A company named Energy Dome has completed a proof-of-concept facility on the Italian island of Sardinia that can store energy for a power grid for up to 10 hours at less than half the cost of a Li-ion battery system. The medium of storage is carbon dioxide. When renewable energy sources such as wind and solar are in abundance, they are used to draw CO₂ from the closed system’s dome and compress it at ambient temperature to a liquid state. When energy is needed, the CO₂ is allowed to expand into an electricity-generating turbine, and then returned to the dome, where it remains until the next charging cycle.