Technology Readiness of 5th and 6th Generation Compliant Foil Bearing for 10 MWE s-CO2 Turbomachinery Systems

Heshmat, H., Walton II, J.F. and Córdova, J.L., “Technology Readiness of 5th and 6th Generation Compliant Foil Bearing for 10 MWE s-CO2 Turbomachinery Systems.” Paper presented at the 6th International Supercritical CO2 Power Cycles Symposium, Pittsburgh, PA, March 2018.

The availability of bearings capable of sustaining long life and maintenance‐free operation in the extreme temperature and pressure environments of supercritical carbon dioxide (s‐CO2) is key to the eventual commercial viability of s‐CO2 power generation systems. The development of advanced compliant foil bearings (CFBs) and tribological coatings capable of operation in such environments under high loads and speeds has been identified as key enabling technology for the development of s‐CO2 turbomachinery. Specifically, Korolon-coated 5th and 6th Generation (Gen 5 and Gen 6) oil‐free CFBs have been demonstrated to operate at temperatures as high as 871oC and journal/runner surface speeds as high as 550 m/sec. Without the speed and temperature limitations of conventional bearings, the aerodynamic and thermodynamic performance of turbomachinery improves significantly. Additionally, the use of foil bearings eliminates the need for liquid lubrication‐support infrastructure, like oil delivery/scavenging pumps, cooling and oil‐coking prevention mechanisms, filtration, and related maintenance, all of which add complexity to the system. It also eliminates the need for oil/gas seals to prevent the formation of unstable s‐CO2 and oil mixtures, which may result in hazardous byproducts such as carbonic acid and other carbonaceous materials. For all these reasons, the use of CFB‐based s‐CO2 rotor/bearings system will result in essentially maintenance‐free and long life turbomachinery.
Despite these clear advantages, CFBs have not yet gained universal acceptance due to two factors. The first is the perception based on old information that they are not yet robust enough for sustained performance in the demanding environments of s-CO2 turbomachinery. The other one is that in contrast to the a-posteriori matching of other types of bearings to existing turbomachinery designs, the successful incorporation of CFBs into s‐CO2 turbomachinery requires an integral approach that treats the pairing as a single rotordynamic and thermal system, with all elements designed from inception for optimal interaction.
Thus, the objectives of this paper are twofold. First, we set out to offer an improved understanding of the rheological and tribological behavior of hydrodynamic CFBs operating in a supercritical fluid. Second, we present results of a tradeoff study and considerations for a recompression s‐CO2 closed cycle system that have led to the conceptual design configuration of a two‐stage, 10 MWe ultra-high speed turbine powered generator. The turbine generator speed is deemed ultra-high since typical power generating systems are operated at least an order of magnitude lower speeds than the selected 55,000 to 60,000 rpm. This is twice the speed proposed for the GE 10 MWe s-CO2 turbine generator system and within 20% of the design speed for the DOE 250 kW supercritical carbon dioxide recompression closed Brayton cycle (RCBC) test assembly (TA) (Pasch, et al. 2014, Kalra, et al. 2014, Talbot, 2016). The interaction of the turbine with other system elements, like compressors, gear box and alternator and insight into the scalability of CFBs to systems between 1 to 100 MWe are also discussed. Based on the operating conditions identified in the tradeoff study, we also present a design for a test engine that will accommodate full‐scale components and will operate with s‐CO2 as the process fluid at realistic speeds up to 60 krpm, pressures up to 20 MPa and temperatures up to 750°C. The test engine will generate relevant data for characterizing the rotordynamic and thermal performance of the CFB-supported test rotor and CFBs. We expect that this work will lay the foundation to ultimately demonstrate the immediate viability of Gen 5 and Gen 6 CFBs for enabling s‐CO2 power generation.

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