Does a solar powered direct air capture plant capture more CO2 from air than a forest planted on the same area?

Question

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.

Answer 1

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.