Direct Air Capture (DAC) uses a lot of energy.

• Current processes use a bit more than 2000 kWh of energy to capture one tonne of CO2.
• This means that capturing one year of global CO2e emissions would require ~120,000 TWh of energy.
• 120,000 TWh is about the same amount of energy that the world uses per year. So we’d have to double the world’s energy supply just to power our giant hoover. [1] [2]

No-one is proposing that we capture all of our emissions with Direct Air Capture. But energy use is clearly critical to this technology.

In this post I’ll break down (A) where these huge energy requirements come from, and (B) what is unavoidable physics, and what is fertile ground for improvement. You won’t need a technical background to understand this post, you’ll just need to know that energy is measured in kilowatt-hours (kWh).

The basics

We need to quickly cover some basics before we dive in. I’ve put these in dropdowns in case you already know the drill

Energy visualised

Let’s compare the energy needed for carbon removal (DAC) to the energy needed for carbon capture (from point sources).

Capturing CO2 from the atmosphere uses waaay more energy than capturing it from the chimney of an industrial plant (where the CO2 is more concentrated).

Let’s first cover the fundamental physical laws at play here.

The theoretical minimum energy for carbon removal

The CO2 concentration in the atmosphere today is about 424 ppm, or 0.0424%. In a coal plant exhaust gas, the CO2 concentration will be around 11%. Therefore, separating CO2 from the atmosphere is a much harder problem, as our needle is buried in a much bigger haystack.

There is a fundamental physical limit on the minimum energy required to separate CO2 from the atmosphere, and it is 130 kWh per tonne of CO2 [3]. (Remember that for our coal plant, this only took 43 kWh per tonne).

This minimum energy cost doesn’t just apply to tech-based carbon removal. Nature-based solutions have to supply this energy too – it just comes in different forms. For example:

• In tree-planting, the energy to absorb CO2 is supplied by sunlight/photosynthesis.
• In enhanced weathering (using alkaline rocks to absorb CO2), the energy is supplied by the chemical bonds inside the rock.

After we’ve captured the CO2, we have to compress it in order to transport it or pump it underground. The theoretical minimum energy for this is 61 kWh per tonne of CO2 [4]. Adding this to the energy for separation, we end up with a total theoretical minimum energy requirement of 191 kWh per tonne of CO2.

Let’s just pause to emphasise this point: no amount of R&D, innovation or technology will allow us to remove CO2 from the atmosphere without spending at least 191 kWh per tonne of CO2.

Real life

Real life is a bummer, and we have to use a lot more energy than the theoretical minimum. In fact, we currently use more than 10x the minimum.

The leaders in deployed DAC systems are Climeworks, Carbon Engineering, and Global Thermostat. Climeworks and Carbon Engineering use processes that consume in the range of 2000 to 3000 kWh per tonne of CO2 [1]. Global Thermostat don’t publicly announce their energy numbers (it’s therefore likely that they’re not any better than the others).

Crucially, these energy requirements are not “what we got on our first try”, but long-term minimums that they expect to achieve under excellent conditions [5].

This is bad news because there are genuine reasons why DAC is harder than point-source carbon capture (more on this below). But it’s also good news, because there is a big opportunity to improve our current processes.

All energy is not equal

Things get more interesting when we consider the type of energy that these processes need.

For processes that have actually been built, like those used by Climeworks, Global Thermostat and Carbon Engineering, the majority of their energy requirement is heat, not electricity.

Most of this heat requirement comes from “regenerating” a capture material. If you read my last piece on carbon capture, you’ll be familiar with this concept. Here’s how it works for DAC.

Whilst this is greatly simplified, it is accurate to the major DAC companies today. The processes of Climeworks, Carbon Engineering, Global Thermostat and even Heirloom (“natural DAC”) are all just embellishments on this principle.

Funnily enough, it’s not absorbing the CO2 (step 1) that’s the hard part. It’s “regenerating” the capture material in step 2. The regeneration step dominates the energy consumption of current DAC technologies.

Climeworks and Global Thermostat use a solid capture material called a “sorbent” to adsorb CO2 (adsorb means that the CO2 sticks to the surface of the material), whilst Carbon Engineering uses a liquid solvent to absorb CO2. Heirloom uses crushed up calcium oxide (limestone with CO2 removed) to naturally soak up CO2.

In all of these cases, it’s temperature that’s used to regenerate the capture material (there are prototypes that use pressure, humidity, or other properties for regeneration, but they haven’t entered the mainstream yet). In fact, for both Carbon Engineering and Climeworks, 80% of their energy requirement is heat [5][6].

So if all of our mainstream DAC processes today use heat, where will we get it from? We pretty much have four options.

1. Waste heat

Industrial plants, power stations and cities all generate a lot of waste heat. Could we use it to power DAC?

Firstly, we need to consider the temperature of heat needed.

Company	Temperature required	Note
Carbon Engineering [5]	900°C	No waste heat at this temperature
Heirloom [7]	900°C	No waste heat at this temperature
Climeworks [6]	80 – 120°C	A bit of waste heat at this temperature
Global Thermostat [8]	85°C	A bit of waste heat at this temperature

Processes that run at high temperatures will have a very hard time finding waste heat to use.

Lower-temperature processes may be able to use some, but it will be limited. If you want to nerd out here, I included some numbers in my sources doc at the bottom [9]. Suffice it to say: whilst waste heat may help get DAC off the ground, it will not power a gigaton-scale industry.

 

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