Analysis of Turbomachinery Losses in sCO2 Brayton Power Blocks

This paper analyzes the contribution of different turbomachinery loss mechanisms to the overall efficiency of a simple recuperated supercritical carbon dioxide (s-CO2) Brayton cycle for output capacities ranging from 100 kW to 1 GW. The optimum turbomachinery specifications suitable for the specified powers are retrieved using a standard design tool that provides information on various turbomachinery losses. The losses are influenced by operating pressures and mass flowrates, which are unknown a priori. An iterative approach is used to arrive at the turbomachinery efficiency and mass flowrate. Earlier studies have shown the dependence of optimal pressures on heat source and sink temperatures alone. This analysis reveals that design-point optimal cycle pressure ratios differ with varying power outputs due to differences in realizable turbomachinery efficiencies. The information on dominant loss mechanisms provides insights on a viable scale of power generation at which s-CO2 Brayton cycles become worthwhile. Poor turbomachinery efficiencies (less than 80%) render the s-CO2 technology commercially unviable at the sub-MW scale. For higher power scales (10 MW and above), axial machines are found to be appropriate, with corresponding turbomachinery efficiencies greater than 85%. The dominant loss mechanisms also help identify issues related to improving turbomachinery efficiencies at the sub-MW power levels, where the cycle efficiencies are not competitive.