Paper on aerodynamic and mechanical design of large gas turbines presented by City, University of London and Baker-Hughes at ASME Turbo Expo, Boston (USA)

Turbine assembly cross section (©Baker Hughes Company, All Rights Reserved)


The results of the collaboration between City, University of London (United Kingdom) and Baker-Hughes (Italy) have been have presented at ASME Turbo Expo in Boston (USA). This joint research has taken place within Work Package 3 – Turbomachinery Design of SCARABEUS, aimed at developing turbine designs able to attain high efficiency when working with Carbon Dioxide mixtures at very high pressures and temperatures.

Turbine design is strongly influenced by the composition of the working fluid because of the impact of this feature on the operating conditions of the cycle that attain peak thermal efficiency, and also the impact of composition on fluid characteristics.


Flow field of three different designs of the exhaust section

The paper presents the results of the unsteady simulations of the last turbine stage and exhaust section to assess unsteady loads on the rotor, as well as aerodynamic losses in the diffuser and exhaust section. Rotordynamics are also studied.

The paper can be downloaded free of charge from the conference website (link). Check the abstract below:

In this paper, the design of a large-scale axial turbine operating with supercritical carbon dioxide (sCO2) blended with sulfur dioxide (SO2) is presented considering aerodynamic and mechanical design aspects as well as the integration of the whole turbine assembly. The turbine is 130 MW, designed for a 100 MWe concentrated-solar power plant with turbine inlet conditions of 239.1 bar and 700 °C, total-to-static pressure ratio of 2.94 and mass-flow rate of 822 kg/s. The aerodynamic flow path, obtained in a previous study, is first summarised before the aerodynamic performance is evaluated using both steady-state and unsteady 3D numerical models to simulate the aerodynamic performance of the turbine. Whole-annulus unsteady simulations are performed for the last turbine stage and the exhaust section to assess the unsteady loads on the rotor due to downstream pressure field distortion and to assess aerodynamic losses of the diffuser and exhaust section. The potential low engine order excitation on the last rotor stage natural frequency modes due to downstream pressure distortion is assessed. The design of the turbine assembly is constrained by current manufacturing capabilities and the proposed working fluid properties. High-level flow-path design parameters, such as pitch diameter and number of stages, are established considering a trade-off between weight and footprint, turbine efficiency and rotordynamics. Rotordynamic stability is assessed considering the high fluid density related to cross coupling effects. Finally, shaft end sizing, cooling system design and the integration of dry gas seals are discussed.

Paper on energy losses in radial turbines presented by City, University of London at ASME Turbo Expo, Boston (USA)

Fan power consumption at reduced mass flow rate for different ACC designs


The researchers at City, University of London have presented their work assessing the different contributions to energy losses experienced by radial turbines working with Carbon Dioxide mixtures in supercritical power cycles. This research work also considers the effect of turbine scale on the breakdown of energy losses, assessing this for turbines in cycles rated at 0.1 MWe, 1 MWe and 10 MWe. The work makes use of meanline design codes and CFD analysis to assess the impact of fluid composition and scale and to compare the data obtained against data from literature.

Meridional profile of radial inflow turbines working with different Carbon Dioxide mixtures and with different scales (plots are not to scale)

The paper can be downloaded free of charge from the conference website (link). Check the abstract below:

Recent studies have indicated the potential of CO2-mixtures to lower the cost of concentrated solar power plants. Based on aerodynamic and cost considerations, radial inflow turbines (RIT) can be a suitable choice for small to medium sized sCO2 power plants (about 100 kW to 10 MW). The aim of this paper is to quantify the effect of doping CO2 on the design of RITs. This is achieved by comparing the 1D mean-line designs and aerodynamic losses of pure sCO2 RITs with those of three sCO2 mixtures containing tetrachloride (TiCl4), sulphur dioxide (SO2), and hexaflourobenzene (C6F6).

Results show that the optimal turbine designs for all working fluids will have similar rotor shapes and velocity diagrams. However, factors such as the clearance-to-blade-height ratio, turbine pressure ratio, and the difference in the viscosity of the fluids cause variations in the achievable turbine efficiency. Once the effects of these factors are eliminated, differences in the total-to-static efficiency amongst the fluids may become less than 0.1%. Moreover, if rotational speed limits are imposed, then greater differences in the designs and efficiencies of the turbines emerge amongst the fluids. It was found that limiting the rotational speed reduces the total-to-static efficiency in all fluids; the maximum reduction is about 15% in 0.1 MW CO2 compared to the 3% reduction in CO2/TiCl4 turbines of the same power.

Among the mixtures studied, CO2/TiCl4 achieved the highest performance, followed by CO2/C6F6, and then CO2/SO2. For example, 100 kW turbines for CO2/TiCl4, CO2/C6F6, CO2/SO2, and CO2 achieve total-to-static efficiencies of 80.0%, 77.4%, 78.1%, and 75.5% respectively. Whereas, the efficiencies for 10 MW turbines are 87.8%, 87.3%, 87.5%, and 87.2%, in the same order. differences in the designs and efficiencies of the turbines emerge


Paper on off-design operation of ACCs presented by University of Seville at ASME Turbo Expo, Boston (USA)

Fan power consumption at reduced mass flow rate for different ACC designs


The researchers at University of Seville have presented their work assessing the part-load operation of supercritical power cycles running on Carbon Dioxide mixtures. As a continuation of the work presented at the 5th European Conference on Supercritical Carbon Dioxide Energy Systems, where the methodology to design ACCs for supercritical CO2 power cycles developed at University of Seville was presented, this paper explores the operation of the cycle when running in off-design. Emphasis is placed on the operation of the ACC because of the strong impact that a wrong operation of this component has on overall cycle performance.

The paper can be downloaded free of charge from the conference website (link). Check the abstract below:

This manuscript, developed in the framework of SCARABEUS project, presents an assessment of the part-load performance of a transcritical Recompression cycle running on a 80%CO2-20%SO2 mixture under different load-control schemes.

The first part of the paper describes the computational platform of the integrated system, implemented in Thermoflex but with profuse use of in-house scripts, in order to accurately describe the off-design performance of key components when operating on CO2 mixtures with non-ideal gas behaviour. These off-design models make use of performance maps for turbomachinery — provided by the SCARABEUS partners — whereas the Conductance Ratio Method employed to model the counter-current heat exchangers is calibrated with in-house tools. The paper is specifically focused on the Heat Rejection Unit, for which a specific design tool accounting for accurate heat transfer between working fluid and cooling medium (air) and for auxiliary power consumption — both in off-design — has been developed by the authors.

In the second part of the paper, different operating strategies of the power cycle are considered, based on keeping one of the following three parameters constant: turbine inlet temperature, turbine outlet temperature or return temperature of molten salts. Globally, plant operation is constrained by the need to keep the temperature of cold HTF returning to the storage system as close as possible to its rated (design) value and by the need to keep turbine outlet temperature below 450°C to avoid the installation of an external cooling system in the low pressure section of this equipment. Therefore, the trade-off between these two parameters and system net efficiency are assessed in the paper. Regarding the Air-Cooled Condenser, the optimal operation strategy of this component found to be based on a combination of Single-speed and Variable Frequency Driver fans.

The results show that the operation at constant turbine inlet temperature leads to the highest net efficiency of the power block, closely followed by the control scheme based on constant return temperature of the heat transfer fluid. Nevertheless, this latter option enables a perfect control on the other two figures of merit. As a consequence, the identification of the best operation strategy must be addressed in future works by means of a thorough techno-economic assessment considering the annual yield of the plant.

Paper on axial turbine flowpath design presented by City, University of London at ASME Turbo Expo, Boston (USA)

Turbine flowpath for a 100 MWe SCARABEUS plant running on mixtures of Carbon Dioxide and Hexafluorobenzene


The researchers at City, University of London have presented their work on the design of the flowpath of large axial turbines for integration into supercritical power cycles operating with Carbon Dioxide mixtures. This research is framed in Work Package 3 – Turbomachinery Design of the project.

The composition of the working fluid has a strong influence on turbine design, as a consequence of two effects. The first one is the impact of working fluid composition on the operating conditions of the cycle that attain peak thermal efficiency. The second one is the impact on the characteristics of the working fluid that are relevant to turbine design. Meanline design codes and CFD tools are combined to assess these effects and to produce optimum flowpath designs.

Impact of loading and flow coefficient on total-to-total efficiency of the turbine

The paper can be downloaded free of charge from the conference website (link). Check the abstract below:

Supercritical CO2 (sCO2) mixtures have been found to be promising for enhancing the performance of power cycles for concentrated solar power (CSP) applications, with up to a 6% enhancement in cycle efficiency compared to a simple recuperated CO2 cycle depending upon the mixture and cycle configuration chosen. Given that turbine efficiency significantly affects the overall plant performance, it is important to confirm whether turbines operating with CO2 mixtures can achieve the same efficiencies compared to pure CO2, whilst exploring whether the use of mixtures introduces any differences in the turbine design. This study aims to investigate the differences in turbine flow path designs produced for pure CO2 compared to CO2 mixtures, whilst taking into account aerodynamic, rotordynamic and mechanical design aspects, as assessed during the mean-line design process. The aim of this study extends to evaluating the effect of key turbine design variables, such as the loading coefficient, flow coefficient and degree of reaction, on the flow path design and overall aerodynamic performance. Multiple flow path designs have been produced for axial turbines operating with pure CO2 and mixtures of CO2 with titanium tetrachloride (TiCL4), hexafluorobenzene (C6F6) and sulphur dioxide (SO2) for installation in a 100 MWe CSP plant. It is found that turbines operating with either pure CO2 or CO2 mixtures result in overall total-to-total efficiencies in excess of 92.5%; where the highest turbine efficiency is achieved for the turbine operating with pure CO2, whilst this reduces by a maximum of 1.1 percentage points for the CO2/TiCL4 mixture. This reduction in efficiency is because the CO2/TiCL4 turbine is limited to a maximum of six design stages in order to meet the imposed mechanical design criteria, whilst the pure CO2 turbine can accommodate thirteen stages leading to higher aerodynamic efficiency. The difference between the two cases is the result of a higher mass-flow rate for the CO2/TiCL4 mixture (66% greater than for pure CO2), which results in high rotor bending stresses and limits the number of stages to comply with the design criteria. It is also found that designing the turbine at loading and flow coefficients of 0.8 and 0.6 respectively, whilst fixing the degree of reaction and pitch-to-chord ratio to values of 0.5 and 0.85 respectively, resulted in an efficiency enhancement of 0.2% with respect to a baseline design produced at loading and flow coefficients of 1.0 and 0.5. This increase is due to being able to increase the number of stages from eleven to fifteen. This indicates that there is not much benefit in modifying key design parameters to improve the turbine efficiency as the 0.2% efficiency enhancement is considered within the margin of accuracy of mean-line flow path design.

Very active participation of the SCARABEUS consortium at ASME Turbo Expo in Boston

Turbo Expo is the flagship conference in the field of turbomachinery for power generation and aircraft and marine propulsion. The conference gathers ~2500 professionals annually, who meet to share the latest development of the scientific and industrial communities through technical, panel and tutorial sessions as well as keynote talks. The conference is organized by the International Gas Turbine Institute of the American Society of Mechanical Engineers.

Turbo Expo has been one of the preferred venues to disseminate the outcomes of the SCARABEUS project, as communicated in previous years on this website, and this year has not been different.

In the first day of this year’s conference, held at the Hynes Convention centre in Boston (MA), Prof. David Sánchez organised a panel session to introduce the “Supercritical CO2 Projects in the International Community”. Seven projects funded by the Horizon 2020 and Horizon Europe programmes of the European Commission were presented in this session, followed by a very interesting panel discussion. The SCARABEUS project broke the ice, setting the stage for the other projects to complete the landscape of supercritical Carbon Dioxide research in Europe. The kind support of Eric Clementoni (Bechtel Marine Propulsion Corporation), stepping forward as improvised Co-Chair of the session due to the absence of Dr. Jason Wilkes (Southwest Research Institute) is gratefully acknowledge.



Intro slide and session schedule of panel on international Supercritical Carbon Dioxide projects



Panellists of session 31-31 (from left): Rafael Guédez (KTH), Silvia Trevisan (KTH), Lorenzo Arcangeli (Baker Hughes), Alberto Traverso (University of Genoa), Renaud Le Pierres (Meggitt), David Sánchez (University of Seville)


The main outcomes of the SCARABEUS project were also presented at another panel session, “Supercritical CO2 Research & Development”, organised by Renaud Le Pierres (Meggitt, UK), where Prof. David Sánchez (Dissemination Coordinator of the SCARABEUS project) shared the stage with distinguished speakers from Europe and North America: Vittorio Michelasi (Baker Hughes, Italy), Nathan Weiland (National Energy Technology Laboratory, USA), Aaron McClung (Southwest Research Institute, USA), Ty Neises (National Renewable Energy Laboratory). With respect to the session above, this session had a much wider scope, not only discussing today research efforts but also looking into the foreseen needs of the energy market in the future and how supercritical Carbon Dioxide technologies can possibly fulfil them.

In addition to these panel sessions, a number of technical papers were presented at ASME Turbo Expo by the SCARABEUS partners. The team at University of Seville presented a very interesting paper discussing how to optimise the design and operation of Air-Cooled Condensers to enhance part load performance with minimum impact on Capital Costs:

  • Rodríguez-de Arriba, F. Crespi, D. Sánchez, L. García-Rodríguez, 2023, Assessment of Part-load Operating Strategies of Supercritical Power Cycles Using Carbon Dioxide Mixtures in CSP Plants, Including Air-Cooled Condenser Optimisation (GT2023-103665), Presented at Turbo Expo, June 26-30, Boston (MA).

The team at City, University of London presented their latest research on the development of axial turbines for Carbon Dioxide mixtures. In this paper, the authors compared the impact of adding certain dopants on the flow path of large axial turbines working with Carbon Dioxide and supercritical inlet conditions:

  • I. Salah, M. T. White, A. I. Sayma, 2023, A Comparison of Flow Path Designs for Axial Turbines Operating with Pure CO2 and CO2 Mixtures, Presented at Turbo Expo, June 26-30, Boston (MA).

These technical papers are currently being processed by the editorial services of ASME and will be made available in Open Access in the next months. Keep an eye on the online digital collection and on this website for further information.