"This giant bubble on the island of Sardinia holds 2,000 tonnes of carbon dioxide. But the gas wasn’t captured from factory emissions, nor was it pulled from the air. It came from a gas supplier… “The facility compresses and expands CO2 daily in its closed system, turning a turbine that generates 200 megawatt-hours of electricity, or 20 MW over 10 hours.”
Compressing gas generates heat, and a significant part of that heat will be lost. Heat dissipation is irreversible, and this lowers efficiency a lot.
BTW the same reason why in industry, pneumatic drives are universally replaced by electric motors: Their efficiency is too low.
There is a thermal energy storage included as s major part. This works because compressing CO2 to 55atm adiabatically heats it up to some 450-ish C, so that heat is pretty high grade, and only the final stage cools it down with heat exchanger open to air. In discharging direction, some heat is taken from outside air to evaporate part of CO2 and heat stored is used up