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)

/in /by

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)

/in /by

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)

/in /by

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)

/in /by

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.

Joint paper by City, University of London, University of Seville and Bakher Hughes published in Applied Thermal Engineering

/in /by

The SCARABEUS teams at City, University of London, University of Seville and Baker Hughes have recently published a paper authored jointly by the three institutions. This scientific publication is the result from the collaboration between Work Package 3 – Turbomachinery Design  and Work Package 5 – Techno-economic, Social and Environmental Assessments. In particular, the team at University of Seville has provided the boundary conditions for the design of the turbines, which has been carried out by City, University of London and Baker Hughes jointly. Then, the impact of the resulting turbomachinery efficiency on cycle performance has been assessed in WP5.

The paper has been published in volume 230 of Applied Thermal Engineering (Elsevier) and it is available in Open Access on the publisher’s website (link). Check the abstract below:

The utilisation of certain blends based on supercritical CO2 (sCO2), namely CO2/TiCl4, CO2/C6H2 and CO2/SO2, have been found to be promising for enhancing the performance of power cycles for Concentrated Solar Power (CSP) applications; allowing for up to a 6% enhancement in cycle efficiency with respect to a simple recuperated CO2 cycle, depending upon the nature of the used blend and the cycle configuration of choice. This paper presents an investigation of the impact of adopting these sCO2-based blends on the flow path design for a multi-stage axial turbine whilst accounting for aerodynamic, mechanical and rotordynamic considerations. This includes assessing the sensitivity of the turbine design to selected working fluid and imposed optimal cycle conditions. Ultimately, this study aims to provide the first indication that a high-efficiency turbine can be achieved for a large-scale axial turbine operating with these non-conventional working fluids and producing power in excess of 120 MW. To achieve this aim, mean-line aerodynamic design is integrated with mechanical and rotordynamic constraints, specified based on industrial experience, to ensure technically feasible solutions with maximum aerodynamic efficiency. Different turbine flow path designs have been produced for three sCO2 blends under different cycle boundary conditions. Specifically, flow paths have been obtained for optimal cycle configurations at five different molar fractions and two different turbine inlet pressure and temperature levels of 250 & 350 bar and 550 & 700 °C respectively. A total-to-total turbine efficiency in excess of 92% was achieved, which is considered promising for the future of CO2 plants. The highest efficiencies are achieved for designs with a large number of stages, corresponding to reduced hub diameters due to the need for a fixed synchronous rotational speed. The large number of stages is contrary to existing sCO2 turbine designs, but it is found that an increase from 4 to 14 stages can increase the efficiency by around 5%. Ultimately, based on the preliminary cost analysis results, the designs with a large number of stages were found to be financially feasible compared to the designs with a small number of stages.

 

Flowpath optimisation methodology used in the paper

Paper on ACC design presented by University of Seville at the 5th European Conference on Supercritical Carbon Dioxide Energy Systems, Prague (Czech Republic)

/in /by

 

Schematic of ACC used as reference for SCARABEUS

 

The researchers at University of Seville have presented their work on the design and operation of air-cooled condensers for integration into Concentrated Solar Power plants using the SCARABEUS technology. This work has been carried out in the context of Work Package 5 (Techno-economic, Social and Environmental Assessments) but also in close collaboration with Work Package 4 (Air Cooled Condenser and Heat Exchanger Development).

Condensation of the working fluid at high ambient temperature is a differential feature of the SCARABEUS CONCEPT, setting it apart from standard supercritical Carbon Dioxide cycles where the working fluid cannot be condensed due to the unfeasibility of cooling CO2 down below the critical temperature. The design of condensers for operation at high pressure (~80 bar) and low temperature is therefore a innovation of the project. Also, the definition of operating strategies in order to reduce the negative impact of auxiliary (fan) power consumption on net cycle efficiency is critical in order to effectively attain the thermodynamic superiority of the SCARABEUS concept.

 

Flow chart of the Air Cooled Condenser design code

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

The SCARABEUS project investigates the use of CO2–based mixtures as working fluid in power cycles for nextgeneration Concentrated Solar Power plants. These fluids exhibit a critical temperature higher than pure CO2, enabling dry condensation of the working fluid even at the high ambient temperatures typical of sites with a high solar radiation. As a consequence, the SCARABEUS power cycle achieves higher thermal efficiency than standard sCO2 cycles, whose performance deteriorates significantly with ambient temperature. In any case, the actual feasibility of this concept is still to be confirmed by a complete techno-economic assessment. To that purpose, it is critical to accurately estimate the power consumption of the Heat Rejection Unit (HRU), which is one of the most important parasitic loads of the system.

Bearing all this in mind, this manuscript presents the design of a horizontal, direct air-cooled condenser (ACC). The bundle geometry proposed is comprised of seven tubes in three passes, with a staggered arrangement. The complete thermal model, developed in MatLab, has been already disclosed by the SCARABEUS consortium in a previous paper, and validated both experimentally in a dedicated test rig and against results obtained by the commercial software Xace®. The novelty in the present manuscript lies in the integration of this thermal model of the tubes with a complete design and integration tool of the whole heat rejection sub-system, including the design of a rotoronly axial fan and supporting frame. The impact of several design parameters (i.e., air temperature rise, acceptable hot pressure drops, tube length) is studied, taking into account auxiliary power consumption, footprint and cycle efficiency as main figures of merit. Two candidate mixtures are taken into account, identified in previous works by the same authors (85%CO2-15%C6F6 and 80%CO2-20%SO2), and a pure sCO2 case is also considered for the sake of comparison. The results show that, for a given gross cycle output, using pure sCO2 yields the smallest ACC with the lowest fan power consumption. Moreover, tube length and air face velocity are found to be the key-parameters driving the design process of an ACC, for which increasing tube length is always beneficial as far as the ACC design is concerned. Finally, various considerations regarding the role played by the optimum design of the ACC within the global optimisation of the power plant are made. It is found that the rationale employed for the design of the ACC may be in conflict with that used from an overall plant optimisation standpoint. It is hence concluded that the definition of the optimal design space of an Air-cooled Heat Exchanger (ACHE) must be included in the global optimisation of the power plant.

Paper on SARABEUS rig design and operation presented by Technical University of Vienna and Kelvion Thermal solution at the 5th European Conference on Supercritical Carbon Dioxide Energy Systems, Prague (Czech Republic)

/in /by

 

Front view of the SCARABEUS test rig at TU Wien

 

 

The researchers at the Technical University of Vienna (Austria) and Kelvion Thermal Solutions (France) have presented their work on the design of and operation of the SCARABEUS test rig at the Austrian academic institution. This work has been carried out in the context of Work Package 4 (Air Cooled Condenser and Heat Exchanger Development) and Work Package 6 (Test Rig and Experimental Validation) of SCARABEUS and is a cornerstone of the project, given the uniqueness of the rig constructed to validate the operation of heat exchangers on Carbon Dioxide mixtures.

Schematic of the SCARABEUS test rig at TU Wien

 

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

At TU Wien, a test facility working with supercritical carbon dioxide (sCO2) was commissioned in 2018. Since then, it has been used for various research tasks. This paper gives an overview about the three configurations of the facility with a focus on design, operation, and results. The authors present the design of components in the three configurations of the test facility: proof of concept of the simple cycle in supercritical and transcritical operation mode, heat transfer measurements, and future work. Special emphasis is given to challenges during engineering and operation. Our most relevant lessons learned are: that a commercial CO2 pump is not sufficient for cycle experiments, how to design a measurement section for heat transfer measurements, and that during experimental research, measurement-concepts and data reduction must be prioritized at all times.

 

New paper by University of Seville presents the exergy analysis of different transcritical Carbon Dioxide cycles for CSP applications

/in /by

 

 

The SCARABEUS team at University of Seville has recently published an assessment of transcritical cycles running on different Carbon Dioxide mixtures in Concentrated Solar Power applications. This assessment makes use of the 2nd Law of Thermodynamics, rather than the 1st Law that is commonly used, with the aim to identify the room for further performance enhancement. Three different dopants are considered: Hexafluorobenzene (for cycles operating at temperatures lower than 600ºC), Titanium Tetrachloride and Sulphur Dioxide.

The paper has been published in Renewable Energy (Elsevier) and it is available in Open Access on the publisher’s website (link). Check the abstract below:

This paper focuses on the thermodynamic comparison between pure supercritical Carbon Dioxide and blended transcritical Carbon Dioxide power cycles by means of a thorough exergy analysis, considering exergy efficiency, exergy destruction and efficiency losses from Carnot cycle as main figures of merit. A reference power plant based on a steam Rankine cycle and representative of the state-of-the-art (SoA) of Concentrated Solar Power (CSP) plants is selected as base-case. Two different temperatures of the energy (heat) source are considered: 575 °C (SoA) and 725 °C (next generation CSP).

Compared to SoA Rankine cycles, CO2 blends enable cycle exergy efficiency gains up to 2.7 percentage points at 575 °C. At 725 °C, they outperform both SoA and pure CO2 cycles with exergy efficiencies up to 75.3%. This performance is brought by a significant reduction in the exergy destruction across the compression and heat rejection process rounding 50%. Additionally, it has been found that the internal condensation occurring inside the heat recuperator for those mixtures with a large temperature glide improves recuperator exergy efficiency, supporting the use of simpler layouts without split-compression. Finally, CO2 blends exhibit lower cycle exergy efficiency degradation than pure sCO2 in the event of an increase in the design ambient temperature.

7th International Seminar on organic Rankine power systems hosted by University of Seville, a SCARABEUS partner – Save the date!

/in , /by

The team led by David Sánchez, Professor of Energy Engineering at University of Seville, will organize the 7th International Seminar on Organic Rankine Cycle Power Systems. This edition of the conference will be held from 4th to 6th of September in the beautiful city of Seville, and will gather the main players across the entire supply chain of ORC power systems.

 

More information about the event is now available in the conference website and LinkedIn account

 

It’s time to mark your calendar for this exciting event!!!

 

 

 

New joint paper by Quantis, University of Seville, Abengoa, Baker-Hughes and Kelvion discusses the carbon footprint of the SCARABEUS concept

/in , /by

A large team from within the SCARABEUS consortium has been assessing the carbon footprint of Concentrated Solar Power plants using supercritical power cycles running on Carbon Dioxide mixtures, in comparison with state of the art power plants relying on steam turbines. This collective work has looked into the contributions of construction and operation to carbon footprint, with a special focus on the singularities introduced by the utilization of an innovative working fluid.

The work was presented at the ASME conference held in Rotterdam (The Netherlands), June 13-17, at a very well attended session where an interesting discussion followed the presentation by Dr. Francesco Crespi, from University of Seville. Life Cycle Environmental Assessment is an ongoing task in SCARABEUS and further results will be published in the coming minths.

The paper is available in Open Access on the publisher’s website (link). Check the abstract below:

The SCARABEUS project, funded by the European Commission, is currently investigating the potential gains brought about by the utilization of carbon dioxide mixtures in supercritical power cycles of Concentrated Solar Power plants, in lieu of the common Rankine cycles based on steam turbines or even pure carbon dioxide cycles. The analysis has already confirmed that it is possible to attain thermal efficiencies higher than 51% when ambient temperatures exceed 40°C, which is unheard of when conventional technology or standard CO2 technology is used. Additionally, this extraordinary performance is achieved with simpler cycle layouts, therefore with lower capital costs. The additives considered include organic and inorganic compounds which are added to the raw carbon dioxide in a variable proportion, depending on the composition of the additive and on ambient temperature. Regardless, it is important to assess whether or not there is an additional environmental advantage in terms of carbon dioxide and other potential hazards brought about by the new chemicals in the system. This is presented in this paper where the results obtained so far by the consortium for the carbon footprint from a Life Cycle perspective are discussed. Along with the assumptions and methodology, the results are compared for three reference plants: state-of-the-art CSP plant based on steam turbines, innovative CSP plant using pure supercritical CO2 technology, and the SCARABEUS concept using supercritical CO2 mixtures. The results are promising as they suggest that it is possible to reduce the carbon footprint of a 110 MWe CSP plant to be significantly less than 27kgCO2/MWh from the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC AR5)