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| Fig. 1: Computer rendering of a DAC facility. The rows of fans, known "contactors," blow air through the facility to a causticiser, which converts filtered air into CaCO3, H20, and Ca(OH)2. After a few high-heat filtration and refinement steps, DAC outputs CO2 ready for compression and sequestration. (Courtesy of NETL) |
Direct Air Capture (DAC) is a technology designed to remove carbon dioxide (CO2) directly from the ambient air. This is typically achieved through a two-step process: first, the ambient air is passed through a sorbent material (such as a liquid solvent or a solid adsorbent) that selectively binds to CO2 molecules. Then, the captured CO2 is separated from the sorbent material, usually through a desorption process, allowing for the collection and storage of the captured CO2.
Controversy surrounding DAC revolves around the significant energy input required for from processing ambient air, which contains CO2 in small concentrations. The concentration of CO2 in the Earth's atmosphere is approximately 0.0416% by volume. [1] In comparison, CO2 concentrations in flue gas from a natural gas-fired turbine power plant are typically around ~4% by volume. [2]
Critics argue that the energy intensity of DAC leaves the solution undeserving of attention and investment. Particularly in cases where the energy source for DAC is not clean or renewable, DAC would result in a paradoxical increase in carbon emissions.
DAC energy inputs vary project to project, so Im using values from the largest operating commercial project, Climeworks Hellisheidi DAC facility in Iceland. The Hellisheidi project captures 4,000 MT of CO2 per year and requires ~8 million kWh of thermal energy and ~2.6 million kWh of electricity to do so. [3] These inputs represent the energy required for air capture from the atmosphere and for the separation of CO2 particles from sorbents. It does not consider the energy inputs for carbon sequestration, which would vary hugely on the sequestration method.
| Total Energy Input | = | Energy Input (thermal energy) + Energy Input (electricity) | = | 8 million kWh + 2.6 million kWh |
| = | 10.6 million kWh/year |
Now, we can calculate the process efficiency:
| Energy Efficiency | = | Captured CO2 Mass Total Energy Input |
||
| = | 4000 tonnes × 1000 kg
tonne-1 10.6 × 106 kWh × 3.6 × 106 J kWh-1 |
= | 1.05 × 10-7 kg J-1 |
The inverse of this number is the energy required to remove 1 kg of CO2 from the air:
| 1 1.05 × 10-7 kg J-1 |
= | 0.952 x 107 J kg-1 |
So, is this energy expense worth it? Interestingly, if coal is burnt, the amount of energy extracted per kg of CO2 in the air is:
| 3.0 × 107 J kg-1 | × ( | 12 44 |
) = | 0.818 x 107 J kg-1 |
Thus, in the case of coal, the energy required to remove CO2 from the air via DAC is greater than the amount of energy produced through the process that emitted that same amount of CO2.
I'll compare this efficiency to that of a post-combustion carbon capture facility that processes the flue gas of a combined-cycle turbine (the most common type of gas-fired power generation in the US). Like the Hellisheidi DAC facility, the 240 MW Petra Nova in Texas is one of the largest operating facilities of its kind. The facility started operating in 2016 but went on a 3-year hiatus over the COVID-19 pandemic as government incentives and low oil prices brought the project's economic viability into question. As both incentives and oil prices recovered, it restarted operations in September of 2023. Though the Petra Nova facility doesn't publish energy input values, it does report that it uses an amine-based gas-treating process to capture ~90% of the CO2 in its flue gas, which was equivalent to 3.543 × 106 tonnes of CO2 over three years. [4] Studies have constrained the energy penalty of amine-based processes to 21% at 90% CO2 capture rates. [5]
| Total Energy Input | = | 240 MW × 0.21 × 1,000 kW/MW × 8,760 hours/year |
| = | 4.415 × 108 kWh/year |
| Energy Efficiency | = |
|
|
| = | 7.430 × 10-7 kg J-1 |
So, using representative examples, post-combustion CO2 capture is currently seven times more efficient than DAC.
The "theoretical" Second-law limit is the absolute minimum amount of energy required to concentrate the CO2 from the air into a tank of 100% CO2 at 1 atmosphere of pressure. The energy required to do this per kilogram of CO2 is:
| E M |
= | R T mν |
× ln(1/c) |
where R is the ideal gas constant, T is the temperature of the environment in degrees Kelvin, mν is the mass of 1 mole of CO2 in kg, and c is the initial CO2 concentration in molecules per molecule:
| E M |
= | 8.314 J mol-1 °K-1
× 300 °K 0.044 kg mol-1 |
× ln( | 1 4.12 × 10-4 |
) | |||
| = | 4.42 × 105 J kg-1 | |||||||
The actual energy required to remove 1kg of CO2 from the air in demonstrated DAC projects, 0.952 × 107J, is greater than 21.5 times this theoretical requirement. From here, we can find a second-law efficiency by dividing the theoretical minimum work by the actual work 1/21.5 = 0.047 (4.7%) . This gives us an idea of how close a given separation technology comes to the thermodynamic minimum.
We can come to a practical energy cost of DAC systems by finding the total energy DAC would require if it represented 10% of the overall carbon dioxide removal portfolio by 2050. [6] This sees DAC capturing 0.5 gigatons CO2/year.
Given that the thermodynamic minimum for carbon capture is 20 kJ/mol of CO2 (126 kWh/tonne), and the second-law efficiency for DAC is 4.7%, we can find DACs absolute minimum energy requirement: [7]
| Energy requirement | = | 126 kWh/tonne 1000 kWh/MWh × 0.047 |
| = | 2.68 MWh/tonne |
| Total energy required | = | 0.5 × 109 tonnes/year ×
2.68 MWh/tonne 1,000,000 MWh/TWh |
| = | 1,340 TWh |
1,340 TWh is equivalent to ~33% of today's US energy usage and ~5.25% of todays global energy production. [8] Needless to say, if DAC projects scale, they will become huge energy consumers. While DAC projects simply need to be proven CO2 negative (through cradle-to-grave life cycle assessments) in order to have positive benefit to the climate, it is unlikely it DAC projects can overcome deployment challenges that arise from its energy intensity. Energy resources, whether renewable or not, are already constrained, so governments would be far more inclined to encourage post-combustion carbon capture projects over DAC projects.
© Kate Bradley. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. M. Maier, "Biogeochemical Cycling," in Environmental Microbiology, 3rd Ed, ed. by O. Pepper, C. P. Gerba, and T. Gentry (Academic Press, 2015).
[2] E. David et al., "Exhaust Gas Treatment Technologies For Pollutant Emission Abatement From Fossil Fuel Power Plants," WIT Trans. Ecol. Eviron. 102, 923 (2007).
[3] G. Kennedy, "W. A. Parish Post-Combustion CO2 Capture and Sequestration Demonstration Project," U.S. Department of Energy, DOE-PNPH-03311, March 2020.
[4] M. Ozkan et al., "Current Status and Pillars of Direct Air Capture Technologies," iScience 25,203990 (2022).
[5] J. Sekera et al., "Carbon Dioxide Removal - Whats Worth Doing? A Biophysical and Public Need Perspective," PLOS Clim. 3, e0000124 (2023).
[6] G. P. Hammond and S. S. O. Akwe, "Thermodynamic and Related Analysis of Natural Gas Combined Cycle Power Plants With and Without Carbon Sequestration," Int. J. Energy Res. 31,1180 (2007).
[7] K. S. Lackner, "The Thermodynamics of Direct Air Capture of Carbon Dioxide," Energy 50, 28 (2013).
[8] "U.S. Energy System," University of Michigan, Pub. No. CSS03-11, August 2023.