Govt may stash carbon dioxide underground

[duncan drennan]

I don't really understand this approach, it seems a bit backwards. It may be slightly more difficult, and take a bit longer, but surely it makes more sense and is more sustainable to focus on energy savings, and reduction? There are plenty of ways that we in SA could be more energy concious, and reduce emissions.

I must say I'm quite curious about the technology behind this. Compressed CO2 is "dry ice", isn't it. Is this the plan?
Also, given that it takes a fair amount of heat to release gaseous CO2 from solid CO2, is a lot of heat released in the comression process? Could that energy also be realised as electricity?

I'm also curious. Not too sure about the harnessing the heat, but I am sure that it will take energy to change the CO2 from gas to solid - how much CO2 is being created as a side product to the energy that is needed to convert the CO2 to a solid?

[Dave A]

This is a bit of a shot in the dark, but here's my logic.

If we take solid CO2 and wish to convert it to gaseous CO2, all we have to do is put it in a tub with some warm frosties. The solid CO2 rather usefully chills the frosties as it sucks heat energy from them so that it can go gaily wandering off into the atmosphere.

Of course, this method of turning solid CO2 into gaseous CO2 is fairly energy efficient, and has some rather useful spin-offs.

Now by my rather primitive understanding of science this means that when converting gaseous CO2 into solid CO2, exactly the same amount of energy is going to have to be sucked out of the CO2 as we put in from our frosties. However, rather than simply refrigerate gaseous CO2 to achieve this effect (which is rather a pain actually because it has to get really cold before gaseous CO2 is going to transform into solid CO2 all by itself), we can take advantage of a particularly interesting feature of CO2 - it doesn't have a liquid phase.

So now we can take gaseous CO2, put it in a big space within something like a piston chamber, and slam it into a really small space where it conveniently turns solid.

*Just a word of caution here to anyone wanting to try to turn steam into ice this way: It's not going to work and you'll probably break your piston because steam (gaseous H2O) first turns into a liquid (which is kinda OK), but then has this irritating quirk of expanding a little to become a solid. Fortunately, this trait proves useful elsewhere. Anyway, I digress and happily CO2 does not even become a liquid so I have no idea why that even came up*

I'm betting that piston chamber gets really hot in the process of slamming the gaseous CO2 into solid CO2. Now we could use that heat to generate steam to drive a turbine which is connected to an electrical generator (by now you have hopefully caught my drift) and in this way recover some of that energy and convert it into electricity. Just like we do with burning coal which is our source of CO2 in the first place.

Even if we can't get this efficient enough to produce a nett positive in terms of energy invested vs energy returned (power to drive piston vs electrical power generated) it still has to be way better than just dumping that energy through evaporator coolers into the atmosphere, thereby adding to the heat of the atmosphere which somehow I recall was vaguely connected to why we were trying to capture the CO2 in the first place.

But given how well solid CO2 chills frosties, I've got a sneaking suspicion there might just be a nett surplus. Which means we get a double bite at the energy potential of coal.

Of course, now we move onto the complexities of storing this solid CO2 underground, but I think I'll save that for another restless night.

[Dave A]

And on a more serious note - we're talking about the latent energy associated with a transition from gas to solid. It should be a fair bundle.