Abstract: An energy storage apparatus that includes at least one inlet for incoming process gas, at least one outlet for expanded process gas and a plurality of energy storage sub-systems configured to be arranged in series with each other and with a compressed gas store. The first of the plurality of energy storage sub-systems includes a first compressor, a first expander, a first thermal store and a first heat transfer device associated with the first thermal store. The second of the plurality of energy storage sub-systems includes a second compressor, at least a second expander, a second thermal store and a second heat transfer device associated with the second thermal store.