Hacker News Reader

They claim 75% efficiency AC-AC [0], and they point out that there’s no degradation with time. What estimates are you using to arrive at the 25% figure?

[0] https://energydome.com/co2-battery/

i didn't do any math tbh, i just took the 25% number from the wikipedia page for cryogenic energy storage. i assumed their efficiency would be lower of the smaller temperature differential, but maybe it will be higher because they are storing part of the energy as pressure rather than temperature

The energy used to liquefy the CO2 is the bulk of the energy stored. They don't throw it away afterwards. The the liquid-gas transition is why this works so much better than compressed air.

of course they are not throwing the energy away. but by using a working fluid that changes phase they are trading away energy efficiency for power density, for the same reason that steam engines are less efficient than stirling engines.

Heat from compression is stored in a thermal energy storage system. Most likely something like a sand container.