Solar fuels come nearer: Direct-from-air CO2 capture cost drops below $100/ton threshold

A technology for direct air capture of carbon dioxide from the atmosphere, with a cost that “fully burdened with interest on capital, ranges from 94 to 232 $/t-CO2 depending on financial assumptions, energy prices, and the specific choice of inputs and outputs,” was the subject of a dense, highly detailed paper published today in a newish energy journal, Joule.

Direct Air Capture is no small thing. The problem with CO2 concentration is that it is too high to support a cool climate, but too low to be easily Hoovered from the sky. The concentration we are worried over is 400ppm, that’s 400 parts per million. That means you have to capture 2500 tons of air for every ton of carbon dioxide — and right there, that’s the reason we have left the job of capturing carbon to plants — not industrial plants, just the garden kind. Flowers work for low wages. 

As has been the case with many advanced biotechnologies, they work, but they cost more than gasoline, and the public doesn’t support higher costs for long, if ever.

It’s a controversy over cost that, in some respects, has been dogging DAC all along. As Carbon Engineering’s David Keith explained to the Digest:

“DAC was born in academic controversy. Early advocates suggested costs could be very low. In reaction to that over-optimism, Rob Socolow led the American Physical Society (APS) in a formal study of DAC that concluded costs were over 600 $/t-CO2 and essentially dismissed the idea. In the years since the APS report, there have been many DAC policy analyses and papers on the chemistry and physics of specific absorbers, but no substantive papers have provided a techno-economic analysis of a complete system.”

The detail is now out there for due diligence wolfpacks to chew on. Prior to this, no paper we’ve seen for direct air capture has had all its major components drawn from well-established commercial engineering heritage, or been described in sufficient detail to allow assessment by third parties.

So, from a cost and detail POV, we may find ourselves at a watershed moment that matters greatly to supporters of algae technology, solar fuels (made from CO2, sunlight and water), electrofuels (made from CO2, water and renewable electricity), or those in favor of scrubbing the atmosphere of enough of this pesky CO2 to reduce the impact of climate change.

Costs have been all over the map, but more and more serious players have emerged

Cost estimates based on simple scaling relationships yield results from 50 to 1,000 $/tCO2. Yet as Keith told The Digest, both “Carbon Engineering, which I founded in 2009, and Climeworks have both build ton-per-day pilot plants, both have grown to over 40 employees, and have attracted considerable press. And we also profiled work by Antecy, here, that focuses on a robust non-amine inorganic solid sorbent, which has several advantages in terms of higher stability, and no environmental risks such as from potentially toxic amine degradation and emissions.

But we’ve been in a certain amount of limbo on costs, and feasibility flowing from that, because of the lack of peer-reviewed data. Some time ago, the US Department of Energy stepped in with financial support, with a contract to perform a techno-economic assessment and publish it in a peer-reviewed journal.

So, why the big drop in cost from $550 per ton to $94?

As the authors explain:

The cost discrepancy is primarily driven by divergent design choices rather than by differences in methods for estimating performance and cost of a given design. Our own estimates of energy and capital cost for the APS design roughly match the APS values. The most important design choices involved the contactor including (1) use of vertically oriented counterflow packed towers, (2) use of Na+ rather than K+ as the cation which reduces mass transfer rates by about one-third, and (3) use of steel packings which have larger pressure drop per unit surface area than the packing we chose and which cost 1,700 $/m3, whereas the PVC tower packings we use cost less than 250 $/m3.

The technology background

As the paper’s authors explained, “Plausible DAC processes use solid sorbents or aqueous basic solutions as the capture media. Solid sorbents offer the possibility of low energy input, low operating costs, and applicability across a wide range of scales. The challenges of solid sorbent designs are first, the need to build a very large structure at low cost while allowing the entire structure to be periodically sealed from the ambient air during the regeneration step when temperature, pressure, or humidity must be cycled. And second, the inherently conflicting demands of high sorbent performance, low cost, and long economic life in impure ambient air.”

The big application: solar fuels

As Keith explained to The Digest, “beyond carbon removal, DAC can be used to make carbon-neutral hydrocarbon transportation fuels, a concept we call “air to fuels”. This is Carbon Engineering’s near-term business focus. We use DAC as a source of atmospheric CO2 to enable carbon-free renewable power to be converted into high energy-density fuels. Solar fuels, for example, may be produced at high-insolation low-cost locations from DAC-CO2 and electrolytic hydrogen using gas-to-liquids technology. This allows displacement of fossil fuels from difficult-to-electrify sectors such as aviation. When integrated into an air-to-fuels process the cost of DAC can be less than 100 $/t-CO2—a price point that enables commercial production of synthetic fuels in today’s low-carbon fuel markets. We have demonstrated the compete air-to-fuels process at our pilot plant.”

The whyfore of solar fuels vs electric cars

With every technological advance, we have to visit the Department of the Painful Tradeoffs and Uncertain Unwanted Consequences. After all, someone thought DDTs were a good idea, and chlorofluorocarbons were a solution to a refrigeration problem before they landed us in ozone hell.

On the negative side, the conversion efficiency — a lot of potential energy is lost or expended in the conversion to fuels.

But there are some avoided negatives, too. For example, the difficulties and costs of changing to mass-scale battery electrical vehicles, the infrastructure cost. Plus, batteries have inefficiency problems too. It has been claimed elsewhere — and Michael Tamor, a Henry Ford Technical Fellow at Ford, ruminated on this topic at a recent DOE Bioeconomy event in Washington — that at least double our current electrical power capacity will be required to be able to charge all battery electrical vehicles. Also the recycling of batteries and its LCA effects remains an issue.

In the end, here’s the great advantage, and it’s infrastructure. With batteries, you have to rebuild the fleet, rebuild the energy delivery system, and rebuild the grid. Fail in any of those and you’ve failed to change the carbon picture. Each of them is massive — together, it’s the biggest industrial transformation ever attempted.

With liquid fuels, you have just the one transition, and that’s the replacement of the energy supply, so long as drop-ins are used. And there’s gradualism — there’s a transition to better energy supply today — and possibly to fuel cells down the line where you get electric motor efficiencies added to the mix.

The CO2 capture backstory

As the paper’s authors explain, “The capture of CO2 from ambient air was commercialized in the 1950s as a pre-treatment for cryogenic air separation. In the 1960s, capture of CO2 from air was considered as a feedstock for production of hydrocarbon fuels using mobile nuclear power plants. In the 1990s, Klaus Lackner explored the large-scale capture of CO2 as a tool for managing climate risk, now commonly referred to as direct air capture (DAC).”

More recently, we profiled that Climeworks opened its first small commercial plant near Zurich, and will capture around 900 tons of CO2 per year. A great step but a tiny one — it would take 25 million of these to capture the world’s annual CO2 emissions, the inventors say.

And here, the Carbon Engineering team demonstrated “Air to Fuels” by directly synthesizing a mixture of gasoline and diesel using only CO2 captured from the air and hydrogen split from water with clean electricity.

The Bottom Line

It’s a watershed event to have a claim out there backed with 12,000 words of peer-reviewed detail. The Due Diligence wolves now have much to chew on, and we’ll be reporting on their response, as well as efforts by Carbon Engineering, Climeworks, Antecy and others to advance the science to commercially-viable costs.