SUPER-CRITICAL CARBON DIOXIDE POWER CYCLE FOR WASTE HEAT RECOVERY UTILIZING HERMETIC OIL-FREE TURBOMACHINERY: CYCLE AND CONCEPTUAL TURBOMACHINERY DESIGN

This paper focuses on the conceptual design of a supercritical carbon dioxide (sCO₂) waste heat recovery (WHR) power drivetrain for application to natural gas (NG) compressor stations. The first goal of this work was to determine the potential benefits from operating a sCO₂ WHR unit with a novel oil-free hermetically sealed dual-spool power drivetrain. A second objective was to identify existing technology gaps and risks that surfaced through the design process. The drivetrain configuration utilizes process gas lubricated bearings and an immersed electric machine, which enables oil-free and leakage-free operation of the power system. The concept adopts a cascaded Brayton bottoming cycle powered from the 510 ^oC and 89 kg/s exhaust of a 34MW aeroderivative gas turbine used in NG compression stations. Operating boundaries for the proposed bottoming cycle reside between 237 bar, 485 ^oC and 85 bar, 133 ^oC. The proposed concept utilizes a dual spool drivetrain configuration possessing a high-speed gas-generator rotor aerodynamically coupled to a low-speed power turbine. The results from the study showed that the oil-free drivetrain configuration has the potential to increase the aeroderivative gas turbine simple cycle efficiency of 41% to a combined cycle efficiency of 51% with a potential of generating an additional 8.5 MWe of emission-free power from the bottoming cycle. Key risks identified for advancing the concept include bearing load capacity, rotordynamics, and thermal management. Finally, the proposed oil-free hermetic sCO₂ power system is compared to an installed Organic Rankine Cycle (ORC) specific case study, which was particularly useful when comparing output power, recovered energy, and improved efficiency of the present concept.