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Comment by nine_k | Hacker News Reader

Can somebody versed in thermodynamics explain me how can it work?

They say that they keep CO2 in liquid form at room temperature, then turn it into gas, and grab the energy so released.

* Isn't the gas be very cold on expansion from a high-pressure, room-temp liquid? It could grab some thermal energy from the environment, of course, even in winter, but isn't the efficiency going to depend on ambient temperature significantly?

- To turn the gas into the liquid, they need to compress it; this will produce large amounts of heat. It will need large radiators to dissipate (and lose), or some kind of storage to be reused when expanding the gas. What could that be?

- How can the whole thing have a 75% round-trip efficiency, if they use turbines that only have about 40% efficiency in thermal power plants? They must be using something else, not bound by the confines of the Carnot cycle. What might that be?

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They store the heat from compression and use it during expansion.

You can see it in the little animation on their website. It's marked TES (thermal energy storage).

It looks like their RTE is based on a 10 hour storage time. The RTE is going to drop after their sweet spot, but if they're just looking to store excess energy from solar farm for when the sun isn't shining that's probably not a huge problem.

Storing the heat is the key part, I suppose, even though they are focusing on storing CO2.

I wonder if something like the paraffin phase transition could be used to limit the temperature of the heat reservoir, and thus the losses during storage.

According to one of their patents, they're just using an insulated container with some incoherent solid to store heat like gravel or ceramic granules.

My hunch is that they're doing this for three reasons.

1. Decompressing the gas can be used to do work, like turning a turbine or something. It's not particularly efficient, as you mention, but it can store some energy for a while. Also the tech to do this is practically off-the-shelf right now, and doesn't rely on a ton of R&D to ramp up. Well, maybe the large storage tanks do, but that should be all. So it _does_ function and nobody else is doing it this way so perhaps all that's seen as a competitive edge of sorts.

2. The storage tech has viable side-products, so the bottom-line could be diversified as to not be completely reliant on electricity generation. The compressed gas itself can be sold. Processed a little further, it can be sold as dry ice. Or maybe the facility can be dual-purposed for refrigeration of goods.

3. IMO, they're using CO2 as a working fluid is an attempt to sound carbon-sequestration-adjacent. Basically, doubling-down on environmentally-sound keywords to attract investment. Yes, I'm saying they're greenwashing what should otherwise be a sand battery or something else that moves _heat_ around more efficiently.

This is more of a compressed-air battery than a sand battery, except that the "air" is CO2 and it's "compressed" enough to cause a phase change.

Heat-based energy storage is always going to be inefficient, since it's limited by the Carnot efficiency of turning heat back into electricity. It's always better to store energy mechanically (pumping water, lifting weights, compressing gas), since these are already low-entropy forms of energy, and aren't limited by Carnot's theorem.

I don't know much about this CO2 battery, but I'm guessing the liquid-gas transition occurs under favorable conditions (reasonable temperatures and pressures). The goal is to minimize the amount of heat involved in the process, since all heat is loss (even if they can re-capture it to some extent).

I suppose that liquid CO2 just requires much less volume to store, while keeping the pressure within reason (several dozen atm). For it to work though, the liquid should stay below 31°C (88°F), else it will turn into gas anyway.

So, in a hot climate, they need to store it deep enough underground, and cool the liquid somehow below ambient temperature.

> they're using CO2 as a working fluid is an attempt to sound carbon-sequestration-adjacent

Um no, that's unfair. CO2 is an easy engineering choice here. It's easy to compress and decompress, easy to contain, non-flamable, non-corrosive, non-toxic and cheap. It's used in many applications for these reasons.

While CO2 is now a great evil among the laptop class, it has been a miracle substance in engineering for roughly 200 years now.

Yes, it is not clear why they have chosen CO2 beyond PR. There are other gas mixtures that would likely have better yields.

> They say that they keep CO2 in liquid form at room temperature, then turn it into gas, and grab the energy so released.

To evaporate something, you need to give it energy (heat). The energy flux through the dome walls is not huge, so CO2 boils away slowly.

> - To turn the gas into the liquid, they need to compress it; this will produce large amounts of heat. It will need large radiators to dissipate (and lose), or some kind of storage to be reused when expanding the gas. What could that be?

Well, you have this giant heatsink called "the atmosphere".

> - How can the whole thing have a 75% round-trip efficiency, if they use turbines that only have about 40% efficiency in thermal power plants?

A quirk of thermodynamics. CO2 is not the _hot_ part, it's the _cold_ part of the cycle.

To explain a bit more, if you confine CO2 and let it boil at room temperature, it will get up to around 70 atmospheres of pressure. You then allow it to expand through a turbine. This will actually _cool_ it to below the room temperature, I don't have exact calculations, but it looks like the outlet temperature will be at subzero temperatures.

This "bonus cold" can be re-used to improve the efficiency of storage or for other purposes.

Looking at the diagram on the web page, seems the key is the water. When expanding, use heat stored in the water to heat the gas. Likewise when compressing CO2 into liquid, use the water to store the excess heat generated?

Marketing. They have marketing.

I am _very_ suspicious the efficiency is anywhere close to 75%.

There are papers that do thermodynamic analysis of similar systems finding something like ~65% efficiency. So 75% might be a bit fluffed up, but not outrageously so.

E.g. if they can use the waste heat for district heating and count that as useful work.

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