Captura's white paper cites about 1,800 kWh per tonne of CO₂ for its DOC system. Our own project estimates use 3,000–3,200 kWh per tonne. Readers sometimes treat this as a contradiction. It is not: the two figures measure different things, and understanding the difference is the first step to comparing capture technologies honestly.
What the 1,800 figure covers
The white paper figure is a system energy number: the electricity the DOC process itself needs to move seawater through the electrochemical stack, swing the pH, and release the CO₂. It is the right number for asking whether the core technology is efficient. It is not the number for asking what a plant will draw from the grid.
What the full-plant figure adds
A complete plant has to do more than the core process. The gap between system energy and full-plant energy comes from steps that are easy to forget when comparing cells and stacks:
- Intake and outfall. Pumping seawater through a real intake, screen and outfall costs energy, and the head loss depends on the site.
- Pretreatment and softening. The diverted stream needs conditioning before it reaches the membranes.
- Degassing and vacuum. Extracting CO₂ from water requires vacuum and gas handling.
- Compression. CO₂ must be raised to pipeline or injection pressure, which is a significant load per tonne.
- Liquefaction, if used. Shipping CO₂ as a liquid adds a further energy penalty.
- Auxiliaries and part-load. Controls, cooling, buildings and operation away from the design point all add consumption.
Each line is small next to the electrochemical stack; together they are not. That is how a system figure of about 1,800 kWh/t becomes a full-plant figure of about 3,000–3,200 kWh/t.
Gross versus net makes it worse
There is a second denominator problem. A 50,000 t/yr plant captures about 50,000 tonnes gross, but net removal is lower — about 43,000 tonnes in our phase 1 estimate — because life-cycle emissions from construction, power supply and transport are deducted. Energy per gross tonne and energy per net tonne therefore differ by roughly 15%. Publishing one without saying which is a common source of confusion.
How to compare technologies
If the question is “which technology is more energy-efficient”, compare on a consistent, full-plant boundary, per net tonne, including compression and storage. Captura's white paper compares its projections with other nascent CDR technologies; that comparison is a projection, not a measured head-to-head. Published DAC energy figures vary widely with design, sorbent and boundary, so a single number from one paper should not be set against a single number from another.
A better comparison also includes power price, load factor and the emissions intensity of the electricity. A plant using 3,200 kWh/t on a low-carbon grid can have a lower net footprint than a plant using 1,800 kWh/t on a coal-heavy one.
Why we publish ranges
Because a single number invites the wrong comparison. Our Economics page gives CAPEX, power price, electricity use and full cost as ranges, with the assumptions stated, and a calculator that lets a reader test their own inputs. The full model, with the line-by-line energy breakdown, is available under NDA.
Three questions to ask any DOC developer
- Does the energy figure include intake, compression and storage, or only the capture system?
- Is it per gross tonne or per net tonne removed?
- What load factor and grid emissions intensity were assumed?
If the answers are not on the page, the number is not comparable — however precise it looks.
Sources
- Captura, technology white paper (2024): ~1,800 kWh/t system energy.
- Carbonjour phase 1 project estimate (internal): 3,000–3,200 kWh/t full plant, gross and net removal assumptions.
- Carbonjour, Economics page, for the published ranges and assumptions.