7. Solving for Legacy CO2: Direct Air Capture
Direct air capture (DAC) is a process for capturing CO2 from ambient air anywhere in the world, and binding it in a stable form for long term storage. Typically, a DAC machine (or plant) uses large fans to continually suck in fresh air, and extracts CO2 by binding it in a chemical solution, which is then transferred to underground deposits for long term storage.
In terms of energy and cost, DAC is the least efficient way to sequester CO2, since it requires electricity and infrastructure to accomplish what plants do for free. However, it is far more efficient and scalable in terms of the land area required per ton of CO2 sequestered. For example, a single DAC facility from Carbon Engineering, one of the industry-leaders pulls down as much as CO2 in one year as 40 million trees, while using a mere fraction of comparative acreage.
DAC is key to removing legacy CO2, because it has no practical upper limit on the CO2 that can be captured, unlike biological methods. Trees slow their growth and die, and soil microbes hit a limit of CO2 uptake. Theoretically, DAC is only limited by Earth’s geological storage capacity for CO2, which is vast enough to store many centuries of human emissions. This means DAC can continue to pull down as much CO2 as needed, and keep sending it away for secure storage.
Furthermore, unlike biological storage, CO2 drawdown from DAC is effectively permanent. Once it’s taken out of the atmosphere, the captured CO2 won’t go back. This allows us to make lasting, long term gains in balancing Earth’s climate budget.
CarbFix injects CO2 into basaltic rocks for long term storage. Credit: CarbFix
Thus, if we’re willing to spend the money, build the industrial DAC plants and diligently store CO2 underground for thousands of years, we can. Let’s explore how large scale deployment of DAC might work.
The need for supporting infrastructure, such as power generation, transportation of captured CO2 and underground storage sites suggests that economies of scale will be necessary. Thus, clusters of DAC facilities - which I will refer to as DAC Bases - seem likely to be the best approach for project cost and development. A single DAC Base might host a few hundred individual DAC plants.
Because we want to store the CO2 safely deep underground, where DAC Bases are built will be somewhat geographic dependent. For simplicity we’ll just focus on the U.S. - which also has some of the best options for storing CO2.
Red and grey areas show viable geological storage deposits for CO2. Credit: U.S. Geological Survey
As the map above shows, there is an even distribution of CO2 storage areas across the U.S. However, the best options are probably in the lesser populated areas of the Great Basin, Rockies, Great Plains and western Texas, where land use and aesthetic conflicts are less likely.
Furthermore, many of these areas, such as the Permian Basin in Texas and the Powder River Basin in Wyoming, are accustomed to large energy developments for coal, gas and oil extraction. Rural communities are in need of investment and new industries to replace jobs and tax revenue due to declining fossil fuel extraction.
Coal Mining
DAC Bases would offer a short term economic boost locally during construction of the plants themselves, as well as energy generation and rail lines. This temporary boom would be followed by a steady flow of Federal funds for ongoing operations and maintenance.
Besides proximity to geological storage, the energy supply to run DAC plants is nearly as important. Each plant consumes significant amounts of energy to power its large fans that constantly suck in air, along generating heat to run chemical processes for binding CO2.
Direct Air Capture plant concept. Credit: Carbon Engineering
This power must be largely clean and low carbon. Even a 50% clean, 50% fossil fuel power supply might make the net CO2 sequestration benefits too marginal to warrant investment. Thus, DAC Bases need to be supplied by clean sources.
Rather than get power from the grid, whose greenhouse gas emissions vary greatly by region, DAC Bases could have their own clean energy mini-grids. Creating this infrastructure would add upfront costs, but over time may be cheaper. There would be no fees for overhead or utility transmission services, and localized power generation is much more efficient per watt, due to losses up to 50% of sending electricity over long distances.
Solar and wind, complemented by battery storage, are excellent options in many areas to supply most of the power for DAC Bases. States with good solar and wind resources, plus access to geological storage, are prime areas to create DAC Bases. These include western Texas, Montana, Colorado, Wyoming, Oklahoma and Kansas.
Solar and wind resources are abundant, cheap and can be rapidly build out. However, some amount of base load energy (i.e. available anytime) will be needed to balance out intermittent solar and wind power. Geothermal power or small hydroelectric dams could fill this need, where geographically available.
Hybrid wind and solar farm. Credit: Clearway Community Solar
It is possible that small nuclear reactors (SMRs) could play a role as well - if SMRs are as cost-competitive, passively safe and quick to construct (a relative term for nuclear energy) as their proponents claim they will be.
Additionally, as the Nuclear Regulatory Commission evaluates these new models in the years to come, they will need sites for pilot reactor builds. Remote DAC Bases could be a good option, providing a valuable outlet for energy generation while being sited far from populations centers, and offering a test case to integrate SMRs with high amounts of wind and solar generation.
Small Modular Reactor Plant Concept. Credit: NuScale Power
Adjacent to DAC Bases would be dedicated rail transport to receive sequestered CO2 and ship it off to geological storage sites. To minimize emissions, these rail lines should run on electric power and connect to the DAC Bases’ clean grid. Additionally, DAC Bases should be built as close as possible to suitable storage sites, to speed up deposits and reduce energy (and power lines) needed for rail transport.
How many DAC Bases would we need? This depends on whether global emissions rise, fall or plateau, and if we rely solely on DAC or include other methods, as proposed in this series.
With only DAC, we’d need 3,000 to 12,000 plants total, each capable of removing 1 million tons of CO2 per year (as Carbon Engineering’s design does). This would be very expensive and demand massive amounts of energy, but would still be cheaper than the climate impacts resulting from not removing CO2, according to Carbon Brief.
We will almost certainly need DAC to get back to a safe climate, but we don’t need it to do all of the work. In a drawdown portfolio strategy, we could utilize the five other sequestration methods covered in this series, to the degree that each is economically scalable and effective.
From a basic buildout of say 2000 plants, DAC Bases could be expanded or kept steady, depending on the success of other, cheaper methods. If tree planting is more successful than estimates, that many less DAC plants need to be built out. Or, if voluntary cash grants for farmers to capture CO2 in their soils aren’t adopted at sufficient rates, DAC plants can be built to pick up the slack.
In this way, DAC is the key piece that ensures we get back to safe levels of CO2, serving as the reliable backbone of the global drawdown portfolio, yet its excessive costs are trimmed by complementary sequestration methods.
