Conceptual Design of an Underwater Direct Seawater Capturing for Enhanced Ocean Carbon Storage

To achieve carbon neutrality, it is essential not only to reduce emissions but also to capture low-concentration carbon dioxide (CO2) already released into the atmosphere. The oceans act as a vast carbon sink, absorbing approximately one-fourth of the earth's CO2, with the amount of carbon contained in seawater being about 120 times denser than that of the atmosphere. In order to indirectly capture low-concentration CO2 from the atmosphere via the oceans, a system for capturing high-concentration CO2 from seawater was investigated. The CO2 capture from seawater was achieved by employing an electrodialyzer to separate dissolved CO2 from acidified seawater and then adsorb it onto an adsorbent. The estimated system collocated with floating offshore wind platforms indicates that, for the planned 20,000 offshore wind turbines in Japan by 2040, each system would require a CO2 capture capacity of approximately 36 tons of CO2 per day. This target requires 2–4 times higher energy efficiency and a reduction in size compared to current capabilities. In the study of such system, a prototype utilizing an electrodialyzer to capture CO2 from real seawater was developed, and its performance was evaluated. The energy efficiency was assessed by varying the applied voltage to the electrodialyzer. When a current density of 13.2 mA/cm² was applied, the system achieved the energy efficiency of 139.6 kJ/mol-CO2, which is a favorable result compared to prior research.