A method for operating a compressed air energy storage system is provided. The method can include compressing a process gas with a compressor train to produce a compressed process gas and storing the compressed process gas in a compressed gas storage unit. The method can also include extracting the compressed process gas from the compressed gas storage unit to an expansion assembly through a feed line. A valve assembly fluidly coupled to the feed line can be actuated to control a mass flow of the compressed process gas from the compressed gas storage unit to the expansion assembly. The method can further include heating the compressed process gas in a preheater fluidly coupled to the feed line upstream from the expansion assembly, and generating a power output with the expansion assembly.