Before carbon capture can clean atmosphere at scale, one bottleneck may decide whether it succeeds - Energy & Green Tech

Credit: Joule (2026). DOI: 10.1016/j.joule.2026.102424

In 2024, global average temperatures exceeded 1.5^o C above pre-industrial levels for the first time. This threshold was set as an aspirational limit by the 2015 Paris Agreement and was considered a line beyond which the impacts of climate change on ecosystems and human vulnerability become stark. Crossing this threshold is a signal that reducing emissions alone will not be enough. Increasingly, scientists, engineers, and policymakers around the globe agree that we will need to actively pull carbon dioxide (CO₂) out of the atmosphere to help reduce the impacts of this pollutant.

The scale of this task is vast. The International Energy Agency projections suggest that reaching net-zero emissions by 2050 will require removing around one billion tons of CO₂ from the atmosphere every year. A billion tons of CO₂ is roughly equivalent to the annual CO₂ output of the entire global aviation industry. This vast amount needs to not only be offset from the system but fully removed from it.

This is the problem that has inspired a collaborative team of researchers at RASEI, including RASEI Fellows Prof. Wilson Smith and Prof. Bri-Mathias Hodge, and is the subject of a recent collaborative report published in Joule.

Two ways to catch carbon

Researchers are exploring a number of ways to pull CO₂ directly from the environment, and this comparative study looks at two of them side by side. The first, direct air capture (DAC), draws air from the atmosphere through a liquid solution that absorbs CO₂, analogous to a large-scale filter. It is the more established of the two approaches, with the world's largest DAC facility currently under construction, a plant in Texas designed to remove 500,000 tons of CO₂ per year.

The second approach examined in this study, direct ocean capture (DOC), is less developed but works with a natural advantage: it is estimated that the oceans absorb around 30% of the CO₂ that human activity produces each year, meaning seawater is already rich in dissolved carbon that originated in the atmosphere. By extracting that carbon directly from seawater, DOC bypasses the need to process enormous volumes of air. In fact, this advantage is one of the main reasons why many researchers are evaluating the feasibility of DOC as a CO₂ removal solution.

Both approaches share a common challenge: Once you have captured the CO₂ from air, you need to do something with it. The regeneration process releases concentrated CO₂ in a usable form, while also recovering the capture solvent. In most current DAC systems, this process requires heating the captured material up to around 900^o C, typically by burning natural gas. This process is energy intensive and creates its own greenhouse gas emissions, somewhat undermining the overall carbon capture process.

To try and understand the impacts of this overall process, the RASEI team modeled what happens when you substitute the heat-based regeneration setup with an electricity-driven alternative called bipolar membrane electrodialysis, or BPMED. Instead of using heat to release the CO₂, BPMED uses electricity to shift the chemistry of the captured solution, enabling the release of CO₂ at ambient temperatures. The key question the team sought to answer was whether this substitution makes economic sense when integrated with DAC and DOC, and under what kinds of conditions.

Building the model

To assess the DAC and DOC pathways, the team built a portfolio of connected models, starting from the physics of how CO₂ is captured and released, moving through the energy demands of each step, all the way up to a full cost analysis. This kind of approach, known as a techno-economic analysis (TEA), links the technical performance of a process directly to its economics. A TEA allows you to not just explore whether something works but also gain insight into whether it is viable at scale and under real-world conditions.

A particular strength of this study is the level at which the models connect these dots. As lead author Dr. Hussain Almajed puts it, the goal was to compare the two approaches "not to say which one is the winner, which one is the loser, but to highlight the trade-offs."

The team pulled data from the California electricity grid, modeled different power supply scenarios, and ran both the DAC-BPMED and DOC-BPMED systems through the same framework. This provided a side-by-side comparison, one that had not previously been explored, that produced some unanticipated observations.

Two technologies, two cost profiles

The comparative study revealed a foundational trade-off rooted in a fundamental difference between DAC and DOC: concentration. Air contains about 120 times less carbon than seawater, requiring large volumes of air to be processed at every iteration. However, once the CO₂ is captured via a liquid solvent, typically a hydroxide, the comparison reverses. A typical liter of DAC solution contains 0.5 to 1.0 moles of dissolved carbon, which is roughly 160 to 320 times higher than the dissolved carbon in a liter of seawater.

That means a DAC plant needs to process far less liquid to recover a given amount of CO₂ compared to DOC, but extracting carbon from such a concentrated solution requires running the BPMED part of the system at high intensity, at high electrical current, which consumes significant energy. The equipment footprint is relatively small, but the electricity bill is high.

DOC works the other way around. Because seawater holds less dissolved carbon compared to a DAC solution, a DOC plant must process vast amounts of seawater to recover the same amount of CO₂. The models estimate that DOC-BPMED would need roughly 20 times more membrane area than the equivalent DAC-BPMED system, representing a significant upfront investment. On the other hand, the electrically driven process can run at a much lower current when handling dilute seawater, using considerably less energy per ton of CO₂ captured.

These differences are obvious in the cost estimates. For a plant capturing 100,000 tons of CO₂ per year, and connected to the current California electricity grid, the modeled cost of capture via DAC-BPMED came in at around $470 per ton of CO₂ in the baseline case. For DOC-BPMED, the equivalent figure was around $1,500 per ton, roughly three times higher. This is driven largely by the upfront cost of all the additional equipment, and not the energy use.

The authors are careful to state that these modeled estimates have a meaningful level of uncertainty built in, and they will shift as the underlying technologies mature. But the overall trends are clear. At present, and with the current equipment costs, DAC-BPMED has a significant cost advantage over DOC-BPMED under this electrically driven regeneration approach.

Unexpected potential routes to profitability

A finding that stood out from these models was an often overlooked commodity side product. The BPMED process works by using electricity to split a salt solution into an acidic stream, which is used to release CO₂, and a basic stream which produces sodium hydroxide (NaOH). Sodium hydroxide is a widely used industrial chemical, a commodity found in a range of industries such as paper manufacturing, water treatment, and chemical synthesis, with an established market value, averaged at around $450 per ton.

In the DOC model, because the plant is processing such large volumes of seawater, it produces considerably more sodium hydroxide than it needs for its operation. The models show that selling that surplus could reduce the cost of the overall CO₂ capture process substantially. In a scenario projecting a largely decarbonized electricity grid by 2050, the revenue generated from sodium hydroxide sales was enough to fully offset the costs of the CO₂ capture process, and in the most optimistic scenario, the process showed a net profit.

The authors were candid about the limits of this finding. The global sodium hydroxide market, even accounting for projected growth, is not large enough to absorb the products from carbon capture at the scale required to make a meaningful dent in atmospheric CO₂.

"Our brief market analysis showed that even if DOC-BPMED supplied 20% of the projected 2050 sodium hydroxide demand, it would still offset less than 0.1% of today's global energy emissions," Dr. Almajed said.

But the principle illustrated by this finding has broader implications. Coupling carbon capture with the production of a valuable commodity, either carbon-based or as a side product, could be a viable route to improving the economics of the whole process. It is an approach that is already being pursued commercially, including by Travertine Tech, a company based in Boulder, Colorado, which captures CO₂ while producing and selling phosphoric acid, gypsum, and cementitious materials.

The electricity issue

Because the BPMED regeneration process is driven entirely by electricity, the source of that electricity matters enormously. This impacts both the cost of the process, and whether it actually delivers a net reduction in atmospheric CO₂. A carbon capture plant powered by fossil-fuel-generated electricity that itself emits CO₂ is self-defeating.

To explore how different electricity generation modes impact the overall process, the team modeled four power supply scenarios: the current California grid, a projected 2050 California grid operating at 95% decarbonization, and two off-grid options: dedicated wind and dedicated solar. Interestingly, the team found that connecting to the grid outperformed both off-grid renewable options on cost, in both the current and the projected scenarios.

The authors suggest that in the model this is down to a matter of reliability; a grid-connected plant can essentially run continuously, spreading its capital costs across more operating hours. A plant running on dedicated solar or wind is constrained by intermittency, which can drive up the cost per ton of CO₂ captured.

Dr. Almajed highlights that this is an area of the model that could be expanded, explaining, "We just looked at solar or wind each by itself; we didn't optimize the off-grid scenarios to include energy storage and batteries."

The policy implication built from the observations across the model is clear, explains Dr. Almajed: "We need to really pursue grid decarbonization. We need cleaner energy to power technologies that are going to help address climate change."

Technologies such as DAC- and DOC-BPMED do not operate in isolation from the broader energy system. The effectiveness of these technologies to help combat atmospheric pollution, both economically and technically, is critically dependent on the grid into which they are plugged. Decarbonizing that grid is not a separate problem. It is a prerequisite.

The future of carbon capture

While there are a lot of valuable observations and ideas that have come out of this TEA, no model is perfect. The team was quick to clarify areas where their model could be refined as technologies and ideas evolve.

"When technologies are in such a nascent stage, the analysis of these models should focus on qualitative rather than quantitative insights," explains Prof. Bri-Mathias Hodge. "While there are a number of areas where the model can be improved, it also suggests where efforts for improvements are best focused, particularly the aspects that have the largest impact on results."

This includes more detailed modeling of the membranes, better data on equipment costs as the technology matures and is more widely deployed, and a more complete optimization of how these carbon capture plants might interact with energy storage or hybrid power systems. Many of these are manageable problems, and work is already underway at RASEI to address some of these areas.

Sometimes, the real value in this kind of analysis is in what it reveals before such refinements are made. By mapping the full system, from the technical fundamentals through the macroscale economics, this study helps to identify where research effort is best directed. Enhancing the concentration of the dissolved carbon in the seawater fed into a DOC plant, for example, could reduce costs by 40–50%, according to the study's sensitivity analysis.

As a technology that is beginning to be deployed and scaled, identifying areas where large improvements in process efficiency can be made could have significant energy and cost savings. Dr. Almajed notes, "The study generated a lot of insights that we didn't even consider at the start of the project."

Removing carbon from the atmosphere at the scale required to significantly impact global emissions is an interdisciplinary problem that spans chemistry, engineering, economics, and energy policy. Analyses such as this don't necessarily resolve that complexity, but they do help to make it understandable, and act as a roadmap to focus efforts. Knowing where the bottlenecks are, and insights into what it would take to impact them, is a great way to start solving the problem. 

Provided by University of Colorado at Boulder 

by University of Colorado at Boulder

edited by Stephanie Baum, reviewed by Robert Egan 

Source: Before carbon capture can clean atmosphere at scale, one bottleneck may decide whether it succeeds