Thermo-fluid dynamic modeling of a supercritical carbon dioxide compressor for waste heat recovery applications

The development of novel technical solutions for the efficient recovery of waste heat is crucial for making accessible the large amount of thermal energy released by industrial processes, thus supporting the EU energy strategy. To this end, the EU-H2020 project CO2OLHEAT aims at developing and demonstrating a novel sCO2 power unit of 2 MW capacity, recovering energy from flue gases at 400 °C. The CO2OLHEAT system features a relatively unconventional multi-shaft configuration where the sCO2 compressor is driven by a dedicated radial expander, while the electrical power is generated via a separated axial turbine. The present study focuses on the design and fluid-dynamic analysis of the CO2OLHEAT compressor. The thermodynamic optimization of the cycle led to an overall pressure ratio slightly above 2.5, delivered with a two-stage centrifugal compressor. As typically found in sCO2 power systems, the thermodynamic state of the fluid at the machine intake (P = 85 bar; T = 32 °C) is close to the critical point and to the saturation curve; therefore, the first stage of the machine demands a dedicated aero-thermodynamic design, which can account for the effects of non-ideal thermodynamics and of the potential onset of two-phase flows. The paper discusses the conceptual aero-mechanical design of the compressor and then focuses on its performance assessment over the full operating range via Computational Fluid Dynamics. Two alternative CFD models are applied, the first one based on the barotropic fluid representation and the second one featuring a complete thermodynamic model, both of them assuming homogeneous equilibrium in presence of multi-phase flows. The experimental validation and the application of the two models are presented and discussed. They are shown to provide similar outcomes, and indicate that the compressor fulfills the system requirement and guarantees wide rangeability. A thorough comparison between the CFD results highlights the implications of the underlying physical differences between the models, thus allowing to properly evaluate their use for the aerodynamic design and analysis of sCO2 compressors.