My last thread on cost of capital was a hit, so I'm going to push the envelope and talk about entropy, and why looking at decarbonization from a second law lens can help cut away a lot of bullshit. Maybe it will be worth it? /1

Everyone naturally, viscerally understands the First Law of Thermodynamics. Energy must balance. Cannot be created or destroyed. It's an accounting identity. Think of the First Law like it's your dad: always fill up the car after a night out. /2

The Second Law, though, is your mom. Eyes in the back of her head and shows up places where you never dreamed possible. The Second Law defines "entropy," a mathematical statement of a system's disorder. To decrease entropy, or increase order, has an energetic cost. /3

It costs energy to reduce entropy. And once you look for it, entropy is EVERYWHERE. Riding your bike down the hill? First law: You can't climb a higher hill. Second law: You can't climb up the same hill. Relevant for decarbonization, entropy shows up in concentration gradients./4

Biological systems, for example, require energy to maintain and enhance concentrations of ions. This is how your cells do work. Think about mixing Scotch and water: you can't unmix the two without a lot of work. That's the Second Law. And it's relevant for decarbonization... /5

...because removing CO2 from the atmosphere at a concentration of 400 ppm (0.04%) is really hard! There's a ton of calculus on this, which I don't know because I'm dumb, but the upshot is that the more dilute the solution, the more energy it requires to concentrate. /6

This seems like common sense, but I'm amazed by how much common sense goes out the window when we see a perpetual motion machine proposed. See, for example, $OXY investing a yard into this project. /7 https://www.reuters.com/article/us-usa-carboncapture-dac-idUSKCN25F1VN

I can't find @CarbonEngr 's energy balance. That's important in a process that's trying to move so much CO2 uphill! I have a feeling that the energetic cost, even with zero-carbon energy, will be unreasonably high for any direct air capture process. /8

Or consider carbon capture from an industrial or powergen process. Remember that pressure drop is a form of energy! Capturing relatively low concentration CO2, say 10-20% CO2, from a flue gas with zero pressure drop available is really hard and expensive. /9

And it's not hard and expensive only because there are engineering challenges. It's hard and expensive because separation costs energy, that cost is built into the physics of our universe, and it's unavoidable. /10

Always, the question to ask before investing in capture tech is, "What is the energy cost?" Not just the capex, opex, but also what is the theoretical limit of separation efficiency. It's surprisingly hard to find this stuff on the open internet. If I were to bet... /11

... I would say that (1) reduced species capture/combustion is the cheapest approach. eg, methane.

(2) Combustion with pure oxygen (and no CO2 separation) is likely superior to CO2 concentration in flue gas. Even though air separation units are expensive. /12

(3) Until we are using very little carbon in our energy mix, there's probably small benefit to carbon capture technologies. "I'll feed my carbon capture with solar power," is inferior to, "I'll use solar power." /13

Let me know if I've missed the boat here. I haven't done real engineering in years, so I may need re-education. Also interested in sources, other discussions on this topic. /14