Carbon capture is an energy problem

And it's not yet clear how we'll solve it

Removing carbon dioxide from the atmosphere is critical to avoiding the worst effects of global warming. It is so important that the IPCC¹ sees the need to remove ~6 billion tonnes annually by 2050. Direct Air Capture (DAC) carbon removal, sifting and storing carbon dioxide from the air, is expected to be a key part of this due to its scalability² and permanence. 

Unfortunately, it is also incredibly energy intensive in its current form.

Current leaders like Climeworks and Heirloom estimate they need ~2-3 megawatt-hours of energy to remove and compress a single tonne of CO₂ for storage.

To put that in perspective: I could drive a Tesla Model 3 from Los Angeles to Nathan’s Famous Hot Dogs at Coney Island, realize I forgot my favorite hot dog eating shirt at home, drive all the way back to get it, and only upon my return to Coney Island would I have used the same energy³ as removing just a single tonne of carbon dioxide. 8,500 miles of driving versus 1 tonne of carbon removed.

Scale this up to 1 billion tonnes⁴ and ~2,000 terawatt-hours of energy will be needed each year for DAC carbon removal. This equates to ~10%⁵ of total current global electricity consumption just to support carbon removal (on top of existing load growth from EV’s, industrial heat, data centers, etc.).

This makes energy use a key constraint on the scaled deployment of Direct Air Capture carbon removal, and a potential bottleneck on broader climate efforts.

In this post, I’ll step through how to think about this energy constraint, and how different startups are trying to tackle it. No matter which approach wins out, a huge amount of low-carbon energy will be needed.

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What makes carbon removal energy intensive?

Direct Air Capture is not a one-step process, and it helps to first unpack where exactly all this energy is going.

Generically⁶, most DAC carbon removal approaches reduce to the following steps:

1. Collection - Air passes over or through a collection medium that traps the carbon dioxide

2. Regeneration - The collection medium is heated⁷ or otherwise processed to release the trapped carbon dioxide

3. Compression - The released carbon dioxide is compressed (often to ~100+ atmospheres) for use or storage underground

The energy required for regeneration in particular is immense, with some approaches requiring temperatures of up to 900 degrees Celsius. Across these steps, ~20% of energy is used in collection (e.g., fans to move air) and compression, while ~80% is used in regeneration. This energy generally comes in the form of electricity (which can be turned into heat), though some approaches use natural gas or geothermal heat directly.

The result is an energy intensive process. Leading players like Climeworks, Heirloom, Carbon Engineering estimate they use ~2-3 MWh per tonne in running this process, though all are working to reduce this.

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$100 per tonne will not be easy

At the same time, many stakeholders believe that a target cost of ~$100 per tonne or lower is necessary to accomplish carbon removal at scale. This puts a hard ceiling on how much our energy can cost for any given energy requirement.

For example, if our carbon removal process requires 2 MWh of energy per tonne, and can cost at most ~$100 per tonne, we must have energy costs below ~$50 per MWh. This is before even considering any other operating or capital costs.

Right now, the average price for industrial electricity in the US is ~$70-80 per MWh, which already puts us above the target cost at ~2-3 MWh per tonne. That grid average also comes with carbon emissions⁸; the power for DAC needs⁹ to be from low-carbon sources like solar, wind, geothermal, or nuclear.

Just as importantly, reducing energy needs or finding cheaper energy leaves more headroom to spend on all the other operating and capital equipment costs¹⁰–the fans, kilns, and compressors themselves, not to mention the permitting, financing, overhead, and land costs (near geologically suitable underground storage). All of this costs money.

Some have claimed that achieving removal at $100 per tonne is simply not possible given current cost structures and energy requirements.

In the face of this impossibility, there are 100+ startups trying to find a way.

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Many ways to try

As you might expect across so many contenders, there are a range of approaches seeking the optimal tradeoff between complexity, capital costs, energy costs, and operating costs. 

To oversimplify, we can center these efforts around energy along two vectors:

1. Use less energy, often at the expense of higher capital costs

2. Use cheaper energy, often from renewables that don’t produce 24/7

Each is taking their own path towards both, with real uncertainty about which will win. No one has yet built a one megatonne DAC plant, let alone a thousand.

So what are some of the main ways people are trying to solve this?

Single loops (Heirloom)

First up is a (relatively) simple single-loop chemical process based around calcium carbonate from limestone (a very cheap material).

Calcium carbonate (CaCO₃) is ground up and heated to release CO₂, becoming calcium oxide (CaO, also known as quicklime). Calcium oxide is highly reactive¹¹, and when combined with water (H₂O) forms calcium hydroxide (Ca(OH)₂) in a process known as slaking. This calcium hydroxide binds CO₂ in the atmosphere, forming calcium carbonate (CaCO₃) and arriving back at the beginning of the process.

Each time the calcium carbonate is heated, the released CO₂ is collected and stored to neutralize its global warming potential.

Source: Heirloom

This process, commonly associated with the startup Heirloom, uses cheap limestone but requires temperatures of 900 degrees Celsius (C) during heating (often referred to as calcination). That said, there are significant potential energy efficiencies possible through heat recycling from calcination and slaking (which is exothermic, releasing heat).

Compared to other pathways, this approach is not as reliant on continuously operated fans, making it well matched to intermittent renewables like solar photovoltaics (PV). As a result, folks like Heirloom are well placed to benefit from falling solar PV costs.

This chemical pathway is also being used by Terraform Industries, somewhat famous for the solar maximalism of their founder Casey Handmer.

Double loops (Carbon Engineering)

One step more complex and we get a double-loop chemical pathway of calcium carbonate and potassium hydroxide (KOH), the latter being a more effective collection medium for atmospheric CO₂ (at the cost of a more complicated process).

Source: Carbon Engineering

Liquid potassium hydroxide (KOH) is reacted with atmospheric CO₂ in large air contactors powered by horizontal fans. This results in a ‘CO₂ rich’ solution of K₂CO₃ and water (H₂O). When mixed with liquid calcium hydroxide (Ca(OH)₂), pellets of solid calcium carbonate (CaCO₃) precipitate out.

While the refreshed liquid KOH heads back to the air contactors, these pellets can then be heated to 900C in a kiln / calciner to release their load of CO₂ for storage, creating calcium oxide (CaO) to be slaked with water (H₂O) back into calcium hydroxide (Ca(OH)₂).

This use of potassium and calcium in a double loop is most commonly associated with Carbon Engineering (CE), a part of Occidental Petroleum.

Interestingly, these designs often include natural gas to generate the heat for calcination. This adds complexity in the form of CO₂ scrubbing and an air separation unit to create O₂ for combustion¹², but allows for cheap (in the US) natural gas to provide the bulk of the energy needed for the process.

This approach appears well-suited for a symbiotic relationship with sustainable fuel manufacturing. The availability of pure O₂ from hydrogen electrolysis makes it more cost efficient by eliminating the need for an air separation unit, and provides an automatic customer for CO₂. While this pathway may not make sense for all DAC, it will likely carve out an important niche in places like Texas with cheap natural gas and nearby chemical manufacturing.

Even more complex looping processes are possible, with folks like Holocene leveraging two overlapping loops of amino acids and guanidine to enable their carbon removal. Other notable startups here include: Phlair, Greenlyte. Each of these trades off the complexity of their systems with input costs, CO₂ reactivity, and energy required for material regeneration, among myriad other factors.

Solid sorbents in a box (Climeworks, CarbonCapture)

Solid sorbent approaches–like those favored by Climeworks and competitor CarbonCapture–trade material handling loops for modules built around sorbent filters which bind CO₂.

Source: Climeworks (via Frontiers in Climate)

This requires higher continuous energy use (at least in current form), with continuously operated fans needed to maintain a pressure differential across the air contactor.

Importantly, the solid sorbent heating cycles run on low grade heat (~80-120C) and could potentially be run directly from geothermal plants, leaving expensive 24/7 electricity to the fans. This could significantly reduce their effective cost of energy as geothermal technology advances.

The real upside potential will come from improved collection materials which, if they are achieved, could significantly reduce energy requirements. This includes non-heat alternatives like the moisture-swing adsorbent approach by companies like Avnos.

Electrochemical cells (RepAir, Mission Zero)

The biggest wildcard, and least mature niche, involves electrochemical cells produced by startups like RepAir and Mission Zero. These use charged cells to attract and bind CO₂, then release the CO₂ when the charge is changed. No heat is required to regenerate the contact medium.

Source: MIT News

They claim energy requirements of ~600-800 kWh per tonne of carbon removed, but it is unclear how scalable these systems are or how expensive they would be to construct upfront (their capital costs). Without additional information, I would put these at the high capital / low energy requirements end of the spectrum. This could win out but, as with all of these technologies, that remains to be seen.

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Taken together, a clear pattern emerges. Approaches with higher energy requirements must be able to rely on cheap, intermittent renewable energy. Those with higher capital costs must have very low energy requirements. Those stuck in the middle will struggle to reach scale.

One path forward is nuclear or geothermal power, whose high availability would favor high capital / low energy approaches. Both sources would allow for continuous operation, both are promising, but neither have proven their ability to scale in a cost effective way.

For my money, I’d bet on the cheap and cheerful DAC plant that runs on ever-cheaper, ever-scaling solar. Because carbon dioxide is diffused globally, DAC plants can go where the sun is, maximizing solar’s advantage over other forms of energy.

Even with reductions in energy usage, whichever approach wins out will likely use prodigious amounts of energy to rebalance our atmosphere. At scale, this will be an important way to use clean energy and ingenuity to combat the worst effects of climate change.

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Big thanks to Rob, Ben, Grant, and Andrew for reading drafts of this. All errors or omissions are my own.

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Further reading:

International Energy Agency (IEA) - Direct Air Capture

Ben James - Energy Fundamentals of Carbon Removal

Austin Vernon - The Future of Carbon Dioxide Direct Air Capture

Related papers:

McQueen N et al (2021) A review of direct air capture (DAC): scaling up commercial technologies and innovating for the future Prog. Energy 3 032001 doi: 10.1088/2516-1083/abf1ce
(overview of Carbon Engineering solvent approach and Climeworks solid sorbent approach)

Keith D, Holmes G, St. Angelo D, Heidel K (2018) A Process for Capturing CO2 from the Atmosphere, Joule, Volume 2, Issue 8, 2018, https://doi.org/10.1016/j.joule.2018.05.006.(Carbon Engineering approach)

Beuttler C, Charles L and Wurzbacher J (2019) The Role of Direct Air Capture in Mitigation of Anthropogenic Greenhouse Gas Emissions. Front. Clim. 1:10. doi: 10.3389/fclim.2019.00010
(Climeworks approach)

McQueen N, Desmond MJ, Socolow RH, Psarras P and Wilcox J (2021) Natural Gas vs. Electricity for Solvent-Based Direct Air Capture. Front. Clim. 2:618644. doi: 10.3389/fclim.2020.618644
(cost estimates for different energy sources for Carbon Engineering approach)

Young J, McQueen N, Charalambous C, Foteinis S, Hawrot O, Ojeda M, Pilorgé H, Andresen J, Psarras P, Renforth P, Garcia S, van der Spek M (2023) The cost of direct air capture and storage can be reduced via strategic deployment but is unlikely to fall below stated cost targets, One Earth, Volume 6, Issue 7, 2023, https://doi.org/10.1016/j.oneear.2023.06.004.
(cost estimates across leading technologies)

1. Intergovernmental Panel on Climate Change, a leading global climate consortium that produces and synthesizes climate measurement, modelling, and forecasting.

2. Other approaches like reforestation and soil carbon sequestration are less expensive, but there are open questions about the permanence of that sequestration and if there are enough target areas globally to support the scale of carbon removal required.

3. Recent Model 3’s get ~3.7 miles per kWh of energy used, implying that the ~8,500 mile hot dog route takes the same ~2-2.5MWh as one tonne of carbon removal.

4. There are varying estimates of what portion of carbon dioxide removal will come from DAC versus other avenues (e.g., afforestation, soil carbon management, enhanced rock weathering). The current IPCC forecast is for 980 Mt to come from DAC by 2050, of the 6 Gt of carbon dioxide removal overall at that point.

5. The IEA estimates that in 2023, the world consumed ~87 petajoules of electricity or ~24,000 terawatt-hours. Removing ~6 gigatonnes of CO₂ each year would take ~12,000 terawatt-hours of energy.

6. Forgive my simplification, I am knowingly conflating the process differences between solvents / sorbents and adsorption / absorption in an effort to focus on attributing energy usage to process steps.

7. Heat is the most common regeneration mechanism, but there are other pathways with different regeneration processes. Later on in the piece I will talk at more length about one: electrochemical cells.

8. The current carbon intensity of the US grid as a whole is ~400 kg per MWh, nearly offsetting all progress at 2 MWh per tonne energy requirements. At present, any form of 24/7 clean power can likely do more good by decarbonizing the electric grid rather than powering DAC at scale.

9. Or have its impact eroded by incremental emissions, raising its effective cost.

10. Some estimates have put capital costs at ~2/3 of the overall cost per tonne removed. Non-energy costs are large, make the energy requirements the ceiling below which everything has to fit.

11. CaO will actually react spontaneously with CO₂ in the air, no slaking required. The slaking step is included to accelerate and control the carbonation process at scale.

12. Combusting CH₄ with O₂ (rather than ambient air) makes the CO₂ scrubbing process much more efficient. This is the same thinking behind Net Power’s pure oxygen natural gas turbine.