A method of operating a compressed air energy storage system, comprising: compressing a process gas with a compressor train to produce a compressed process gas; directing the compressed process gas to a compressed gas storage unit and storing the compressed process gas in the compressed gas storage unit; extracting the compressed process gas from the compressed gas storage unit through a feed line; actuating a valve assembly to control a mass flow from the compressed gas storage unit to an expansion assembly, wherein the valve assembly is fluidly coupled to the feed line upstream of the expansion assembly and downstream from the compressed gas storage unit; heating the compressed process gas in a preheater before directing the compressed process gas to the expansion assembly, wherein the preheater is fluidly coupled to the feed line upstream from the expansion assembly; and generating a power output with the expansion assembly, the expansion assembly comprising a rotor shaft, a generator coupled to the rotor shaft, an air turbine assembly coupled to the rotor shaft, and a gas turbine assembly coupled to the rotor shaft, wherein generating the power output with the expansion assembly comprises controlling a total mass flow of the compressed process gas from the compressed gas storage unit to the gas turbine assembly, wherein controlling the total mass flow of the compressed process gas to the gas turbine assembly further comprises actuating a bypass control valve to regulate the distribution of the total mass flow through the air turbine assembly and a bypass line, wherein the bypass line is coupled to the feed line upstream of the air turbine assembly and further coupled to a line fluidly coupled to a combustor of the gas turbine assembly downstream of the air turbine assembly. compressing a process gas with a compressor train to produce a compressed process gas; directing the compressed process gas to a compressed gas storage unit and storing the compressed process gas in the compressed gas storage unit; extracting the compressed process gas from the compressed gas storage unit through a feed line; actuating a valve assembly to control a mass flow from the compressed gas storage unit to an expansion assembly, wherein the valve assembly is fluidly coupled to the feed line upstream of the expansion assembly and downstream from the compressed gas storage unit; heating the compressed process gas in a preheater before directing the compressed process gas to the expansion assembly, wherein the preheater is fluidly coupled to the feed line upstream from the expansion assembly; and generating a power output with the expansion assembly, the expansion assembly comprising a rotor shaft, a generator coupled to the rotor shaft, an air turbine assembly coupled to the rotor shaft, and a gas turbine assembly coupled to the rotor shaft, wherein generating the power output with the expansion assembly comprises controlling a total mass flow of the compressed process gas from the compressed gas storage unit to the gas turbine assembly, wherein controlling the total mass flow of the compressed process gas to the gas turbine assembly further comprises actuating a bypass control valve to regulate the distribution of the total mass flow through the air turbine assembly and a bypass line, wherein the bypass line is coupled to the feed line upstream of the air turbine assembly and further coupled to a line fluidly coupled to a combustor of the gas turbine assembly downstream of the air turbine assembly.