a method for operating an air - conditioning system for an aircraft includes compressing ambient air with a compressor driven by a shaft in communication with a motor and a turbine . the method includes forwarding the compressed air to an aircraft cabin for circulation and removing circulated compressed air from the aircraft cabin . the method also includes forwarding the circulated compressed air to the turbine , depressurizing the circulated compressed air in the turbine and capturing energy created by depressurizing the circulated compressed air .