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I looked into air liquifaction a few years ago when there was talk of air cars but i think i found that liquid air technically doesn't exist at room temperature since 0°C is above the critical temperature of its major constituents. the storage is therefore limited due to continuously increasing pressure, or there's a requirement for cryogenic cooling, and if the cryo fails the tank could explode or have to release all your stored energy. Heat is a more useful thing to store overall, i think.