Elasto-Hydrostatic Bearing for sCO2 Applications

Summary. Supercritical CO2 (sCO2) power cycles are superior to traditional water based, airbreathing, direct-fired, open Brayton cycles or indirect-fired, closed Rankine cycles in terms of efficiency and equipment size. They hold great potential in fossil fuel power plants, nuclear power production, solar power, geothermal power, and ship propulsion. DOE has invested in multiple sCO2 Power Cycle projects, including Sandia National Laboratories’ (SNL) 1MW Recompression Closed Brayton Cycle project and National Energy Technology Laboratory’s (NETL) Supercritical Transformational Electric Power (STEP) project. To unlock the potential of sCO2 power cycles, technology readiness must be demonstrated on the scale of 10 – 600 MWe and at sCO2 temperatures and pressures of 350 – 700 ºC and 20 – 35 MPa. Amongst many subcomponent challenges, the lack of bearings for sCO2 turbomachinery is outstanding. Oil film journal and thrust bearings are widely used in power generation turbomachinery. They need lubrication oil, which cannot operate in high temperature and solvency sCO2 conditions. One compromise is to isolate oil film bearings from the sCO2 environment, which mandates a thermal management system for reducing bearing temperatures. The compromise negatively affects the sCO2 efficiency and increases complexity of the equipment. Magnetic bearings do not require lubricants. But its windings can hardly survive at ~700°C sCO2 temperatures. Moreover, the starting and stopping of magnetic bearings require auxiliary mechanical bearings. Compliant foil bearings can operate in sCO2 environment. However, at sCO2 temperatures (350 – 700 ºC) and pressures (20 – 35 MPa), the load capability and reliability of metal foils in compliant foil bearings are simply not sufficient for long-term sCO2 turbomachinery operations. CO2 lubricated hydrostatic bearings have been studied and developed for sCO2 applications. While providing load capacity, stiffness and damping benefits, the conventional CO2 lubricated hydrostatic bearings have several intrinsic limitations: 1) Inflexible Feeding CO2. So far, the CO2 lubricated sCO2 hydrostatic bearings uses the CO2 either directly from sCO2 turbomachinery or from a CO2 tank, which makes the bearing inflexible for load variation and excessive vibration. 2) Limited Anti-Vibration Capability by Large Recesses. Large recesses of compressible lubricant, such as CO2 in a hydrostatic bearing can hardly accommodate vibrational operating conditions. 3) Low Temperature Bearing Materials. Traditional hydrostatic bearing materials, such as Babbitt, brass, and bronze cannot survive in temperatures of 350 – 700 ºC. 4) Expensive Machining. Precision machining of bearing land and recess is expensive. To that end, we propose an elasto-hydrostatic (EHS) bearing (Figure 1) for sCO2 cycles by leveraging Ultool’s strong expertise in sCO2 elasto-hydrodynamic (EHD) seal, inexpensive sCO2 pump and back-pressure regulator (BPR). The proposed EHS bearing eliminates bearing recess and introduces elastically deformable bearing material, such as impregnated carbon graphite. The feeding sCO2 will be regulated by the sCO2 pump or BPR. The benefits are: 1) Feeding sCO2 will be regulated for stable operations. 2) Carbon graphite can operate at sCO2 temperatures. 3) No recesses will improve anti-vibration capability. 4) Simpler structure can be easily machined. Phase I will focus on the feasibility of the EHS bearing and a 2” prototype EHS bearing will be developed in Phase II. Key Words Supercritical Carbon Dioxide, Nuclear Power Cycle, Turbomachinery, High Pressure/Temperature, Bearing, Elasto-Hydrostatic Bearing, Loading Capacity, Antivibration.