Search

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. [ABSTRACT FROM AUTHOR]

To overcome the degradation of the cycle efficiency of a supercritical carbon dioxide (S-CO2) Brayton cycle with dry cooling, this study proposes an improved design of an S-CO2 centrifugal compressor. The conventional air centrifugal compressor can achieve higher efficiency as backsweep angle increases. However, the structural issue restricts the maximum allowable angle (−50~−56°). In this study, an S-CO2 centrifugal compressor performance was examined while changing the backward sweep angle at impeller exit to study if the previous optimum backsweep angle for an air centrifugal compressor is still valid when the fluid has changed. It is shown through an analysis that an S-CO2 centrifugal compressor can achieve the highest efficiency at −70° backsweep angle, which is greater than the typical design value. The S-CO2 centrifugal compressor is less restricted from a structural integrity issue because it has low relative Mach number regardless of the low sound speed near critical point (Tc = 304.11 K, Pc = 7377 kPa). It is also shown in the paper that the variation of compressibility factor does not impact on its total to total efficiency since its Mach number is still lower than unity. Finally, it is also shown that a backward sweep impeller can achieve higher pressure ratio and operate stably in wider range as the mass flow rate is decreased. As further works, the suggested concept will be validated by the structural analysis and the compressor performance test. [ABSTRACT FROM AUTHOR]