. This thesis explores offshore implementation, and the changes that may be necessary in order to complete it.

First, a design using modular solid sorbent DAC units is proposed onboard the deck of a floating offshore 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. Two dimensional (2D) and three dimensional (3D) com- putational fluid dynamics is used. The pressure drop through a representative com- mercial scale unit was found using 2D simulations. Various adsorbents commonly used in practice are examined, with a pressure loss curve obtained for each one. 3D simulations analyze the impact of ambient wind conditions for local CO2 mixing, as well the reduction in necessary fan work. Wind speeds at hub height (150m) greater than 24m/s, allowed fans to be turned off in upstream contactors; passively blowing air through the contactors using ambient wind. Thermal energy had a similar rela- tionship; high wind speeds induced increased CO2 mixing, thus greater concentration entering downstream units.

Second, design and energy demand changes from operating DAC offshore were ex- plored. Parallel technologies offered insights, and further experimental work is sug- gested to bridge knowledge gaps. A configuration for air pre-treatment to remove aerosol salt particles contained in sea air is examined. Using a wire mesh demister pad to capture salt particles contained in the air, liquid solvent DAC systems had a greater change in pressure loss and fan energy requirement. An additional 79% pressure loss, and 194.4kWh/t-CO2 of fan energy was incurred. Air pre-treatment is more pertinent with regards to solid sorbent DAC, resulting in an additional pressure loss of 20-28%, and fan energy input of 38.1kWh/t-CO2. Solid sorbent DAC is likely more susceptible to performance changes as a result of aerosol salt particles contained in sea air, and aqueous based DAC more sensitive to dynamic motions encountered on floating platforms. Size, modularity, and lack of feed stock requirements lend solid sorbent DAC better to implementation far offshore, onboard floating platforms. Aqueous based DAC systems are likely restricted to shore/shore side deployment due to large unit sizes; and feed stock and waste transportation limitations.