Comparative energy efficiency of DACCS vs DWCCS (CO2 removal from seawater)?

[Jasper Sky]

Can anyone point me to a comparison of the energy input requirements per ton of CO2 removal from air via DACCS versus from seawater? 

It seems to me that if a life-cycle analysis were to conclude that capturing and removing carbon from seawater is inherently more energy efficient than a similar process in which carbon is removed from air, then DWCCS will eventually displace DACCS from global CDR markets entirely. I have not yet been able to find a comparison of the LCA of the two processes. 

[John Buttles]

I have wondered that and have a response but bear in mind that I am not a chemist/chemical engineer.  My understanding is that even though the concentration of CO2 volumetrically is higher in the liquid phase, there is no significant energy advantage to the removal process in (sea)water.  The energy consumption for mCDR will be similar to DAC.   The confirmation I see for this is the energy consumption numbers that end up in the various reports, press-releases, website seem to be in the same general range. If the 150X concentration were an important process edge, surely the reported energy numbers would be dramatically different but don’t get reported that way.

 

John.

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[Jasper Sky]

Thanks, John. The concentration difference is 140x, if I understand correctly, but seawater is heavier than air, so pumping seawater presumably requires more energy than pumping air.

Nevertheless: it seems implausible that the energy required per ton of CO2 sequestered will be *identical.* And energy input will be the main driving factor of costs. It seems to me that if there is even just a (say) 20% or even a 10% difference in overall costs per ton of CDR, driven largely by a differential in irreducible energy requirements between DAC and DWC, then the more inherently efficient process will drive the other process out of the market. Figuring out NOW which of the two approaches is irreducibly more efficient, based on engineering physics first principles, is really important, because it could prevent tens of billions of dollars in malinvestment. If we can calculate in advance, through detailed LCA models of both processes, that (say) DWC will be more inherently efficient than DAC, then we can abandon DAC now and invest instead in moving DWC along its TRLs and learning curves. The question I'm posing is therefore not merely theoretical - it is very pragmatically important.

[Jim Baird]

Thermodynamic Geoengineering will drive DAC and DWC out of the market. It cools the surface by converting surface heat and shifting the converted heat to a depth of 1000 meters from where it returns and can be recycled. This first prevents and then reverses the offgassing of CO2 from the ocean to the atmosphere as the ocean is  warmed. And is about 3 times cheaper than the consumption of fossil fuels when the IMF’s $7 trillion assessment of the environment cost of doing business burning fossil fuels is added to the true cost of the energy.  Even absent the subsidies TG is cheaper.

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[Klaus Lackner]

Thermodynamically, CO2 in the air and in the surface ocean are close to equilibrium.  Therefore, the theoretical energy requirement for collecting CO2 from the ocean or the air is essentially the same.  Since the ocean lags behind the air (CO2 is net flowing into the ocean), in most places there is a tiny and irrelevant advantage to collecting CO2 from the air.  However, there are places where the ocean exhales CO2 and there the thermodynamic balance slightly favors ocean water.

Therefore it comes down to the question of which implementation is easier, and there is no good a priori answer to that.   In terms of material one needs to sift through one in 2500 molecules in the air bore one encounters a CO2.  In the ocean this ratio is about one in 25,000.   However the moles per cubic meter in the ocean is about 100 times higher than in the air.  If your process depends on the mass processed, air is easier. If it depends on the volume processed ocean water is easier.

 

The biggest difference is the methods one might use.  Ocean water lends itself to electrochemical separation which you cannot do directly in air.  On the other hand, electrochemistry has not shown itself to be very energy efficient method when working with tiny electrochemical potentials.

 

In short, I would pay attention to both options, and not prejudge which one will turn out to be easier. You will likely see innovation on both sides.

 

Klaus

 

 

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[Anton Alferness]

Klaus - 

Good distillation. From a total value perspective, how do you perceive the potential ecosystem co benefit DOC has over DAC?

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[David Hawkins]

How would locational considerations affect the choice?  For example, proximity to sequestration sites, to demand centers if one is making synthetic fuels, to energy resources (eg, Sahara Desert), to process materials (sorbents--or are these too small to matter?).

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[Michael Hayes]

Electrochemical mCDR can have ocean energy conversion tech built into the platforms, such as TG. Coupling mCDR methods can allow for shortcomings in one method to be compensated for by other mCDR techs. Direct Air Capture can be included in a 'basket' of mCDR techs, or system of systems of mCDR methods. 

Coupling CDR techs for possible synergies is finally being called for in other forums, and I've called for such repeatedly in this forum for years. It likely is time to move away from asking which method is best, as a stand alone effort, to which methods have the highest ability to support other CDR methods as a comprehensive CDR system of systems.

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[Chris Harding]

Jasper, 

CO2 Mineralization from seawater or DWCCS: 

'On a grid-wide basis (U.S. average emissions factor of 743 g/kWh of electricity) and setting solar electricity (emissions factor of 25 g/kWh of electricity) (3) as a baseline zero-carbon electricity reference, it is important to ask whether renewable energy should be used to (1) displace fossil fuel-based power generation or (2) used to power NETs. The answer is complex, but in short, it depends on the energy intensity of the CO2 removal process. For the case of NGC-based electricity generation, in general, a NET needs to feature an energy intensity of no more than 1.34 MWh per t of CO2 removed. The conclusion is that the sCS2 process as outlined herein, if deployed in a format where the energy in the cogenerated H2(g) can be recovered (0.8–1.2 MWh per t of CO2 removed), features an energy intensity that is sufficiently low to be worthy of deployment as a renewable energy powered NET, rather than displacing sources of NGC-powered electricity generation by renewable energy generation.' [1]

According to [2], the cost of DAC, currently, is 1200 dollars/tonneCO2. With that said, the author did not include MIT's electrochemical process for DAC that operates at ~0.41 MWh/tonnneCO2[3]. 


References: 

[1] La Plante, E. C., Simonetti, D. A., Wang, J., Al-Turki, A., Chen, X., Jassby, D., & Sant, G. N. (2021). Saline Water-Based Mineralization Pathway for Gigatonne-Scale CO2 Management. ACS Sustainable Chemistry & Engineering, 9(3), 1073–1089. https://lnkd.in/gdzCZgBM

[2] Greenhouse Gas Removal Technologies. (2022). United Kingdom: Royal Society of Chemistry.

[3] Seo, H.; Hatton, T. A. Electrochemical Direct Air Capture of CO2 Using Neutral Red as Reversible Redox-Active Material. Nat. Commun. 2023, 14, 313.

On Wed, Mar 6, 2024 at 11:58 AM Jasper Sky <jasp...@gmail.com> wrote:

Can anyone point me to a comparison of the energy input requirements per ton of CO2 removal from air via DACCS versus from seawater? 

It seems to me that if a life-cycle analysis were to conclude that capturing and removing carbon from seawater is inherently more energy efficient than a similar process in which carbon is removed from air, then DWCCS will eventually displace DACCS from global CDR markets entirely. I have not yet been able to find a comparison of the LCA of the two processes. 

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Chris Harding

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[Mike Landmeier]

>>  If the 150X concentration were an important process edge , surely the reported energy numbers would be dramatically different but don’t get reported that way.

This is widely reported, especially in the paper published by the Captura group.  Unfortunately, it is very misleading.

The CO2 concentration is NOT higher in seawater than in the air.  Henry's law tells us that the CO2 concentration in seawater is about 13 ppm, which is much less than the 400 ppm in air.  Now, the 150x refers to the concentration of carbonates in the seawater relative to the concentration of CO2 in the air. Apples and Oranges.  Carbonates are NOT CO2.

Carbonates (HCO3—and CO32-) can be converted to CO2 in the presence of excess hydrogen ions (H+), but seawater has a pH of 8.2. The concentration of H+ ions in seawater is 0.007 uM, which is much less than the CO2 concentration in the air (400 uM).

Moving mass to capture CO2 in the ocean is a dead end. We know this because the leading mCDR companies use existing infrastructure (such as desalination plants) to hide moving water costs. Operating along the coastline at a meaningful scale will change ocean chemistry, which is unacceptable.

My work suggests that mCDR is a malinvestment. But then, so are the leading DAC technologies that are popular today (Carbon Engineering, Adsorption). These DAC technologies destroy exergy, which is bad, Bad, very BAD.

Klaus Lackner (who is participating in this thread!!!!) did an excellent analysis of passive direct air capture. This is the way!

There is no passive CO2 capture in seawater. The diffusion of CO2 in the air (especially when enhanced by wind) is far greater than the diffusion of H+ ions in seawater.  

Regarding energy, Passive Direct Air capture will utilize massive membrane-based air contactors without fans to remove CO2 from the air using less than 1.5GJ per ton. Maybe someday :(

Here is a simple visualization of passive direct air capture. Massive artificial trees, 400m tall.

https://www.dropbox.com/scl/fi/1odzd84re9lq2qhaft3ff/Forest.jpg?rlkey=xuv6lu35509p93v4jvq9zxdts&dl=0

Mike

[Greg Rau]

A pretty definitive comparison of DAC and electrochemical "IOC" is here: https://www.cell.com/joule/pdf/S2542-4351(20)30085-4.pdf

As I recall, depending on assumptions and location, DAC usually wins.

Greg  

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.

[Chris Harding]

Hello, 

From the article, I would say depending on the location and assumptions, IOC can beat DAC by a significant margin. 

The UCLA Equatic system produces hydrogen as well, and that offsets the costs of ex-situ CO2 mineralization (IOC), which is the quote I shared.  

I am not being bellicose, I am just pointing out that there is usually a divergence of opinion among experts. As you mentioned, DAC can be an overall winner as well. 

Cheers, 

Chris Harding

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[Jasper Sky]

As Chris points out, there's a divergence of opinions amongst experts in evidence here - to a non-trivial extent. I'm now at a loss as to what to report to my employer (a global development bank) about what category of highly scalable CDR method is inherently more energy efficient per ton of CO2 removed, which presumably is largely going to be a function of irreducible process energy input, and which in turn will drive financial costs per ton.

It's remarkable that after years of work by many smart people on CDR methods, there's such a radical divergence of opinions even in something I'd have thought is eminently calculable: the irreducible process energy input for a given CDR approach.

It seems to me that what's needed here is an effort to create very detailed technological process models of each CDR method, with special attention to LCA energy requirements, and a common modeling approach to make model intercomparison straightforward.

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[Michael Hayes]

Jasper, I recommend that the development bank leverage it's resources by following and investing in what the current DoE R&D spending is on CDR:

 https://carbon180.medium.com/with-a-new-funding-opportunity-doe-is-offering-100m-to-cdr-pilot-projects-75aba5555f04

I spend money on "multi-pathway" tech development as it will likely help find synergies that competing CDR 'experts' are reluctant to entertain at any working level:

• $25M for up to five multi-pathway carbon removal testbed facilities (up to $5M per project).

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[Dan Miller]

One reason that it is hard to calculate the energy required is that most forms of DAC use a great deal of “Earth Energy” to drive some or most of their cycle.

For example, most of the thermal-swing processes like Climeworks use free air to cool the sorbent after the CO2 is removed using heat. And in the case of Climeworks, they use low-grade geothermal heat for that part of the cycle. The only “manmade electricity” is used for balance of plant (BOP) needs.

Klaus Lackner’s original moisture-swing system used dry air and water to run the cycle (except for BOP).

Think of it this way, if someone tells you that they have calculated the minimum electricity or gas needed in joules to dry a pair of jeans in a dryer, I’ll just take those jeans outside and hang them on a clothesline!

Dan

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[Jasper Sky]

Re: using 'Earth energy' rather than commercial energy for DAC, a few questions:

1. Am I right in thinking that around 20% of total energy input requirements for DAC are for running the fans to push air through the system?, and

2. 80% of the energy is for heating the sorbent so that it ejects CO2 molecules for downstream concentration?

3. What then is the additional energy requirement for compressing the CO2?

4. and what is the energy requirement for transferring it into geological storage?

Can y'all provide some estimates of the thermodynamically irreducible amounts of energy required for each of the four steps, plus some estimates of the realistic amounts of energy required in a real-world optimized system (which will be larger than the irreducible amounts)?

I would think that there are opportunities to harvest "Earth energy" to deal with the first and second of these energy requirements. 

1. For moving air through a DAC collector system, would it suffice to set up DAC equipment in locations where air is naturally funneled through at a brisk pace? We've all been in canyons or tunnels or in places on the street between skyscrapers in a major city, where there's a permanent wind because of the topology of the location. Maybe a natural or artificial canyon could be built that's miles long and in which no fans are necessary to move air, with natural pressure differentials forcing air through the canyon instead, thereby eliminating the energy cost of moving air. Such locations could allow very large-scale DAC systems. Have any papers been written that play with ideas for scaling up DAC systems along these lines?

2. If the sort of large tunnel-DAC described here is equipped with a connection to a deep-well geothermal system, the energy input requirement for moving the CO2 molecules off the sorbent material could be supplied as direct geothermal heat on a continuous basis without much commercial energy input other than what's necessary to circulate a working fluid through the geothermal system. 

3. That leaves the challenge of compressing the CO2 and 4. pushing it down a hole somewhere. For these purposes, I suppose geothermal energy could again be harnessed to generate power to run compressors and pumps, although it's not clear that it would be especially advantageous to use geothermal over (say) wind, solar, or hydro power. 

[Mike Landmeier]

  Hi Jasper.

I recently did a deep dive exploring the issues you listed.  I can walk you through some online data that will help answer your questions.

The Carbon Engineering documents provide the best information for understanding DAC energy. They documented their development process over a decade.

The best Carbon Engineering document is this one.

A Process for Capturing CO2 from the Atmosphere - ScienceDirect

Studying the all-natural gas pathway is easiest as CO2 from the air is commingled with CO2 from combustion.

Page one clearly shows their process uses 8.81 GJ of energy to capture and sequester 1 ton of CO2 from the air. So, that's the headline number.  

Figure 2 provides more detail. The air contactor is shown on the lower left. 9.2MW is needed to capture 112 tCO2 per hour. 9200kwh/112 = 82 kwh/tCO2 = 0.29 GJ/tCO2.

Skipping item 2 for now, CO2 sequestration is the Compression, transporting, and storing of CO2 in a geological formation.  This entire process (items 3 and 4) is handled by compressing CO2 to 151 bar as shown in the upper right.  The compressor shows 22MW to compress 171 tCO2 per hour.  This is 22000/171 = 128.6 kWh/tCO2 = 0.46 GJ/tCO2.

The rest of the energy is used to regenerate the solvent.  Regen Energy is 8.81 - (0.29 + 0.46) = 8.06 GJ/tCO2.

This quick and simple analysis gives you a ballpark number to work with.  Just be aware that this process uses natural gas to power the system.  This energy sequesters 1.4 tons of CO2, 1 ton of which was captured from the air.

This document does not include the energy costs of replacing lost water or sorbent.

>> I would think that there are opportunities to harvest "Earth energy"

CO2 capture has been under development for nearly 100 years.  However, directly using earth energy has had very little success.

>> 1. For moving air through a DAC collector system, would it suffice to set up DAC equipment in locations where air is naturally funneled through at a brisk pace?

This is Passive Direct Air Capture.  I am a big fan, but the concept is underdeveloped. Klaus Lackner has written extensively about the advantages of PDAC.  See his work from 2015-2017.

He is also supporting the Mechanical Trees project, which uses Moisture Swing.

The only other PDAC project I am aware of is SmartDAC/Circulair in Europe.  It's running, but nothing has been published for a few years.

>> 2. If the sort of large tunnel-DAC described here is equipped with a connection to a deep-well geothermal system

The DAC / Geothermal combination is somewhat new.  See Fervo Energy for details.

>>  I suppose geothermal energy could again be harnessed to generate power to run compressors and pumps

Geothermal electricity is portable. I see no need to directly couple DAC with geothermal. Electricity from solar or wind will power pumps just fine.

Mike

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[Dan Miller]

See responses in bold below.

Dan

On Mar 8, 2024, at 5:23 PM, Jasper Sky <jasp...@gmail.com> wrote:

Re: using 'Earth energy' rather than commercial energy for DAC, a few questions:

1. Am I right in thinking that around 20% of total energy input requirements for DAC are for running the fans to push air through the system?, and

As you mention below, it is possible to place the units in areas of high wind flow. Otherwise, fans can consume significant energy.

2. 80% of the energy is for heating the sorbent so that it ejects CO2 molecules for downstream concentration?

This is normally the biggest energy user but, again, geothermal heat of ~120ºC is available almost everywhere with reasonable drilling depth. Also, industrial heat pumps are now being developed that can deliver ~120ºC heat from lower heat sources (similar to home heat pumps that are 500% efficient). 

3. What then is the additional energy requirement for compressing the CO2?

Energy to compress is significant, but is not required. Climeworks uses Carbfix to turn the CO2 into “club soda” and then inject it into basalt formations that turn the CO2 permanently into carbonates over a short period of time.  Such basalt formations are present in many places on Earth. https://www.carbfix.com/

4. and what is the energy requirement for transferring it into geological storage?

For saline aquifer injection, I believe most of the energy is in the compression stage. But see (3) above regarding Carbfix.

As an overall note, it is always cheaper and takes less energy to not emit CO2 in the first place. But for past emissions, the limiting factor is not the energy required for DAC since you can always build renewable energy systems with the DAC to get the needed energy (for BOP, etc.). The issue is you must compare the cost of doing DAC with the cost of not doing DAC. Since not doing DAC may lead to the loss of all coastal cities and the collapse of civilization, the ~$2 trillion/year to do 40 Gt/y of DAC is a real bargain! And we may find cheaper ways to remove CO2 from the atmosphere (ocean fertilization, etc.)

The same thinking should be applied to SRM. While SRM may have negative side effects, those must be compared to the negative side effects of not doing SRM, which will be orders of magnitude larger.

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[Jasper Sky]

Thanks, Dan, and thanks, Mike L. It seems to me, then, that the energy input per ton of CO2 put forward by Carbon Engineering, 8.8 GJ/ton, can be regarded as an upper limit, no? 

Would I be correct in thinking that Carbon Engineering's liquid sorbent, with its high-temperature requirement, is inherently less energy-efficient than the solid sorbent approach favored by some other companies?

I would think it should be possible to create a very detailed techno-economic model of a modular DAC system that could be deployed just about anywhere, with preference for locations sitting directly atop high-capacity saline aquifers where the carbon can be disposed of. This leads to follow-on questions about the plausible volume of such reservoirs, and how long a given DAC plant can operate before it runs out of nearby geological storage capacity. 

Again, my sense is that the details of DAC and of rival technologies like direct ocean capture remain hotly contested amongst experts (or near-experts), and that we do not yet have a clear sense of what the energy and materials requirements of the rival technologies are. 

I'm ultimately primarily interested in CDR technologies capable of removing 40 to 60 GtCO2 per annum, because I think we (the world) is going to realize (as Dan put it) that the cost of doing this is much lower than the cost of not doing net negative emissions at that scale. I think we ought to aim at going from peak atmospheric CO2, which we'll attain in the first net-zero year perhaps around 2060 CE, back down to about 350 ppm by 2100 CE and 275 ppm by about 2130 CE, to limit cumulative sea level rise and avoid the loss of our great coastal cities (and of the Netherlands and Vietnam). I'm hoping we can yet stop the meltdown of the Greenland and West Antarctic Ice Sheets, though if I had to place a bet, I'd bet that we're too late to stop their loss unless we apply robust SRM geoengineering in the polar regions ASAP.

If CO2 ppm peaks at about 460 ppm in about 2060, then we'd need to remove 2.75 ppm CO2 from the atmosphere annually to get back down to 350 ppm by 2100, necessitating, if I calculated correctly, 47 GtCO2 per annum net negative emissions, and if we assume residual GHG emissions entailing offset requirements of 7 GtCO2, then we'll need to achieve 54 GtCO2 CDR per year. (I've assumed 17 GtCO2 CDR will be needed to reduce atmospheric CO2 by one ppm, which uses a factor of 2.2x the mass of CO2 in one ppm atmospheric CO2, to take into account outgassing of CO2 from the oceans and biosphere in the years after we get past net-zero annual emissions.) 

There are some marginal CDR methods that can only contribute a couple of GtCO2 per year (land biochar?) that are worth doing because of co-benefits. But getting to CDR quantities on the order of 40 to 60 GtCO2 per annum will necessarily entail CDR methods that are almost arbitrarily scalable. It's possible that people are correct who say we'll need a diverse basket of CDR technologies to get to several tens of GtCO2 per year, but I'm skeptical of that view. The cheapest arbitrarily scalable solution will tend to drive all the others out of the market, barring valuable co-benefits of some less scalable CDR methods. 

It seems to me that we have no clear idea as yet what the most likely candidates are for hugely scalable, cheap, energy-efficient, low-ecological-cost CDR solutions. It might be some form of DACCS which uses mostly geothermal energy, or it might be some marine CDR system. We need highly detailed, continually improved Open Source techno-economic models of each candidate CDR approach to help us identify and improve plausible scalable CDR methods, and to help guide investment (and avoid malinvestment into inefficient options and dead ends). Agree or disagree?

[Chris Van Arsdale]

The raw energy requirements for DAC are probably secondary to the type of energy, in terms of techno-economic feasibility/investment.

For example, a 3MWh/ton cost for low-grade thermal system will likely be a lot cheaper than a 1MWh/ton electrical device.

As mentioned in the thread, it is possible to get 120C geothermal... It is even easier to get 80C water (black selective surface in the sun).  At 80C the energy is almost free (there is a major step function above ~90C, and another above ~140C). Temperature and partial pressure can often be traded with these sorbents. Also keep in mind that it is also expensive to store renewable power overnight, but much easier for heat, so the BOP amortized costs are going to be very different.

There are also the more research-y ideas, like the photochemical angle that might completely change the unit economics regardless of the energy requirements.

- Chris

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[Jasper Sky]

I agree with you, Chris, that direct heat from geothermal sources (or via hot brick energy storage systems like Rondo heat batteries, for soaking up excess solar/wind renewable energy when grid demand is below power supply on sunny or windy days) could make DACCS a lot cheaper than a system with a high electricity input requirement. I still need to see detailed models of DACCS (i.e. the precise irreducible energy and materials requirements, and calculations about the geological storage capacity that will likely be accessible to any single DACCS plant without requiring transporting compressed CO2 off-site, etc.), and similar models of rival technologies. Any CDR technologies that are all straightforwardly scalable to the >50 GtCO2/year scale are of high interest, and all such technologies will end up competing with each other for CDR dollars; in the medium term, one of those technologies will win (likely whichever technology has the lowest combined cost per ton CO2 removed, in terms of energy, materials, and land, and hence in $/ton). That technology will drive all the other CDR technologies out of the market, with the exception of minor CDR methods that don't scale very well but do have significant co-benefits, like biochar. 

I'm not aware of 'the photochemical angle' - can you point me to an article on that?

[Chris Van Arsdale]

On Fri, Mar 8, 2024 at 8:58 PM Jasper Sky <jasp...@gmail.com> wrote:

I agree with you, Chris, that direct heat from geothermal sources (or via hot brick energy storage systems like Rondo heat batteries, for soaking up excess solar/wind renewable energy when grid demand is below power supply on sunny or windy days) could make DACCS a lot cheaper than a system with a high electricity input requirement. I still need to see detailed models of DACCS (i.e. the precise irreducible energy and materials requirements, and calculations about the geological storage capacity that will likely be accessible to any single DACCS plant without requiring transporting compressed CO2 off-site, etc.),

I would say this is all still in the early stages of development. It's like PV circa 1985. Will monocrystalline win? poly? thin film dark horse?

From an irreducible energy standpoint... Technically the entropy of mixing is the theoretical limit (which, if I am not super addled) should be about ~100-200kWh/ton-CO2 (before compression). Realistically, getting below 1MWh/ton would be impressive.

There are a number of papers on geologic storage, the options are pretty widely available. In human scale terms, the amount is enormous. But, in volumetric terms for the earth... It's about 1 large mountain. There is also a near infinite capacity below ~1500m in the ocean.

- Chris

[Chris Harding]

Jasper, 

Although an expensive book, it is well worth the money[1]. A cost comparison of CDR technologies, where it is possible, is is listed in Chapter 11, Table 11.1. 

References: 

[1] Greenhouse Gas Removal Technologies. (2022). United Kingdom: Royal Society of Chemistry.

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