Blower Energy of Direct Air Capture Systems

Fig. 1: Air volume vs capture efficiency.

Rising ocean acidification levels indicate that Earth's largest carbon sink is insufficient to sustainably dampen anthropogenic CO₂ emissions. [1] New technologies have emerged that claim to be able to act as artificial carbon sinks that draw CO₂ directly out of the atmosphere. Commonly referred to as "Direct Air Capture" (DAC), this type of technology has attracted the attention (and funding) of climate-conscious parties. This report will examine the electricity loads and costs of implementing this technology at scale. Every DAC technology is different, and to avoid besmirching any particular company, the scope of this report will be limited to the electricity costs associated with blowing enough air through a system to draw down 1 gigaton of CO₂.

The earth's atmosphere is around 78% nitrogen, 21% oxygen, 0.9% argon, and 0.1% trace gases. [2] This yields an average molar mass of 18 g/mol. Given that the atmospheric concentration of CO₂ in the air is 405 ppm and the density of air is 1.29 kg m^-3, the concentration of CO₂ in air is 0.029 kg m^-3. [3] In order to capture 1 GT of CO₂ in one year, a DAC system would need to process at least 34 trillion cubic meters of air per year or 25 million cubic meters per second. In actuality, DAC systems have capture efficiencies below 100%, so they would have to process even more air. Fig.1 shows a plot of air volume vs efficiency for 1 GT capture.

The power (P) required to blow air through a DAC device is a function of the pressure drop across the device, volumetric flow rate (Q), and blower efficiency (u). [4]

It is important to understand what pressure drop is in order to understand how it affects energy considerations in direct air capture systems. When fluid flows across an orifice, it experiences friction and will lose energy in the form of a pressure drop proportional to the square of the fluid velocity and inversely proportional to the diameter of the orifice as per the Fanning Equation. [5] There are two ways to optimize the volumetric flow rate of a DAC system: increasing the velocity of the fluid or increasing the size of the system. As stated before, increasing the fluid velocity increases the pressure drop of the system and therefore requires an increase in the energy required by a blower to make up for the energy loss due to friction. Fig. 2 shows a plot of pressure drop versus blower energy for 1 GT/year capture. Note that this is only the electricity required to blow air across the device; more energy will be required to compress/liquefy CO₂ and regenerate adsorbents and solvents.

Fig. 2: Blower energy vs pressure drop.

The cheapest renewable energy source is currently onshore wind at around $33/MWh. [6] A DAC system with a 50% capture efficiency, 75% blower efficiency, and a 250 Pa pressure drop would carry an operating expenditure of $1.4 billion just for the blower assuming there was even enough electricity available from wind power. Lowering the pressure drop would linearly decrease the power required, but would increase the capital expenditure.

Increasing the size of the DAC system's gas-solid or gas-liquid contactor would increase the amount of air the system could process without increasing the pressure drop but would also massively increase the footprint and capital expenditure. By example, Eucalyptus trees can sequester the most carbon dioxide of any tree at up to 40 tons CO₂ per hectare per year and can live for centuries using only the wind to supply carbon. [7] However, for 1 GT/year scale storage, eucalyptus trees would need to be planted over 25 million hectares (an area larger than the state of Minnesota) and must be constantly monitored during hot months to prevent wildfires. Land values vary, but assuming an average price of $3,800 per hectare, the cost of the land to plant these trees would be about $95 billion. [8] None of this is meant to undermine the ecological value of reforestation efforts; rather, it is meant to show that reaching gigaton scale carbon sequestration is difficult and expensive to do with trees (or any system with a negligibly low pressure drop).

This analysis reveals that even though the increase in the atmospheric concentration of CO₂ can have massive climate effects, the magnitude of [CO_2atm] is too still too low for DAC to be a viable technology at scale.

© Antone Cruz. 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.

References

[1] S. C Doney et al., "Ocean Acidification: The Other CO₂ Problem," Annu. Rev. Mar. Sci. 1, 169 (2009).

[2] W. M Haynes, CRC Handbook of Chemistry and Physics (CRC Press, 2016).

[3] K. Scrable, G. Chabot, and C. French, "World Atmospheric CO₂, Its C-14 Specific Activity, Non-fossil Component, Anthropogenic Fossil Component, and Emissions (1750 - 2018)," Health Phys. 22, 291 (2002).

[4] R. A Wallis, Axial Flow Fans and Ducts (Wiley-Interscience, 1983).

[5] M. Stewart, Surface Production Operations Volume III: Facility Piping and Pipeline Systems. (Gulf Professional Publishing, 2015).

[6] "Renewable Power Generation Costs in 2021," International Renewable Energy Agency, 2021.

[7] B. Bernal, L.T. Murray, and T. R. H. Pearson, "Global Carbon Dioxide Removal Rates From Forest Landscape Restoration Activities," Carbon Balance Manage. 13, 22 (2018).

[8] L. Qiu, "Farmland Values Hit Record Highs, Pricing Out Farmers," New York Times, 13 Nov 22.