Two Stage Radial Compressor for a Kilowatt Scale Supercritical Carbon Dioxide Power Block: Design Considerations

Abstract

Potential applications of kilowatt (kW) scale supercritical Carbon dioxide (sCO2) Brayton power systems include exhaust waste heat recovery in diesel engines and bottoming cycles for biomass gasifier-driven gas turbines. However, the practical realization of the kW scale single-stage sCO2 turbomachinery is associated with several challenges. While a hermetically sealed arrangement is most beneficial, it is fraught with large windage losses in the motor rotor cavity. The high windage losses in the single-stage setup are primarily due to high shaft speeds. These issues limit the commercial utility of kW scale sCO2 Brayton power cycles. In order to mitigate these issues, the design of a two-stage unshrouded radial compressor unit for a kW scale sCO2 Brayton cycle is described in this study. The working pressures are 103 bar/ 170 bar, and the compressor mass flow rate is 2 kg/s. These optimal operating pressures are obtained taking into consideration both thermodynamic power and motor/disc windage losses. The two rotors are mounted on a single shaft in a double-ended configuration where the axial thrusts act in opposite direction. The compressor blade profiling is carried using a standard design tool. The performance of the compressor is assessed using a 1D mean-line calculation to estimate the overall efficiency. Using this approach, an overall compressor efficiency of ∼ 71.8 % (including motor and disc windage losses) is attained. A 3D CFD simulation is carried out at the design point to validate 1D data. Rotodynamic analysis is carried out to determine the first and second shaft critical speeds of the rotor-shaft configuration. Structural analysis is carried out to ensure that the maximum Von-Mises stresses are well below the material limits.