Design of a 130 MW axial turbine operating with a supercritical carbon dioxide mixture for the SCARABEUS project

Abstract

The application of supercritical carbon dioxide (sCO2) mixtures in power generation cycles has been shown to improve the cycle efficiency for concentrated solar power applications. That could be achieved through mixing CO2 with dopants to raise the critical temperature of the working fluid to allow condensation at ambient temperatures in solar filed locations. This enables converting the supercritical power cycle into a transcritical power cycle with lower compression power and hence higher thermal efficiency. This paper presents the flow-path design of a commercial scale axial turbine operating with an 80-20% mix of CO2 and sulfur dioxide (SO2) by mole as the SO2 dopant is found to be promising to increase the cycle thermal efficiency while offering a good thermal stability at the proposed turbine inlet temperature. The turbine design process for non-conventional working fluids is challenged by the availability of validated loss models, as well as the nature of the working fluid. Aerodynamic and mechanical constraints have been integrated into an in-house mean-line design code to derive the basic flow path at a turbine inlet conditions of 700 C and 240 bar, with a total-to-static pressure ratio of 2.94 and gross power output of 130 MW. It was found that suitable stress and rotodynamic constraints lead to a 14-stages design with 310 mm hub diameter and 1700 mm axial length which cannot be considered compact as mentioned in other sCO2 turbines case studies from the literature. The 3D blade geometry was then generated based on the initial mean-line design parameters along with preliminary assumptions that are evaluated and optimised utilising a computational fluid dynamics (CFD) model. The blade shape optimisation was carried out using a single-stage, steady-state model applied to the first and last turbine stages of the multi-stage design. The resulting modifications were extrapolated to the other turbine stages to evaluate the performance of the whole turbine. The final turbine design has shown a total-to-total efficiency of 93.3% with maximum stress that is less than 260 MPa and a mass-flow rate within 2% of the intended cycle mass-flow rate.