Direct Ocean Capture (DOC) and Direct Air Capture (DAC) both remove CO₂ from the atmosphere, and both need energy, equipment and storage. The difference starts with the feedstock: air contains about 426 parts per million of CO₂, while seawater carries dissolved inorganic carbon at roughly 150 times that concentration. Captura's white paper makes this the first line of its case, and it is the right place to start.
Concentration is a physical advantage
Every capture system has to move a large volume of a dilute medium past a capture surface. If the medium is 150 times more concentrated, the equipment can be smaller, or the same equipment can process more carbon in the same time. That is not a marketing claim; it is the reason DOC can use a compact electrochemical stack rather than a large air contactor.
No sorbents, no additives
DOC uses pH swings to convert dissolved inorganic carbon into CO₂, then returns the water with its alkalinity restored. Captura's process uses bipolar membrane electrodialysis, diverts about 0.5% of the flow for softening, and adds no sorbents or chemical additives. DAC, by contrast, depends on solid sorbents or liquid solvents, each with its own supply chain, degradation and regeneration energy. That is a real difference in system complexity, not just in chemistry.
The energy boundary problem
Comparisons of energy use are where most public claims go wrong. Captura's white paper cites about 1,800 kWh per tonne of CO₂ for the DOC system. Our own project estimates use 3,000–3,200 kWh per tonne for the full plant, because they include intake, compression and liquefaction. DAC systems also need compression and storage, and published DAC energy figures vary widely with design and boundary.
The honest comparison is therefore full-plant to full-plant, including everything up to the storage wellhead. On that basis, the gap between DOC and DAC is narrower than the concentration ratio suggests, and site-specific power price matters more than the headline number. This is why our Economics page publishes ranges rather than a single figure.
Where DOC wins
- Concentration. Smaller equipment for the same mass of carbon.
- Land and infrastructure. DOC can share intake and outfall with power plants, desalination plants and ports, instead of needing a large air-contact field.
- No sorbent supply chain. Fewer consumables to manufacture, ship and replace.
- Coastal industrial symbiosis. Waste heat, renewable power and storage can be arranged around an existing industrial site.
Where DAC wins
- Location independence. DAC does not need a coast, a specific current regime or a discharge permit.
- Simpler MRV. CO₂ is measured at the equipment; ocean-based removal requires ocean models and near-field and far-field monitoring, as the Isometric DOCS protocol sets out.
- Mature methodology landscape. DAC has a longer record of registered projects and third-party methodologies.
- Modularity. DAC plants can be sited close to storage or to cheap power without a coastline.
MRV is where the two diverge most
For DAC, the quantity of CO₂ captured is measured at the plant boundary; the hard questions are energy provenance and storage permanence. For DOC, the captured CO₂ is also measured, but the net removal depends on what the ocean does with the returned water. That is why DOCS requires a validated biogeochemical model, four spatial domains and a monitoring plan with stop conditions. The MRV burden is heavier, and it is also the reason a DOC project cannot be evaluated with a DAC checklist.
Siting implications in China
For a Chinese project, the DOC constraints are concrete: a coast with fast, well-mixed water; a short intake; a grid connection that can absorb a flexible load; and a storage route within reasonable distance. The Lingao project is built around exactly those four elements. DAC would face a different constraint set — more land, no seawater, but the same storage and power questions.
The conclusion is not “DOC is better”
It is that the two technologies are suited to different sites. DOC turns a coastline into an advantage; DAC turns location independence into an advantage. A portfolio that includes both is more robust than an argument about which one wins. What matters for buyers is that each tonne is measured on a consistent boundary, under a methodology that survives due diligence.
Sources
- Captura, technology white paper (2024), including the ~1,800 kWh/t system energy figure and the concentration comparison.
- Isometric, Direct Ocean Capture and Storage v1.0 (October 2025).
- Carbonjour project estimates for the Lingao phase 1 (internal, ranges published on the Economics page).