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The principles involved in the. railcar-based method of Direct Air Capture (DAC) as explained in the July 20, 2022 Vol 6,Issue 7 of the. scientific. magazine Joule and now being developed. by Texas based 'CO2 Rail' show-

  1. the use of 'slipstream' from moving transport being used to duplicate the fans currently used in schemes such as Climeworks to produce sufficient airflow over the adsorption material thus saving considerable electrical energy and
  2. the conversion and storage of waste heat from 'regenerative braking' in diesel /electric rail in batteries to power the DAC process thus largely eliminating the cost (in association with rail car solar panels) of the heat energy currently required in extraction.

As envisaged, the method represents a considerable advance on current DAC methods in that it saves energy currently wasted and utilises existing transport infrastructure thus raising the possibility of DAC being economic and reaching gigaton levels. The .pdf in the longer Joule articles, while giving detailed workings does not mention the weight of the plant to be placed on railcars though a considerable advantage of miniaturisation seems to have been achieved.

Would it be possible (weight considerations allowing) to adapt such a method to commercial airliners to enable them to harvest significant quantities of CO2 inflight?

  1. Turbofan engines generate significant quantities of waste heat which conceivably could be captured for a similar DAC purpose.(As all answers to-date(26/01/23) have made this point it seemed best to address them in an Edit rather than in individual replies) Question partly relates to recovering waste heat from turbofan engines In High bypass Turbofans approximately 80% of the thrust is supplied by cold slow-moving air and only about 20% by residual ( hot exhaust). Answers so far have portrayed hot turbofan exhaust as sole provider of thrust and heat capture as virtually eliminating it with fundamental changes to flight characteristics. There would appear to be nothing new about suggesting the possibility of such heat recovery or describing it as 'waste heat' Examples of such heat recovery research-and such description- for turbofan exhaust can be found by an internet search and .pdf's obtained by author can be posted on request. Perhaps the 4 providers of answers might care to revisit their answers in the light of the above?
  2. The study of airflows over aircraft surfaces. -being an integral and essential element of aircraft design and aerodynamics-should make it feasible to incorporate some form of 'slipstream capture' through cowlings or otherwise without interfering with overall aerodynamic performance. ................
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    $\begingroup$ Adding to the answers already given, the key difference is that the train is creating the heat and exhaust as a peripheral and undesirable consequence of moving the train forward. Exhaust = waste. The jet engine produces heat and exhaust as the means of propulsion, not the unwanted byproduct of propulsion. Capturing the exhaust by definition means capturing the thrust. Exhaust = mojo. $\endgroup$
    – Max R
    Jan 21, 2023 at 19:52
  • $\begingroup$ Re edit 27th of January, the fact remains that the job of a gas turbine is to push the aircraft through the sky, so pulling energy from the exhaust impacts performance in a domain where single digit and even fractional percentage performance boosts are pursued, if there was energy there they would already be using it. It is plausible that some materials science advance may drop the mass of a heat capture system enough to make sense, but seems probable that powering the aircraft with it rather than carbon capture is still better overall. $\endgroup$ Jan 27, 2023 at 1:33

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It is very hard to see this making any sense for an aircraft.

The major obstacle to CO2 capture is that it is energy intensive, so the key part of the proposed design is to recapture otherwise wasted braking energy in trains*.

Aircraft generally do not 'waste' energy if at all possible since fuel is a major operating expense, they climb to cruise height and then fly optimally there and these days follow optimal descent trajectory as well turning that height based potential energy into kinetic energy. There is a small period during final descent and landing where air and wheel brakes turn energy into heat but it is a very small part of the flight time so any carbon capture technology would produce more C02 in additional burned fuel to carry it than captured in the final minutes of flight, even disregarding the economic and safety factors.

The heat energy in a jet turbine is already being used to push the aircraft along, trying to extract any sort of serious energy will reduce thrust. If there is a magic way to extract additional energy from the engines it would be better spent improving aircraft fuel efficiency than powering carbon capture.

In terms of savings from 'free' airflow, with any sort of chemical process system like this the aim is the minimum flow speed that would allow the system to work due to the way friction losses increase with the square of speed. An aircraft is much faster than ideal, making this much less free, likely manifesting as more drag losses than burning the same fuel to power a fan to slowly move surface air through processing.

If serious about carbon reduction from flights, the answer is not to fly.

*I have not attempted to recreate the maths but would strongly suspect if adding regenerative braking, solar panels and batteries to a train it would make more sense powering the train to reduce fuel burn than powering carbon capture (capturing carbon by not burning it in the first place).

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  • $\begingroup$ @EdwardHogan per the link, if you double your speed, the drag your engines have to overcome goes up by four, speed increase by four is drag by 16 (4*4=16), so any drag from the ductwork on a plane is burning something like 16 times the fuel the same duct would on a train. Clever design helps but why not do the same clever design on the train... Putting intake/exhuast on the wings is generally not a good idea since messing with the airflow will reduce lift. $\endgroup$ Jan 25, 2023 at 12:13
  • $\begingroup$ I guess this is for regenerative braking on trains without overhead or 3rd rail power. Because in those, regen braking power goes back to the grid. Other forms of energy storage like flywheels or compressed air could also be used on wheels, but none of them in the air $\endgroup$
    – Chris H
    Jan 31, 2023 at 16:35
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Capturing CO₂ is normally done by letting it react with some absorber substance at one temperature and then letting this substance sweat the CO₂ out in a controlled environment and a higher temperature.

Engines create thrust by accelerating air. In order to harvest the CO₂ from that air, it first needs to be decelerated and cooled down. Similarly, the air flowing around the airplane needs to be brought to the airplane's speed, which produces drag. Aerodynamics has the goal of doing as little as possible of this "bringing up to speed".

Do you see the problem now? In order to process the exhaust and/or slipstream air, no net thrust will remain and additional drag will be created. Any attempt at capturing CO₂ will make the airplane into a poor glider.

And that is before we consider the energy needed for heating and cooling the absorber substance.

It is slightly more realistic to harvest brake energy. Modern landing gear concepts include electric hub motors so the airplane can taxi without letting its engines run. This saves a bit of fuel overall. The same system can be used to harvest energy, but only a fraction of what needs to be dissipated during braking.

As a rule of thumb, you can decelerate by running the electric motor as a dynamo during deceleration with about as much torque as your motor produces when accelerating the airplane. Also, the hubs already contain the brakes, so any additional motor cannot be large and powerful enough to absorb all braking energy during the runout but only a small fraction of it.

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This is probably the most wrong avenue. Aircraft are rather small and need to be lightweight. Moreover, they generate thrust by expelling the offgas out their turbines.

If one wanted to "harvest" CO2 there are two much more promising avenues

  • Capture the offgas on any stationary plant where "stuff" is burned, ideally being burned using oxygen rather than ambient air. Stationary plant, or buildings in general, have no functionaly weight or size restriction similar to aircraft. Furthermore, capturing offgas from most plants (powerplants, garbage incinerators, industrial furnaces, etc.) does not hamper their primary purpose. Capturing the offgas from an aircraft effectively grounds it.
  • If, for any reason, one wants to do direct air capture, why do it on a small, moving aircraft? You can build a direct air capture plant in a windy location with access to cheap energy and be much better off that trying to capture CO2 from the ambient air surrounding an aircraft.
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If you want to consider a balloon or dirigible, why not?

However, CO2 scrubbers are just as easily land based. Recovery of CO2 with caustic soda yields a saleable product: baking soda.

There is no need to go airborne to recover it. Perhaps better off not to make it in the first place.

But aircraft rely on relatively light and energy dense liquid hydrocarbon fuel, and probably will well into the future. In the process of combustion, oxygen adds to hydrocarbon. The products of combustion weigh more than the fuel. Add to that the extra weight of absorbent material. Realisticly, a heavier than air craft cannot bear the weight of "bringing home its own carbon".

Burning liquid hydrogen is a somewhat viable alternative, but the fuel is extremely cold and will require its own infrastructure. Hydrogen is also not nearly as energy dense.

But viewing the issue globally, those who are making CO2 emissions could go a bit more "net carbon zero" with a ground based recovery system.

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