[yoan9224]

The round-trip efficiency comparison (60-75% vs lithium-ion's ~90%) is interesting but somewhat misleading without context. For grid-scale storage, the relevant question isn't efficiency in isolation - it's lifecycle economics including capex, degradation, and replacement cycles.

Lithium-ion has superior efficiency but degrades significantly after 5,000-7,000 cycles, typically reaching 80% capacity in 7-10 years. If CO2 batteries can maintain performance for 20+ years with minimal degradation (which the article suggests), the lower efficiency becomes less relevant. You're trading 15-25% energy loss for potentially 2-3x longer operational life and no lithium supply chain dependencies.

The real breakthrough is duration-flexible storage. Lithium-ion economics break down beyond 4-hour discharge rates because you're paying for both energy capacity and power capacity. CO2 systems decouple these - the turbine size determines power output, the storage tank size determines duration. That makes them ideal for seasonal storage patterns where you might charge for days during high renewable production and discharge slowly over weeks during winter lulls.

What's missing from the article: what's the round-trip efficiency at different discharge rates? Does efficiency drop significantly when discharging over 12 hours vs 4 hours? That would determine whether these make sense for daily solar smoothing vs weekly wind intermittency vs seasonal storage.

[whimsicalism]

I think it is generally polite to flag when you are using an LLM to write your comment, some people tire of reading the same style of writing over and over - even if the content of your comment is interesting!

[nayuki]

Oh good point; I wouldn't have noticed if you didn't point it out. The last ~5 comments from yoan9224 are all in 4-paragraph format. A few comments before that are in 3-paragraph format. They all look suspiciously uniform in writing style, and very mechanical.

[nine_k]

The system actually sort of uses the atmosphere as an ambient heat sink (when compressing) or heat source (when expanding).

I wonder if that heat could be stored in a more sensible way, e.g. as heated water in a tank near the bubble. This could improve the efficiency figures at short repeating patterns (charding at high noon, discharging through the night).

[anotherpaul]

As far as I understand they do try to keep the heat around for the next decompression. As of course they need it. But I could not find what type of heat storage they use. Ultimately they "only" seem to need to store it for 12h, right?