Solid sorbent systems are less developed and more expensive than liquid sorbent systems, but their modularity, relative simplicity, and most importantly their significantly lower energy requirements are placing them on a trajectory to become the leading technology for DAC systems.

Summary

Direct Air Capture (DAC) systems use liquid and solid sorbents with distinct cost and efficiency profiles. While liquid sorbents are proven technology with lower capital costs ($150/tCO₂ vs $1,000/tCO₂ for solids), solid sorbents show promise through lower energy needs (3.8 – 6.6 GJ/tCO₂ vs 6.57 – 9.9 GJ/tCO₂ for liquids) and operational costs ($4.10 – $47.60/tCO₂ vs $41.00 – $78.00). Existing systems like Carbon Engineering's liquid sorbent cost $94 – $232/tCO₂ using fossil fuels, while Climeworks' operational solid sorbent system costs ~$600/tCO₂ using geothermal energy. Solid sorbents' lower temperature requirements (80 – 120°C vs 400 – 900°C for liquid sorbents) and modular design position them as future leaders despite higher upfront costs, though capture efficiency remains lower due to material degradation and downtime.

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Liquid sorbents and solid sorbents are the two main methods employed in Direct Air Capture (DAC) systems. Liquid sorbents are a proven technology that has been used in industrial gas scrubbing for many decades, while solid sorbents are a relatively recent development in the field of DAC, which are beginning to show promise in addressing the largest issue with liquid sorbents, namely the high energy requirement in the desorption/regeneration phases ¹. The main competing factors when considering liquid and solid sorbents are the capital costs, operational and maintenance costs, energy requirements and costs, and capture efficiency.

Capital Costs

Both liquid and solid sorbents have high initial capital costs; however, liquid sorbent systems tend to have a significantly lower capital cost than solid sorbent systems. Recent estimates for capital costs are as high as $1,000/tCO₂ for solid sorbent systems depending on the adsorbent type, while for liquid sorbent systems, costs can be as high as $150/tCO₂ ². The reason for this high cost difference is that the technology for solid sorbent adsorbents is still in the development phase, and they are therefore still considered specialty equipment ². Another factor is the high capture efficiency of the liquid sorbent systems, which facilitates a lower cost per tonne of CO₂ captured.

Operational and Maintenance Costs

Solid sorbent systems generally have a lower operational and maintenance cost compared to liquid sorbent systems due to their lower complexity design and operating conditions that are less damaging to the machinery. Some solid sorbent materials require pre-heating or moisture removal before the adsorption phase, while other forms of sorbent materials only require an energy input in the desorption phase. Estimates for running costs of solid sorbent systems range from as low as $4.10/tCO₂ to as high as $47.60/tCO₂, with steam generation maintenance costs contributing to over 90% of the total running cost in the high estimate ². For liquid sorbent systems, the operation and maintenance costs are distributed between maintenance, labor, waste removal, and electricity (not including the energy used for desorption). Running costs for liquid sorbent systems range from $41.00 to $78.00/tCO₂ ².

Energy Requirements and Costs

Energy requirements and costs become the main differentiating factor between liquid and solid sorbent systems. Liquid sorbent systems typically require a much greater energy input in the desorption/regeneration phases, requiring temperatures between 400-900°C across multiple phases¹. Solid sorbent systems tend to be much more energy efficient, with the desorption phase requiring temperatures as low as 80-120°C depending on the sorbent material used, meaning that lower quality waste energy can be utilized ¹. Estimates for the energy requirements of DAC systems generally range from 6.57 - 9.9 GJ/tCO₂ for liquid DAC systems, and 3.8-6.6 GJ/tCO₂ for solid DAC systems ². The sourcing of this energy also plays an important role in the overall cost of the system, with energy costs currently being lowest when sourced from fossil fuel sources such as coal and natural gas, and highest with renewables and nuclear energy ². To give a comparison of operational costs of current state-of-the-art DAC systems, Carbon Engineering, which operates liquid sorbent DAC systems, has a quoted cost of between $94 - $232/tCO₂, while Climeworks, which operates solid sorbent DAC systems, has a quoted cost of around $600/tCO₂ ². It needs to be reiterated that these costs are highly dependent on the type of energy sourced, with Climeworks' solid sorbent plant sourcing most of their energy from geothermal sources, while Carbon Engineering sources their energy from fossil fuel sources that tend to be cheaper ². It should also be noted here that there is a direct negative trade-off when using cheaper fossil fuels for DAC as they produce their own carbon emissions, with some estimates suggesting that DAC plants that utilize fossil fuels produce as much as half the carbon dioxide emissions that they capture ².

Capture Efficiency

Capture efficiency is considered a less important factor when considering DAC systems as the costs and considerations of the high energy requirements tend to overshadow the capture efficiency. However, in general, liquid sorbent systems tend to have a higher capture efficiency due to their ability to run almost continuously along with the large reactive surface area provided by the liquid scrubbing process. Solid sorbent systems tend to have a lower capture efficiency related to degradation of the solid sorbent material and the shutdown period required in the desorption/regeneration phases ¹.

Sources

Footnotes

1. Sodiq, A., Abdullatif, Y., Aissa, B., Ostovar, A., Nassar, N., El-Naas, M., & Amhamed, A. (2023). A review on progress made in direct air capture of CO₂. Environmental Technology & Innovation, 29, Article 102991. https://doi.org/10.1016/j.eti.2023.102991 ↩ ↩² ↩³ ↩⁴

2. Ozkan, M., Nayak, S. P., Ruiz, A. D., & Jiang, W. (2022). Current status and pillars of direct air capture technologies. iScience, 25(4), Article 103990. https://doi.org/10.1016/j.isci.2022.103990 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹