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Optimisation of cycle and turbine designs for transcritical cycles of CO2-based working fluids

Aqel, O. (2023). Optimisation of cycle and turbine designs for transcritical cycles of CO2-based working fluids. (Unpublished Doctoral thesis, City, University of London)


Since their inception, CO2 power cycles have gained prominence for their excellent performance and compactness. Among their benefits, CO2 power cycles may reduce the levelised cost of electricity (LCoE) of Concentrated Solar Power (CSP) plants. Integrating them into CSP plants may require doping CO2 with other molecules to adapt the working fluid to dry cooling in locations with a high ambient temperature. The implications of doping CO2 on the design of the cycle and its components have not yet been fully explored, nor has there been an investigation into its implementation in small- to medium-scale power plants (0.1 – 10 MW). The aim of this thesis is to determine the implications of doping CO2 on the design of a simple recuperated transcritical cycle and the design of radial inflow turbine for small- to medium-scale CSP plants.

The study focuses on three dopants: TiCl4, SO2, C6F6. First, a thermodynamic model of a transcritical cycle was developed to compare the effect of doping CO2 with different dopants on the thermal efficiency of the cycle, the recuperator size, the expansion process, and the design of the turbine. The sensitivity of cycle conditions, specifically turbine boundary conditions, to the dopant fractions was also analysed. The maximum achievable cycle efficiencies were found to be 48.1%, 46.5%, and 42.2% for dopant molar fractions of 0.17 of TiCl4, 0.21 of SO2, and 0.17 of C6F6.

The effect of the choice of dopant and its molar fraction on the performance of recompression cycles was also investigated by considering six additional dopants. It was found that the benefit of a recompression cycle diminishes as the aggregate molecular complexity of the working fluid increases. For simple dopants, such as H2S and SO2, the recompression cycle will outperform the simple recuperated cycle, regardless of the dopant molar fraction. On the other hand, more complex dopants may achieve thermal efficiencies in simple recuperated cycles that are comparable to a recompression cycle. The dopant molar fraction at which both cycles achieve a similar performances depends on the molecular complexity of the dopant; the more complex the dopant, the lower the molar fraction at which this occurs.

Having established the intrinsic thermodynamic differences between the dopants, the consequences on the design of radial turbines was explored across power scales. This required the development of a mean line model based on experimentally for mulated loss equations, which was validated using data from the literature and com putational fluid dynamics simulations. Performance estimates from the mean line model were then used to update cycle design in a conjugate optimisation model that accounts for the achievable turbine performance of the mixtures across scales. This led to the identification of dopant properties that are advantageous to small-scale power plants.

In terms of radial inflow turbine design, variations in the achievable total-to-static efficiency amongst the fluids stem from variations in their clearance-to-blade height ratio, their pressure ratios, rotational speed limits, and, to a lesser degree, differences in their viscosity. Although doping CO2 has little effect on the aerodynamic behaviour of CO2, it is the consequence of the change in cycle conditions along with the design limitations of radial inflow turbines that lead to differences in the performance of the turbines amongst the fluids.

Finally, the modelling of binary mixtures requires the intramolecular interactions between the mixture components to be considered, usually through an equation of state supplemented by a binary interaction coefficient. The uncertainty in cycle and turbine designs associated with the choice of the equation of state and its calibration was found to be dependant on the mixture and power capacity. Cubic equations of state showed the most consistency in thermodynamic model results considering the choice and calibration of the fluid model, thus they are recommended for when using thermodynamic models to compare CO2-based mixtures.

Overall, this thesis successfully identifies the source of the differences between CO2-based working fluids, in terms of cycle design, turbine design, and modelling uncertainty.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QD Chemistry
T Technology > TJ Mechanical engineering and machinery
Departments: School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
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