Direct ocean capture

GESAMP (2019) describes DOC as conceptually ‘one of the simplest [marine] CDR techniques’, However, it has not been applied at scale, and significant questions about potential feasibility and scalability remain. There are several suggested electrochemical DOC techniques, some of which are already in use at a smaller scale for different purposes (see NASEM, 2022). One often mentioned technology, that is also featured in GESAMP (2019), combines electrodialysis with a bipolar membrane.

There have been laboratory and prototype and model studies on this technique (see mainly: Eisaman et al., 2012; de Lannoy et al, 2017; Eisaman et al., 2018; Digdaya et al, 2020), although it is not the only one. Kim et al (2023) for example recently announced they found a far cheaper method for DOC without the use of expensive membranes.

In the United States, ARPA is funding a project on DOC and supports programs at California Institute of Technology, the University of North Dakota, and MIT (arpa-e.energy.gov/technologies/exploratory-topics/direct-ocean-capture). DOC is also developed by commercial companies Heimdal (https://www.heimdalccu.com/), which has built a pilot plant in Hawaii and aims to build larger ones in the following phase, and Captura (capturacorp.com), who built a first pilot plant in 2022 and aim to have a next one ready in 2023.

Although proponents of DOC often highlight that the technique would be far easier scalable than air capture technologies because of the large area the oceans offer and the greater concentration of carbon in seawater as opposed to air, many questions remain.

 NASEM (2022) gives a medium to high confidence for potential scalability of DOC, with the caveat that energy requirements may limit scaling. GESAMP (2019) note that due to the energy requirements, the potential area for DOC could be limited.

It has been suggested to combine DOC with tidal or wave energy, desalination systems, or experimental Ocean Thermal Energy Conversion. This last method generates power through temperature differences in the ocean, and is itself also still in a developmental stage (See GESAMP, 2019). If coupled with Ocean thermal energy conversion, deployment would be most efficient in tropical areas, as thermal energy generation works best there.

Costs remain very uncertain, and will be to a large degree dependent on the means of energy generation. At the moment costs are relatively high compared to other carbon capture methods, but could come down significantly in the future (Digdaya et al, 2020). Eisaman et al (2018) cost assessment found that even when combined with a desalination plant, this lowest cost assessment would likely cost $604 per tCO2, with a lowest estimate of $373. Heimdal notes that their first Hawaian version has a cost of 475$ per tonne of CO2, and that they hope to drive this down to $200 in their next version. Kim et al, (2023) recently claim to have developed an improved system that could operate at $50–$100 per ton CO2.

NASEM (2022) notes that hydrogen might be a by-product of electrochemical direct ocean capture, and could offer substantial side benefits and thereby reduce costs.

Webb et al. (2021) explored the governance issues around electrochemical direct ocean capture methods through alkalinity addition (see Ocean alkalinity enhancement), but there seem to be no direct studies on governance of direct ocean capture as described here.

Although DOC features in major reports like NASEM (2022) and GESAMP (2019) it is one of the lesser explored ocean CDR technologies, and is studied by several organizations and institutions, it remains one of the lesser known and understood CDR techniques.