Performance and Life Characteristics of Hybrid Gas Bearing in a 10 MW Supercritical CO2 Turbine

U.S. Department of Energy (DOE) has recently sponsored research programs to develop Megawatt scale supercritical CO2 (sCO2) turbine for use in concentrated solar power (CSP) and fossil based applications. From rotordynamics perspective, the extremely high power densities associated with this sCO2 turbine (similar to those of rocket turbo pump) and the long shaft of the turbine require mid-span support from bearings to achieve the high rotational speeds needed to achieve desired turbine efficiency targets. The mid-span bearings, if oil lubricated, need two sets of seals to isolate the working fluid from lubricating oil and represent a combined parasitic load of as much as 5% of the power plant output. These parasitic loads can be avoided by the use of hybrid gas bearing (HGB) which uses turbine working fluid itself as the lubricant for generating hydrostatic + hydrodynamic pressure and hence can operate in a high temperature, high pressure sCO2 environment. A successful and reliable implementation of HGB is expected to provide significant efficiency improvement in the sCO2 turbine for modular plants in 10MW scale. However, the behavior of hybrid gas bearing with supercritical CO2 as the working fluid has never been studied before. Hence a careful investigation of its hydrodynamic and rotordynamic characteristics with perturbations associated with sCO2 fluid and sCO2 turbine is warranted. In this paper the use of HGB to bring the critical speed ratio of highly power dense 10 MW sCO2 turbine into a manageable regime is investigated. Further, the performance characteristics of HGB in high pressure, high temperature sCO2 are investigated with a coupled fluid-structure interaction model. Differences between load capacity and stiffness characteristics for traditional operation in air versus operation in highly dense and viscous sCO2 are highlighted. Using Bayesian probabilistic models, insights into key design parameters needed to optimize HGB performance are obtained. A 3D fracture mechanics and crack propagation model is also developed to predict life of HGB under typical sCO2 turbine mission cycle for CSP application.