Using pressurized CO2 (which changes to a liquid under pressure, even at ambient temperature) to store energy. The pressurized gas is used to run a turbine to recover the energy. The CO2 gas is recaptured in a domed building with a bladder inside, and is re-pressurized to complete the cycle and “recharge” the storage. They claim 80% round-trip efficiency. Simple, but does require a large building to store the CO2 gas.