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Comment by mdoliwa | Hacker News Reader

Stupid question, if we find a commercially viable way of converting CO2 into fuel is there a danger of "global cooling" because people will get too greedy with this?

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No: the atmospheric CO2 is at the bottom of the energy hill. In order to turn it in to fuel you need to pump in all the energy originally extracted from the fuel, then some more.

After you've re-expended the entire 20th century's worth of fossil fuels energy equivalent in sucking CO2 out of the atmosphere, not burned it, and stored it in a country-sized propane tank, then maybe we're back to 19th-century temperatures.

The earth is not a zero sum game, we get energy from the sun. Photosynthesis reverses the process turning co2 into sugar and o2.

And? So?

That means co2 is not the bottom of the energy hill.

"Bottom of the energy hill" is a chemist's short hand way of saying that we can't practically extract energy from CO2 reacting with anything useful, meaning chemically this is at the bottom end of reactions that happen without energy input. To convert it into something reactive, you need to invest energy to split it up again into its constituent parts, which is what happens during photosynthesis.

"Bottom of the energy hill" is a useful term in a lab. However the earth is not a chemistry lab. We cannot ignore photosynthesis outside of a lab as it is a factor that exists.

> We cannot ignore photosynthesis outside of a lab as it is a factor that exists.

I apologize if this wasn't clear. It's an expression about energy potentials, and as such it's not just useful in a lab, it's an essential piece of information about the substance.

For example in the original context of this comment thread, a person asked whether there was a danger of people exhausting our atmospheric CO2 because they got too greedy with this technology - a question that can in fact not be answered meaningfully without talking about energy deltas. The fact that CO2 cannot be practically processed in a way that releases energy is the only pertinent information when talking about this.

In the photosynthesis case the bottom of the energy hill is helium in the sun’s core.

Well and also the CO2 isn't acting as a fuel, it acts as a battery or resource.

Plants make sugar from CO2, and also burn that sugar back to CO2 (plants respire overnight - they're not pure CO2 consumption machines).

- CO2 isn't the only greenhouse gas.

- This process produces hydrocarbon fuel. It will be burned and the carbon will return to the atmosphere.

I suppose propane could become a chemical feedstock. Then the carbon could theoretically be tied up for a while.

Even if it winds up back in the atmosphere, it's displaced an equivalent amount of new CO2 going into the atmosphere.

Pulling enough CO2 to make a kilogram of propane and then burning that kilogram of propane for electricity is still better than burning a kilogram of freshly fracked propane.

One offsets consumption. One just adds consumption.

Only if your source of electric doesn't add CO2. If you burn coal to make propane you are worse off CO2 wise than using regular propane. We have a lot of wind and solar, but most places they are still a minor part of the electric mix (if this applies to you, it wouldn't be hard to get a lot more wind/solar in your grid).

Only if more fuel is created than is used, which would seem to be a waste of energy. On the whole, it should be a balanced loop and should net off.

Burning the propane will release the same amount of Co2 again... So I doubt it will change much

The application of this is not in energy production or - realistically - CO2 capture on a global scale. This takes huge amounts of energy to do and requires a sophisticated chemical synthesis in the background in order to replenish the catalyst.

Pretty much the only reason to do this would be because you're specifically interested in generating propane. For example, it could be very useful for ISRU on other planets, or to generate propane "for free" from a solar setup.

If your goal is energy production, you'd just use the output of solar panels directly without this costly step in the middle. If you want to store energy locally, electrolyzing water into H and O would be hugely more cost effective. But propane is a more dense fuel that would be useful for mobile applications such as ships and cars, and can also be used as a raw material in chemical synthesis.

Theoretically it would be possible, but we would just release some carbon from other sources, there's a lot of it in various rocks if fossil fuels somehow expired (we still have a lot of those).

Taking the carbon from the atmosphere will always be more expensive than from some carbon-rich rocks that form a huge fraction of the Earth's crust.

In fact, our most effective ways to take carbon out of the atmosphere today all involve a step of letting some mineral turn into one of this carbon-rich ones, and extracting it from there.

It'll always be more expensive to extract carbon from the atmosphere, but if it produces a valuable byproduct and the power to run the process is very cheap (excess solar) it might still pencil out.

I fail to see anything that doesn't apply equally to the rocks.

When you have surplus photovoltaic capacity — an increasingly common situation — you could turn it to electrolysis. You still have to pay for the equipment, but it sounds like it could be pretty cheap.

It would cut out the cost of mining, and possibly a lot of transportation costs, if you can use the products nearby.

Yes, and before humans the earth was on a cooling trend, though only measurable on a geological time scale (millions of years). A little plant matter gets converted to charcoal and then coal every years. Though biology mostly trys to get at that carbon first.

That would be an easy problem to solve. We are very good at heating up the atmosphere.

More CO2 should also stimulate plant growth, CO2 is a bit of a limiting factor there.

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