A compressed air energy storage system comprising: a compression system fluidly coupled to a storage compartment and configured to compress a first quantity of gas for storage in the storage compartment, the compression system comprising a compression path configured to convey the first quantity of gas therethrough; an expansion system fluidly coupled to the storage compartment and configured to expand a second quantity of compressed gas from the storage compartment, the expansion system comprising an expansion path configured to convey the second quantity of gas therethrough; a first path fluidly coupled to the compression path and configured to convey the first quantity of compressed gas to the storage compartment; a second path fluidly coupled to the expansion path and configured to convey the second quantity of compressed gas from the storage compartment to the expansion system; and a carbon dioxide separation unit fluidly coupled to one of the first path, the second path, the compression path, and the expansion path, wherein the carbon dioxide separation unit is configured to remove a quantity of carbon dioxide from one of the first quantity of compressed gas and the second quantity of compressed gas; wherein the expansion system is configured to decrease a pressure of the second quantity of compressed gas to ambient pressure, the expansion system comprising a plurality of expander units (208, 210) fluidly coupled to the expansion path; and wherein each expander unit is configured to decrease the pressure of the second quantity of compressed gas by a respective pressure less than a difference between the pressure of the compressed gas and ambient pressure the compressed air energy storage system being characterised in that the first quantity of gas comprises flue gas; and the compressed air energy storage system further comprises a third path fluidly coupled to the expansion system and to the compression system, the third path configured to convey a quantity of carbon dioxide-enriched gas from the expansion system to the compression system. a compression system fluidly coupled to a storage compartment and configured to compress a first quantity of gas for storage in the storage compartment, the compression system comprising a compression path configured to convey the first quantity of gas therethrough; an expansion system fluidly coupled to the storage compartment and configured to expand a second quantity of compressed gas from the storage compartment, the expansion system comprising an expansion path configured to convey the second quantity of gas therethrough; a first path fluidly coupled to the compression path and configured to convey the first quantity of compressed gas to the storage compartment; a second path fluidly coupled to the expansion path and configured to convey the second quantity of compressed gas from the storage compartment to the expansion system; and a carbon dioxide separation unit fluidly coupled to one of the first path, the second path, the compression path, and the expansion path, wherein the carbon dioxide separation unit is configured to remove a quantity of carbon dioxide from one of the first quantity of compressed gas and the second quantity of compressed gas;