Direct Air Capture Explained in 10 Questions

Direct Air Capture ("DAC") systems are an emerging class of technologies capable of separating carbon dioxide (CO2) directly from ambient air at large scale. Want to learn more about how DAC systems work and how they can help fight climate change and create a circular economy? We've got 10 Q's and A's below to get you started: 

1.      How do DAC systems work? DAC systems can be thought of as artificial trees. Where trees extract CO2 from the air using photosynthesis, DAC systems extract CO2 from the air using chemicals that bind to CO2 but not to other atmospheric chemicals (such as nitrogen and oxygen). As air passes over the chemicals used in DAC systems, CO2 "sticks" to these chemicals. When energy is added to the system, the purified CO2 "unsticks" from the chemicals, and the chemicals can then be redeployed to capture more CO2 from the air. Check out the video below explaining how Climeworks's DAC system works:

2.      What type of carbon management technology is DAC? DAC systems can be classified as carbon "recycling" or carbon "removal" technologies, depending on what happens with the purified CO2 that the DAC system produces.

  • Recycling: CO2 produced by DAC can be recycled into fuels or other products that release CO2 back into the atmosphere quickly after their use (such as greenhouses, carbonated beverages, etc.). As a carbon recycling tool, DAC systems can provide an important component of a circular economy, where the sky is mined for the raw inputs used in subsequent manufacturing processes.
  • Removal: CO2 produced by DAC that is sequestered in geologic formations underground or in materials that do not allow CO2 to escape into the atmosphere (such as cements or plastics) can generate negative carbon emissions.
Above: a visualization of what a commercial-scale DAC plant might look like, via Carbon Engineering.

Above: a visualization of what a commercial-scale DAC plant might look like, via Carbon Engineering.

3.      Are DAC systems classified as energy- or manufacturing-sector technologies? Unfortunately, DAC systems defy easy industry classification. DAC systems can be used to generate the inputs for manufacturing processes. But DAC systems also can operate in similar fashion to energy-sector carbon capture and storage (CCS) technologies. As a result, DAC systems can be considered an energy-sector technology, a manufacturing-sector technology--or both--depending on how it is used.

4.      What are the pros and cons of DAC as a carbon management technology?

  • Pros: Because DAC systems do not need to be sited directly at power plants, they can be sited close to sequestration/manufacturing sites, eliminating the sometimes costly CO2 transportation step associated. In addition, DAC systems take up a relatively small land footprint. A study by the American Physical Society showed that a square kilometer of DAC machines could generate around 1 million tons of CO2/year (meaning that 3 sq-km of DAC projects could offset the same amount of coal power that the Topaz Solar Field does using over 25 sq-km of land)
The APS report shows that DAC systems can take up relatively little land compared to other renewable energy technologies such as solar or wind.

The APS report shows that DAC systems can take up relatively little land compared to other renewable energy technologies such as solar or wind.

In addition, DAC systems require no biomass inputs, so there is little competition for agricultural land (as there is with other leading carbon removal approaches).

  • Cons: High costs compared to other greenhouse gas abatement approaches.

5.      What organizations are building DAC systems today? The idea of separating CO2 from air is not new, and has been done on submarines and in space applications for decades (it would be impossible to breathe in these closed environments without CO2 capture from air). That said, large-scale DAC systems used for carbon management purposes are only beginning to emerge today, and there are no commercial-scale deployments of DAC systems as of this writing. Today, there are four leading commercial DAC system development efforts, along with one academic center pursuing DAC research:

a.      Carbon Engineering: Based in BC, Canada, Carbon Engineering is pursuing a liquid potassium hydroxide based system. They have a pilot plant in Squamish, BC set for an October, 2015 launch date.

b.      Climeworks: Based in Zurich, Switzerland, Climeworks is employing a novel sorbent coupled with a temperature swing to release the captured CO2. Climeworks has inked a commercial partnerships for CO2 recycling with Sunfire and Audi, and are building a 1,000 ton-per-year plant in Germany to supply a greenhouse with CO2 for its operations.

c.      Global Thermostat: Based in CA, USA, Global Thermostat is pursuing a DAC technology based on proprietary amine sorbents with a temperature swing for regeneration. Global Thermostat has a pilot plant up and running at the SRI headquarters in Menlo Park, CA.

d.      Infinitree: Based in NY, USA, Infinitree is using a humidity swing process for concentrating CO2. They are targeting the greenhouse market for initial customers. This technology is based on the DAC system developed by now-bankrupt Kilimanjaro Energy (formerly Global Research Technologies).

e.      Center for Negative Carbon Emissions at ASU: based in AZ, USA, this academic group headed by professor Klaus Lackner is developing a DAC technology based on a humidity swing process.

Global Thermostat's pilot plant in Menlo Park.

Global Thermostat's pilot plant in Menlo Park.

DAC for carbon management purposes is a relatively new pursuit because separating CO2 from air is challenging to do in an economically viable way. The main reason for this is that it takes a significant amount of energy and air to separate and concentrate CO2: CO2 exists in the atmosphere in very dilute concentration compared to other chemical elements (CO2 comprises 0.04% of the atmosphere compared to about 78% for nitrogen, and 21% for oxygen). Finding chemical agents that are sticky enough to bind with the few CO2 molecules that exist in the air—but are also not too sticky so that they will easily release the CO2 in the chemical regeneration step—has proven challenging.

6.      How is DAC related to other carbon capture and storage (CCS) systems? In many ways, DAC systems are quite similar to other CCS systems, especially in regards to the chemicals used to capture CO2. Capturing CO2 from ambient air, however, is thermodynamically more challenging than capture from energy systems, as coal power plants generate exhaust gas with around 15% concentration of CO2, natural gas power plants around 5%, and ambient air has around 0.04%. This relatively dilute stream of CO2 in the air requires DAC systems to deploy novel engineering designs, as traditional CCS systems would require a prohibitive amount of energy to capture CO2 directly from the air.

7.      How much energy is required for DAC? It depends on how efficient the air capture process is, and what ending concentration of CO2 is required. To get 100% pure CO2 stream at the maximum possible efficiency, the American Physical Society report cites that it takes approximately 497 kJ of energy to generate 1 kg of compressed CO2. In other words, for every million tons of compressed CO2 generated from a maximally efficient DAC system, a power plant running at 100% capacity factor of 10 MW is required. To get to the billion ton scale of CO2 capture viewed by many experts as climatically significant, DAC systems would thus require about 10 GW of power, equal to about 3 times the capacity of the largest nuclear plant in the US.

A visualization of what an "artificial forest" of DAC machines might look like when coupled with renewable energy, via the ASU Center for Negative Carbon Emissions.

A visualization of what an "artificial forest" of DAC machines might look like when coupled with renewable energy, via the ASU Center for Negative Carbon Emissions.

8.      How much does DAC cost? At commercial scale, no one really knows. Estimates range from around $60/ton of captured CO2 at the low end (for only CO2 capture) to $1000/ton of CO2 at the high end (for both capture and regeneration) according to a recent National Research Council study (on page 72). The eventual cost of DAC systems will likely depend on how efficient manufacturing for DAC systems becomes. Because there are no commercial scale deployments of DAC systems, however, it is very difficult to estimate how quickly costs will come down. It is likely that the first commercial-scale DAC projects will cost several hundreds of dollars per ton of concentrated CO2, but as manufacturing improves over time, these costs are likely to come down significantly, especially if DAC is manufactured modularly like many startups are attempting to do. It is also likely that operating costs will come down overtime as novel chemical structures are developed that cost less and/or require less material than existing capture chemicals.

DAC system costs are likely to come down with larger scale deployments, much like other clean energy technologies such as wind energy have, especially if DAC systems are manufactured modularly. 

DAC system costs are likely to come down with larger scale deployments, much like other clean energy technologies such as wind energy have, especially if DAC systems are manufactured modularly. 

9.      What are the revenue opportunities DAC? In the future, carbon markets or regulations can provide large sources of revenue for DAC system operators. Without carbon prices, DAC systems are likely to find the largest revenue opportunities by providing CO2 for manufacturing fuels, or for use in enhanced oil recovery (as many oil fields are located far from CO2 pipelines, making them ideal candidates for flexibly-sited DAC systems). Smaller, high value markets (such as greenhouses, carbonated beverages, etc.) can provide early revenue opportunities. 

Audi's "e-diesel" uses Climeworks's DAC system. Transportation fuels can provide an early revenue opportunity for DAC companies.

Audi's "e-diesel" uses Climeworks's DAC system. Transportation fuels can provide an early revenue opportunity for DAC companies.

10.   Are there any policies related to DAC today? Very few. The US Federal government has provided a $3M solicitation from the DOE to support the development of DAC systems, and there is language providing $250k for research and development in the Senate Energy and Water Appropriations Bill report language. In addition, the provincial government of Alberta in Canada has provided grant support for DAC companies through the CCEMC. DAC will benefit from ongoing policy advances around the utilization and geologic storage of CO2, and potentially from the development of carbon markets that are considering traditional CCS as a compliance option. Nevertheless, DAC systems would likely require specific policy treatment in any carbon regulatory system, and so far there has been very little discussion about how to incorporate DAC into any of these existing/potential policy structures.

Bonus question: Want to learn more? Check out our list of links related to DAC, and share your own favorite resources in the comments section!

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Thanks to Avi Ringer, Matt Lucas, and Daniel Sanchez for helping to prepare this post.