The energy requirements for the compression and liquefaction of CO₂ are nearly equivalent to the minimum energy requirements for the initial capture of the CO₂ being liquefied, which presents another significant hurdle for direct air capture.

Summary

Capturing and storing CO₂ requires substantial energy investments, particularly for compression and liquefaction needed for transportation and storage. Different climate scenarios (RCP 2.6, 4.5, and 8.5) require removing between 866-3523 gigatons of CO₂, with compression energy demands ranging from 312-1268 exajoules - equivalent to 21-84 times the United States' 2019 electricity production. While some CO₂ utilization exists, most applications like enhanced oil recovery still require a degree compression. Current compression technologies operate near their theoretical efficiency limits, making this energy demand a major barrier for large-scale carbon removal efforts. The storage phase alone approaches the idealized energy requirements of initial capture, highlighting significant challenges for climate change mitigation strategies.

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Carbon capture, and specifically direct air capture (DAC), is a highly energy-intensive process that would require diverting a significant portion of the world's generated electricity capacity, even when just considering the theoretical minimum energy requirements for capture. However, capturing CO₂ is only the first step in the process of carbon removal. Once carbon dioxide (CO₂) is captured from the atmosphere, it needs to be stored in a way that is both safe and efficient. This can involve transporting the gas over long distances via pipeline or in trucks, as well as preparing the gas for injection. This often involves the process of compression and cooling to bring the captured CO₂ to a liquid or supercritical state. As CO₂ at atmospheric conditions is a stable gas, in order to convert it into liquid or any other state, energy is required through the form of work.

Energy for CO₂ Compression

In most cases, liquefied CO₂ is the preferred physical state for carbon dioxide removal after it has been captured. This is because liquefied CO₂ is denser than the gas, which means that it can be stored in smaller volumes, making it easier to transport. In many situations, such as enhanced oil recovery (EOR) and subterranean geological storage, it is desired that the CO₂ is in its supercritical state, which is a state of matter that has a similar density to liquid CO₂, but with lower viscosity and friction like a gas, making it easier to disperse through rock formations ¹. Estimates for the energy requirement vary in the literature as it often depends on specific ambient conditions, such as what temperature and pressure does the CO₂ begin at, and the desired final state. Estimates lie between 100-150 kWh per ton of CO₂ for a range of carbon capture technologies including CCS and DAC ^{2 3}. This value includes thermodynamic efficiencies of the whole compression and liquefaction process of around 60%, which are well established and are unlikely to improve.

We can use these figures to estimate the energy requirement for compression using the International Panel on Climate Change (IPCC) Representative Concentration Pathways (RCPs). Previously, we calculated the amount of CO₂ that would need to be removed from the atmosphere to meet the RCP 2.6 pathway, as well as the amount of CO₂ that would need to be removed in the RCP 4.5 and RCP 8.5 pathways to achieve our stated goal of "limiting the atmospheric concentration of CO₂ to 450 ppm by 2100" as shown in table 1.

Table 1: Atmospheric CO₂ removal required to achieve 450 ppm or less for each RCP scenario

Scenario	Projected 2100 CO₂ concentrations (ppm)	Estimated average global temperature Rise Range (°C)	CO₂ Removal required (Gt)
RCP 2.6	400	0.9 – 2.4	1300
RCP 4.5	525	1.7 – 3.3	866.4
RCP 8.5	900	3.2 – 5.7	3522.6

Using the lower estimate of liquefaction of 100 kWh/ton of CO₂, we can calculate the total energy required for compression:

• RCP 2.6:

$$\begin{align*} \text{Energy compression}_{ (2.6)} &= 1300 \text{ Gt} \times 10^9 \times 100 \text{ kWh/ton} \\ &= 1.3 \times 10^{14} \text{ kWh}\\ &= 468 \text{ EJ} \end{align*}$$

• RCP 4.5:

$$\begin{align*} \text{Energy compression}_{ (4.5) } &= 866.4 \text{ Gt} \times 10^9 \times 100 \text{ kWh/ton} \\ &= 8.664 \times 10^{13} \text{ kWh}\\ &= 312 \text{ EJ} \end{align*}$$

• RCP 8.5:

$$\begin{align*} \text{Energy compression}_{ (8.5) } &= 3522.6 \text{ Gt} \times 10^9 \times 100 \text{ kWh/ton} \\ &= 3.5226 \times 10^{14} \text{ kWh}\\ &= 1268 \text{ EJ} \end{align*}$$

As is shown, the energy requirement for liquefaction is extremely high, nearly approaching the calculated minimum energy requirements for the initial capture of the CO₂. We can put these values into perspective by comparing this energy figure to a value of 15.07 EJ, the total electrical energy supplied in the United States in 2019 ⁴.

• RCP 2.6: Equivalent to more than 31x the total electrical energy supplied in the United States in 2019
• RCP 4.5: Equivalent to more than 21x the total electrical energy supplied in the United States in 2019
• RCP 8.5: Equivalent to more than 84x the total electrical energy supplied in the United States in 2019

Conclusion

The energy requirement for compression and liquefaction of carbon dioxide is extremely high, and given that compression pump technologies are a mature field, it is unlikely that this value will see a significant reduction in the future. Different options of gas utilization may assist in reducing the energy requirement for compression by not requiring a compressed gas state such as in agricultural settings. However, CO₂ utilization only sits around 3.7 Gt/yr in 2020, most of which is used for enhanced oil recovery (EOR) which does require significant gas compression ⁵. Even if more utilization options existed, some degree of compression would need to be performed in order to necessitate transportation of the CO₂. The necessity of compression is another significant barrier that has to be overcome in order to make DAC a viable solution in addressing climate change.

Sources

Footnotes

1. Aminu, M. D., Nabavi, S. A., Rochelle, C. A., & Manovic, V. (2017). A review of developments in carbon dioxide storage. Applied Energy, 208, 1389-1419. https://doi.org/10.1016/j.apenergy.2017.09.01 ↩

2. Fasihi, M., Efimova, O., & Breyer, C. (2019). Techno-economic assessment of CO₂ direct air capture plants. Journal of Cleaner Production, 224, 957-980. https://doi.org/10.1016/j.jclepro.2019.03.086 ↩

3. Romeo, L. M., Bolea, I., Lara, Y., & Escosa, J. M. (2009). Optimization of intercooling compression in CO2 capture systems. Applied Thermal Engineering, 29(8–9), 1744–1751. https://doi.org/10.1016/j.applthermaleng.2008.08.010 ↩

4. International Energy Agency. (2021). Key world energy statistics 2021. IEA. https://www.iea.org/reports/key-world-energy-statistics-2021 ↩

5. Anwar, M. N., Fayyaz, A., Sohail, N. F., Khokhar, M. F., Baqar, M., Yasar, A., Rasool, K., Nazir, A., Raja, M. U. F., Rehan, M., Aghbashlo, M., Tabatabaei, M., & Nizami, A. S. (2020). CO₂ utilization: Turning greenhouse gas into fuels and valuable products. Journal of Environmental Management, 260, 110059. https://doi.org/10.1016/j.jenvman.2019.110059 ↩