In one embodiment, the volume 26 is

In one embodiment, the volume 26 is the cabin of an aircraft and the third medium A3 is cabin discharge air, which is air leaving the volume 26 that would typically be discharged overboard.In some embodiments, the system 20 is configured to extract work from the third medium A3.In this manner, the pressurized air A3 of the volume 26 can be utilized by the system 20 to achieve certain operations.[0029]The environmental control system 20 includes a RAM air circuit 30 including a shell or duct, illustrated schematically at 32, within which one or more heat exchangers are located.Within the heat exchangers 34, 36, 38, ram air, such as outside air for example, acts as a heat sink to cool a medium passing there through, for example the first medium A1 and/or the second medium A2.[0032]As shown, the compression device 40 includes a com pressor 42, a bleed air turbine 44, a fresh air turbine 46, and a power turbine 48 operably coupled to each other via a shaft 50.The bleed air turbine 44, the fresh air turbine 46, and the power turbine 48 are mechanical devices that expand a medium and extract work therefrom(also referred to as extracting energy).[0034]In the illustrated, non-limiting embodiment, the expansion device 52 is a two-wheel air cycle machine including a turbine 54 and a fan 56 operably coupled via a shaft 58.However, it should be understood that any suitable expansion device, including an air cycle machine having any number of wheels(i.e. three-wheel or four -wheel)are also within the scope of the disclosure.