Development of a Turbine Concept for Supercritical CO2 Power Cycles

Abstract

Supercritical CO2 (sCO2) is considered as a working fluid in future Concentrating Solar Power (CSP) applications
to increase the efficiency of the power cycle. Due to the limited experience, the turbine of an sCO2 power cycle is in literature referred to as the component with the highest risk. In this thesis, a design methodology will be established and used to gain knowledge in sCO2 turbine design by designing and analysing a turbine concept which can be used to generate sCO2 turbine experimental data for validation purposes. The high density of the fluid in combination with the low power output of the turbine results in small turbine dimensions, a high rotational speed and a large axial load when using existing design wisdom. Therefore, the effect of limiting the rotational speed and axial load on the turbine design is evaluated.

The turbine design methodology includes a preliminary design module, an aerodynamic design module and a mechanical design module. The aerodynamic design module consists of a steady-state and transient solver to evaluate the importance of taking into account transient effects when estimating the aerodynamic performance. According to literature, pressure loads can have a significant effect on the stresses in sCO2 turbines. Therefore, the mechanical design module is able to take into account the pressure loading from the steady-state and transient aerodynamic design.

The low power in combination with the limited rotational speed pushes the turbine design away from the desired specific speed range. During the preliminary design, the radial inflow turbine was found to give the best performance while meeting the turbine requirements. The degree of reaction of the turbine is close to
0, to limit the axial load due to the pressure difference across the rotor. As a result of the low specific speed, the turbine efficiency of 74% is lower than usually seen for radial inflow turbines. As no sudden peaks in entropy generation are seen across the rotor channel, it can be concluded that the high curvature of a low reaction radial inflow turbine does not seem to have a detrimental effect on the turbine performance. The main contribution to the losses can be found in the rotor channel as a result of the tip clearance losses and high flow velocities compared to reference turbines, while the stator losses are comparable. A good agreement is found between the steady-state and transient aerodynamic analysis. Based on this, it can be concluded that a steady-state analysis can give a good approximation of the turbine performance, but evaluating more designs would be required to generalise this statement to all small-scale low reaction sCO2 turbomachinery. Due to the low degree of reaction of the turbine, the stresses originating from the pressure load are found to be less than 1% of the total stresses. Based on this, it can be concluded that only taking into account centrifugal loads for a low reaction sCO2 turbine gives a good estimation of the stresses in the turbine during operation.

As this research focuses to a large extend on establishing the methodology, for future research it is recommended to perform a more thorough optimisation of the turbine shape, in particular to reduce the losses in the rotor channel by for example introducing a shrouded design. Moreover, it is recommended to analyse the turbine in combination with the other subsystems in the power block to also take into account the interaction between the subsystems.