A method for generating mechanical work via a closed-loop Rankine cycle, the method comprising: passing a working fluid through a heat exchanging device to increase the temperature of the working fluid; passing the working fluid through a first volumetric fluid expansion stage to decrease the temperature and pressure of the working fluid and to create a third mechanical work; passing the working fluid through a second volumetric fluid expansion stage to decrease the temperature and pressure of the working fluid and to create a first mechanical work; passing the working fluid through a third volumetric fluid expansion stage to decrease the temperature and pressure of the working fluid and to create a second mechanical work; condensing the working fluid; and returning the working fluid to the first volumetric fluid expansion stage; characterized in that each of the first, second, and third volumetric fluid expansion stages (20-1, 20-2, 20-3) comprises: a housing with a fluid inlet and a fluid outlet; first and second interleaved counter-rotating non-contacting rotors (30, 32) disposed in a cavity of the housing, each of the rotors (30, 32) having lobes that are straight, twisted or helically disposed along the length of the rotors, each of the rotors (30, 32) having the same number of lobes; and an output shaft driven by said first and second rotors (30, 32); wherein the working fluid stream is admitted at the fluid inlet at a first pressure and is discharged from the outlet port at a second pressure lower than the first pressure, wherein the flow of the working fluid stream through the housing provides for rotation of the rotors (30, 32). passing a working fluid through a heat exchanging device to increase the temperature of the working fluid; passing the working fluid through a first volumetric fluid expansion stage to decrease the temperature and pressure of the working fluid and to create a third mechanical work; passing the working fluid through a second volumetric fluid expansion stage to decrease the temperature and pressure of the working fluid and to create a first mechanical work; passing the working fluid through a third volumetric fluid expansion stage to decrease the temperature and pressure of the working fluid and to create a second mechanical work; condensing the working fluid; and returning the working fluid to the first volumetric fluid expansion stage; characterized in that each of the first, second, and third volumetric fluid expansion stages (20-1, 20-2, 20-3) comprises: a housing with a fluid inlet and a fluid outlet; first and second interleaved counter-rotating non-contacting rotors (30, 32) disposed in a cavity of the housing, each of the rotors (30, 32) having lobes that are straight, twisted or helically disposed along the length of the rotors, each of the rotors (30, 32) having the same number of lobes; and an output shaft driven by said first and second rotors (30, 32); wherein the working fluid stream is admitted at the fluid inlet at a first pressure and is discharged from the outlet port at a second pressure lower than the first pressure, wherein the flow of the working fluid stream through the housing provides for rotation of the rotors (30, 32). a housing with a fluid inlet and a fluid outlet; first and second interleaved counter-rotating non-contacting rotors (30, 32) disposed in a cavity of the housing, each of the rotors (30, 32) having lobes that are straight, twisted or helically disposed along the length of the rotors, each of the rotors (30, 32) having the same number of lobes; and an output shaft driven by said first and second rotors (30, 32); wherein the working fluid stream is admitted at the fluid inlet at a first pressure and is discharged from the outlet port at a second pressure lower than the first pressure, wherein the flow of the working fluid stream through the housing provides for rotation of the rotors (30, 32).