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I couldn't find a fair comparison. Let's the compare the following two:

  • A direct air capture plant which is surrounded by solar panel farm which provides the energy to run it. It obviously runs only during the day. In a typical implementation a DAC plant captures CO2 into an alkali solution eg. KOH or NaOH as carbonate ions. Which then reacted by Ca(OH)2 to precipitate the insoluble CaCO3 (limestone) and restore the KOH/NaOH solution. The limestone is then subjected to high heat to release CO2 and store it in a tank which then can be trucked/piped away. The reactants are recycled. So power is needed to run the fans for air filtration and power is needed to bake out the limestone. (There may be less energy intensive processes I don't know about.)

  • A forest planted into the same area: it captures CO2 as it lives. Let's assume we collect all carbon containing parts: eg. the fallen leaves and other shedding and process them. One way to process it is using hydrothermal liquefaction which turns the organic matter into crude-oil like substance which provides a way to remove the carbon from it. What remains is a nutrient rich sludge that can be returned to the forest.

There are a lot of comparisons on the web but they often compare the net DAC plant area without the area needed for the energy source with the carbon sequestered in the wood of the forest only without considering the fallen leaves and stuff.

My question is which of the two can collect more CO2 per area per year? Are forests better at capturing CO2 than artificial solutions.

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  • This is not a criticism of the question, but the DAV process seems convoluted. It makes me wonder why not leave manufactured limestone as is & try to use as gardening or building material?
    – Fred
    Jul 27, 2023 at 9:36
  • @Fred I calculated that to create sodium hydroxide (NaOH) needed to react with 1 tonne of carbon dioxide, you need 3600 kWh of electricity for the chloralkali process. Most of the estimates of direct air capture put the energy usage per tonne of CO2 at far smaller values. Therefore, you want to recycle the chemicals (which releases CO2 as gas), rather than to get it as solid carbonates.
    – juhist
    Jul 27, 2023 at 18:26
  • Also, to create calcium hydroxide, you need to mix calcium oxide with water. The way calcium oxide is created is by heating calcium carbonate (requires heat energy), which then releases carbon dioxide. So if you want enough calcium hydroxide to capture the carbon dioxide, you need to release the same amount of carbon dioxide to produce the calcium hydroxide needed for that.
    – juhist
    Jul 27, 2023 at 18:30
  • If we want to store massive amount of CO2 as calcium carbonate, we need a massive source of calcium oxide or a massive source of calcium hydroxide, which we don't have.
    – juhist
    Jul 27, 2023 at 18:32

1 Answer 1

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A quick googling for energy need of direct air capture gives these estimates:

  • 1200 kWh/tonne-CO2
  • 2000 kWh/tonne-CO2
  • 8.8 GJ/tonne-CO2 (2400 kWh/tonne-CO2)
  • 14 GJ/tonne-CO2 (3900 kWh/tonne-CO2)

Let's use 2500 kWh/tonne-CO2 as a mid estimate.

A solar panel of 1 square meter at an optimal angle has 200 watts of nominal power at an efficiency of 20% (typical today), yet provides only about 23 watts of average power due to sunlight being available only during daytime, due to angle of sun varying, and due to clouds. However, this is for optimal angle. If the optimal angle is 45 degrees, then one square meter on the ground gets about 23*cos(45*pi/180) = 16 watts.

A square meter on the ground therefore gives 140 kWh per year.

A square meter can therefore extract 140/2500 = 0.056 tonnes of CO2 per year (56 kg).

Forest grows maybe at a rate of 5-10 cubic meters per hectare per year. That's 0.0005 - 0.001 cubic meters per square meter per year. One cubic meter of wood captures about one tonne of CO2, so there you have it: direct air capture with solar power gets you 56 kg, the same area of forest gets you 0.5 - 1 kg. That's nearly 100x difference.

Yet the costs are likely to have astronomical differences too. The wood that isn't used as long-lived sawlogs to create lumber is today used as pulpwood and energy wood and has a price of around 20 euros per cubic meter, or 20 euros per tonne of CO2. I suspect this is an order of magnitude below the current costs of direct air capture and the required solar panels and battery energy storage to power it, and direct air capture gives CO2 as gas, whereas wood gives CO2 converted to solid carbon. Far more convenient to store solid materials than to store a gas (CO2).

Also note that this assumes the only surface area used by direct air capture is the solar panels. In reality, you need enough airflow through the devices, which means the airflow itself is going to require quite large surface area too.

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