An adiabatic compressed air energy storage (ACAES) system operable in a compression mode to compress air and in an expansion mode to expand air, the ACAES system comprising: a compressor system configured to compress air supplied thereto, the compressor system comprising: a plurality of compressors (22, 24); and a compressor conduit fluidly connecting the plurality of compressors together and having an air inlet and an air outlet; an air storage unit connected to the air outlet of the compressor conduit and configured to store compressed air received from the compressor system; a turbine system configured to expand compressed air supplied thereto from the air storage unit, the turbine system comprising: a plurality of turbines (36, 38); and a turbine conduit fluidly connecting the plurality of turbines together and having an air inlet and an air outlet; and a plurality of thermal energy storage units (42, 44) configured to remove thermal energy from compressed air passing through the compressor conduit and return thermal energy to air passing through the turbine conduit, each of the plurality of TES units (42, 44) being positioned on the compressor conduit along a length thereof between the air inlet and the air outlet of the compressor conduit and on the turbine conduit along a length thereof between the air inlet and the air outlet of the turbine conduit; the ACAES system characterised in that the compressor conduit and the turbine conduit are arranged such that each TES unit (42, 44) of the plurality of TES units is configured to operate at a first pressure state during the compression mode of operation and at a second pressure state different from the first pressure state during the expansion mode of operation. a compressor system configured to compress air supplied thereto, the compressor system comprising: a plurality of compressors (22, 24); and a compressor conduit fluidly connecting the plurality of compressors together and having an air inlet and an air outlet; a plurality of compressors (22, 24); and a compressor conduit fluidly connecting the plurality of compressors together and having an air inlet and an air outlet; an air storage unit connected to the air outlet of the compressor conduit and configured to store compressed air received from the compressor system; a turbine system configured to expand compressed air supplied thereto from the air storage unit, the turbine system comprising: a plurality of turbines (36, 38); and a turbine conduit fluidly connecting the plurality of turbines together and having an air inlet and an air outlet; and a plurality of turbines (36, 38); and a turbine conduit fluidly connecting the plurality of turbines together and having an air inlet and an air outlet; and a plurality of thermal energy storage units (42, 44) configured to remove thermal energy from compressed air passing through the compressor conduit and return thermal energy to air passing through the turbine conduit, each of the plurality of TES units (42, 44) being positioned on the compressor conduit along a length thereof between the air inlet and the air outlet of the compressor conduit and on the turbine conduit along a length thereof between the air inlet and the air outlet of the turbine conduit; the ACAES system characterised in that the compressor conduit and the turbine conduit are arranged such that each TES unit (42, 44) of the plurality of TES units is configured to operate at a first pressure state during the compression mode of operation and at a second pressure state different from the first pressure state during the expansion mode of operation.