With the increasing emphasis on reducing the CO2 emission while improving power generation efficiency, new power cycles are being developed. One of those promising power cycles is a supercritical CO2 (S-CO2) power cycle. To generate over 10MW electricity with S-CO2 power cycle, a magnetic bearing can be a good option for the hermetic type turbomachinery. However, from several studies on the magnetic bearing, the instability issues under high density fluid and high speed operating conditions were repeatedly mentioned. The instability in the magnetic bearing was observed to be related to the fluid conditions, mostly pressure and density. Because of this issue, the magnetic bearing sometimes cannot maintain enough clearance for the rotor leading to physical contact and consequently damaging the system. Thus, these instability issues should be thoroughly studied and be resolved for the successful and steady operation of the power system. The instability due to fluid force around the rotating shaft can be modeled with the Reynolds lubrication equation. The steady lubrication force analysis model is developed based on this equation. The model results imply that the lubrication performance is quite sensitive to the thermal condition of the CO2 especially density gradient around the shaft. Based on the modeling results, an experimental system is designed to investigate the issue. To study the instability issues experimentally, an impeller of the operating S-CO2 compressor is removed and the discharge line is blocked. Therefore, the main instability factor in this experiment will be the interaction between the rotor and the bearing only. The shaft position can be measured with inductive sensors. The forces exerted from the electromagnet is calculated from the electric current data which is applied by the controller. From these experimental data, the lubrication force is calculated. These results are compared with the analytical lubrication model to verify the model. From this study, it is expected that it will be possible to define the unstable operating conditions and suggest the required magnetic bearing performance for S-CO2 conditions.

A supercritical CO2 (S-CO2)-cooled Brayton cycle is under development for distributed power applications for remote regions. In order to successfully develop it, issues of controlling shaft levitation with bearings have to be solved. From several studies, magnetic bearings have been suggested for reliable levitation performance with reduced cost and complexity. However, several studies on magnetic bearing show that instability issues under high-pressure fluid and high-speed operating conditions may exist. The purpose of this research is to provide background for understanding the instability of magnetic bearings under S-CO2 conditions and propose functional requirements of the magnetic bearing. Thus, the rotating shaft with magnetic bearings operating under high pressure fluid was first analyzed. To test the theory, a magnetic bearing test rig was constructed. By comparing experimental data to the analysis results, the analysis results were verified. Therefore, the analysis results can be used for predicting instability in the future and can contribute to the development of better magnetic bearing controllers.

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A supercritical CO2 (S-CO2) Brayton cycle is a compact and simple power conversion system with competitive efficiency. However, the strong real gas effects of S-CO2 pose challenges to the design of a cycle and its components. In particular, designing turbomachinery for expansion and compression processes has to accurately reflect real gas effects. Windage loss is one of the major losses that affects the motor load and heat generation in turbomachinery. The windage loss has a substantial impact on the overall turbomachinery efficiency especially in an S-CO2 power cycle since the windage loss is reported to be the dominant loss mechanism due to high fluid density and high rotational speed. Therefore, an accurate windage loss model reflecting the real gas effect of S-CO2 is essential to obtaining an optimal design of turbomachinery as well as maximizing the performance of an S-CO2 power cycle. In this study, existing windage loss models are first compared to the recently obtained data from S-CO2 windage loss experiments conducted by the KAIST research team under S-CO2 conditions in order to understand the turbomachinery performance uncertainty caused by the windage loss models. This is followed by proposing a new windage model which explains data better.