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

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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.

A new joint paper between City, University of London (CUL) and Baker Hughes (BH) discussing the blade shape optimisation of the SCARABEUS turbine operating with sCO2-SO2 mixture

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City, University of London and Baker Hughes have been working on the turbomachinery design for the SCARABEUS project. A 130 MW axial turbine is designed to produce a net power output from the cycle of ~100 MWe. The design process starts by identifying the aerodynamic and mechanical design constraints based on industrial and academic experience, while these constraints are applied to produce the preliminary design using the available loss correlations. A further design assessment is conducted using numerical CFD/FEA simulations to optimise the turbine blades for performance maximisation and to ensure a safe and reliable operation.

This work was presented at the ASME conference held in Rotterdam (The Netherlands), June 13-17, by Abdelrahman Abdeldayem, from City, University of London on behalf of the team.

The paper is available in Open Access on the publisher’s website: https://doi.org/10.1115/1.4055232

Abstract

Within this study, the blade shape of a large-scale axial turbine operating with sCO2 blended with dopants is optimized using an integrated aerodynamic-structural three-dimensional (3D) numerical model, whereby the optimization aims at maximizing the aerodynamic efficiency whilst meeting a set of stress constraints to ensure safe operation. Specifically, three candidate mixtures are considered, namely, CO2 blended with titanium tetrachloride (TiCl4), hexafluorobenzene (C6F6), or sulfur dioxide (SO2), where the selected blends and boundary conditions are defined by the EU project, SCARABEUS. A single passage axial turbine numerical model is setup and applied to the first stage of a large-scale multistage axial turbine design. The aerodynamic performance is simulated using a 3D steady-state viscous computational fluid dynamic (CFD) model while the blade stress distribution is obtained from a static structural finite element analysis simulation (FEA). A genetic algorithm is used to optimize parameters defining the blade angle and thickness distributions along the chord line while a surrogate model is used to provide fast and reliable model predictions during optimization using a genetic aggregation response surface. The uncertainty of the surrogate model, represented by the difference between the surrogate model results and the CFD/FEA model results, is evaluated using a set of verification points and is found to be less than 0.3% for aerodynamic efficiency and 1% for both the mass-flow rate and the maximum equivalent stresses. The comparison between the final optimized blade cross section has shown some common trends in optimizing the blade design by decreasing the stator and rotor trailing edge thickness, increasing the stator thickness near the trailing edge, and decreasing the rotor thickness near the trailing edge and decreasing the rotor outlet angle. Further investigations of the loss breakdown of the optimized and reference blade designs are presented to highlight the role of the optimization process in reducing aerodynamic losses. It has been noted that the performance improvement achieved through shape optimization is mainly due to decreasing the endwall losses with both the stator and rotor passages.

New SCARABEUS paper by University of Seville discusses the benefits obtained from utilizing Carbon Dioxide – Sulphur Dioxide mixtures in (supercritical) Recompression cycles

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The latest SCARABEUS research carried out by the team at University of Seville has just been published in Applied Thermal Engineering journal. Starting off from previous research by the the same partner, this paper provides a discussion on further efficiency gains that can be attained when Carbon Dioxide is blended with Sulphur Dioxide in supercritical or transcritical power cycles, with boundary conditions representative of those in a Concentrated Solar Power plant. The paper reveals that, unlike with other dopants tested previously by the consortium, this mixture performs best when in a transcritical Recompression cycle.

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

This paper investigates the interest and potential of using working fluids based on Carbon and Sulpur Dioxide mixtures (CO2-SO2) in a transcritical Recompression cycle. In order to assess the actual thermodynamic potential of the concept proposed, the influence of dopant (SO2) content is assessed for two different turbine inlet temperatures (550ºC and 700ºC). The results obtained are compared with other CO2 mixtures already proposed in literature (CO2– C6F6 and CO2-Ti Cl4) and for two alternative cycle layouts (Recuperated Rankine and Precompression).

The results pf the analysis reveal that, at high ambient temperature, the Recompression cycle operating on CO2-SO2, with Sulphur Dioxide content between 20% and 30%(v), is a very interesting option for Concentrated Solar Power plants, able to achieve thermal efficiencies 45% and ¿51% at 550ºC and 700ºC respectively. At a minimum cycle temperature of 50ºC, the proposed configuration leads to thermal efficiency gains of 6% and 2% with respect to the Brayton and Recompression cycles working on pure CO2. This performance enhancement of the Recompression cycle with CO2-SO2 is comparable to or higher than that enabled by other CO2 mixtures proposed in literature, but with significantly higher specific work (smaller footprint) and temperature rise across the solar receiver (lower installation costs).

 

The potential and technical challenges of the SCARABEUS project presented at the 7th Supercritical Power Cycles Symposium held at Soutwest Research Institute, TX

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The potential and technical challenges of the SCARABEUS project presented at the 7th Supercritical Power Cycles Symposium held at Soutwest Research Institute, TX

The team at University of Seville, on behalf of the consortium, presented the potential of the SCARABEUS project to overcome the main limitations experienced by all power cycles in the usually warm environments where Concentrated Solar Power facilities are located. High ambient temperatures are inherent to these sites and they set an intrinsic limit to the achievable thermal performance of the power block, which translates into larger solar fields and worse economic performance.

SCARABEUS is exposed to the same constraint but it also exhibits a much higher resistance to performance deterioration when ambient temperature increases. Nevertheless, there is no such thing as free lunch and this comes at the cost of technical and economic barriers that the consortium is currently working to overcome. If you want to find out more, check the collective symposium paper here.

 

PAPER FINAL

 

SCARABEUS participates to the European Corner at the 6th International Seminar on Organic Rankine Cycle Power Systems

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The 6th edition of the International Seminar on ORC Power Systems was held in October, organized by the Technical University of Munich and the Knowledge Centre on Organic Rankine Cycle technology (KCORC). It was a very exciting event, with lots of interesting presentations and panels on different aspects of the energy industry and technologies, including some excellent works on the utilization of Carbon Dioxide mixtures in supercritical power cycles.

The conference was held virtually but, akin to the previous edition of the conference, a European Corner to raise awareness of the large research projects on the topic funded by the European Commission was organized. SCARABEUS was one of such projects. The Exploitation Manager, Noelia Martínez Sanz (Abengoa), prepared some materials to offer an interactive environment to get to know about the potential and challenges of the technology. These are shared on the SCARABEUS website now so you can enjoy the same experience!

 

 

New SCARABEUS paper discusses the benefits brought about by the utilization of Carbon Dioxide mixtures in supercritical power cycles applied to Concentrated Solar Power plants

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The SCARABEUS consortium (University of Seville, University of Brescia, Politecnico di Milano and LEAP) has just published in Energy journal their recent research in the area of cycle performance. This research paper provides a through analysis of the underpinning reasons why the utilization of mixtures enables higher maximum (thermal) efficiency and it also reveals the best combinations of cycle layout and working fluid composition.

The paper, whose abstract is pasted below, is now available in Open Access on the publisher’s website (link). Check it out now!

The present paper explores the utilisation of dopants to increase the critical temperature of Carbon Dioxide (sCO2) as a solution towards maintaining the high thermal efficiencies of sCO2 cycles even when ambient temperatures compromise their feasibility. To this end, the impact of adopting CO2-based mixtures on the performance of power blocks representative of Concentrated Solar Power plants is explored, considering two possible dopants: hexafluorobenzene (C6F6) and titanium tetrachloride (TiCl4). The analysis is applied to a well-known cycle -Recuperated Rankine- and a less common layout -Precompression-. The latter is found capable of fully exploiting the interesting features of these non-conventional working fluids, enabling thermal efficiencies up to 2.3% higher than the simple recuperative configuration. Different scenarios for maximum cycle pressure (250–300 bar), turbine inlet temperature (550–700ºC) and working fluid composition (10–25% molar fraction of dopant) are considered. The results in this work show that CO2-blends with 15–25%(v) of the cited dopants enable efficiencies well in excess of 50% for minimum cycle temperatures as high as 50ºC. To verify this potential gain, the most representative pure sCO2 cycles have been optimised at two minimum cycle temperatures (32ºC and 50ºC), proving the superiority of the proposed blended technology in high ambient temperature applications.

 

Carbon Dioxide mixtures at the 4th European sCO2 Conference for Energy Systems, Prague

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The 4th edition of the European sCO2 Conference for Energy Systems, held virtually on March 23-24, gathered some forty excellent works presented by international authors. The number of attendees and quality of works presented confirmed that the sCO2 community is vibrant and the future of the technology looks bright.

A number of very interesting papers dedicated to CO2 blends triggered the interest of the SCARABEUS consortium:

  • 1 Valencia Chapi, R., Fierros-Peraza, O., Coco-Enríquez, L., Muñoz-Antón, J., Modeling and study of a printed circuit heat exchanger for Brayton power cycles using supercritical CO2 mixtures as working fluid (Universidad Politécnica de Madrid)
  • 2 Ayub, A., Di Marcoberardino, G., Invernizzi, C.M., Iora, P., Advanced thermodynamic power cycles utilizing carbon dioxide based mixtures as working fluids for high temperature waste heat recovery (University of Brescia)
  • 3 Rath, S., Mickoleit, E., Gampe, U., Breitkopf, C., Jäger, A., Study of the influence of additives to CO2 on the performance parameters of a sCO2-cycle (TU Dresden)

These added to two works by University of Seville and City, University of London, presented on behalf of the consortium:

  • Aqel, O., White, M., Sayma, A., Binary interaction uncertainty in the optimization of a transcritical cycle: consequences on cycle and turbine design (City, University of London)
  • Crespi, F., Rodríguez-de Arriba, P., Sánchez, D., Ayub, A., Di Marcoberardino, G., Invernizzi, C.M., Martínez, G.S., Iora, P., Di Bona, D., Binotti, M., Manzolini, G.,  Thermal efficiency gains enabled by using supercritical CO2 mixtures in Concentrated Solar Power applications (University of Seville, Politecnico di Milano, University of Brescia, LEAP)

Different CO2 mixtures were proposed for Concentrated Solar Power and Waste Heat Recovery applications. Rath et al. performed a vast screening of 135 candidates out which five were selected: Krypton, Xenon, Carbonyl sulfide (COS), Propane and Sulfur hexafluoride for WHR systems. For the same application, Ayub et al. studied CO2-Novec mixtures in three different layouts, concluding that a 3 percentage point gain with respect to pure CO2 seems possible. Regarding CSP, Crespi et al. investigated the use of CO2-C6F6 and CO2-TiCl4 mixtures, coming to the conclusion that it is possible for the power block to achieve and even exceed 50% thermal efficiency even under semi-arid boundary conditions, provided that the suitable cycle layout is selected for each working fluid-was achievable. Also for CSP plants, Valencia-Chapi et al. modelled a printed circuit heat exchanger and studied its performance for different CO2 mixtures, noting that heat transfer coefficients of the mixtures were higher than those of pure CO2; this favours lower heat exchang areas. Finally, the turbine of large power blocks running on CO2-C6F6, CO2-H2S and CO2-NOD (non-organic dopant) were studied by Aqel, White & Sayma.

Common to all work was the emphasis on the suitable fluid modelling of CO2 mixtures. In the work of Ayub et al., binary interaction parameters of different CO2 mixtures (Novec 5110, Novec 649, R134a, HFO1234yf and HFO1234ze(E)) were estimated using experimental VLE data from literature and then applying Peng-Robinson with Van der Walls mixing rule to estimate thermodynamic properties. The same fluid model was used by Crespi et al. for CO2-C6F6 and CO2-TiCl4 mixtures. Valencia-Chapi et al. modelled CO2 mixtures using the Aungier-Redlich-Kwong real gas model. Aqel, White & Sayma studied the influence of four different Equations of State and of the uncertainty in the estimates of binary interaction parameters on cycle performance and turbine geometry. Rath et al. use a predictive model to calculate mixture properties based on the best available EoS for the pure components.

In addition to the works by Aqel et al. and Crespi et al., which describe the latest results obtained by the SCARABEUS consortium, a series of works developed by other projects funded by the European Commission (sCO2 Hero and sCO2 Flex) were presented at the conference. Moreover, several interesting topics regarding sCO2 cycle performance, turbomachinery and heat exchanger design and novel sCO2 cycle configurations have been thoroughly discussed. The entire set of presentations and papers are available online in the conference repository (https://sco2.eu/conference-repository/4th-conference-online/) so, if you wish to take a closer look at some of the works, just follow this link and enjoy!

 

New Open Access publication by University o Brescia and Politecnico di Milano provides information about characterisation of supercritical CO 2 blends

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In the last Open Access paper published by UNIBS and POLIMI, a new methodology for
the thermal stability test of CO2 blends have been developed and tested.
The method proposed relies on the study of the thermodynamic behaviour of the
working fluid from the variation of the van der Waals coefficients. The comparison of
the estimated coefficients a, b and the molar mass (MM) from the regression of the
experimental data (in the gas phase), starting from the virgin fluid isochoric line, and
after different thermal stress test, can be representative of potential decomposition of
the investigated fluid. As a consequence of the thermal stress, the substance
decomposes in a mixture of different unknown species that, for simplicity, is assumed
as a pure fluid characterised by different coefficients a, b and MM. Moreover, starting
from the obtained parameters, the isothermal compressibility k T can be used as a
proper index to highlight the impact of the thermal degradation on the power cycle.

An example of the new method is briefly discussed for a mixture of carbon dioxide and
perfluorohexane, with molar fractions of 80% and 20% respectively. In Figure 1 , the
virgin fluid measurements are along mixture density value of 99.4 kg/m 3 , in the gas
phase, while measured p-T points after each thermal stress are represented. The best
fit of the experimental values, using the van der Waals equation of state, yields the
values in Table 1 , assuming a pure fluid behaviour of the mixture, while Table 2 shows
the resulting isothermal compressibility k T at different temperatures. Since
measurements at 250°C and 300°C are in agreement with the fresh mixture, the values
were included for the calculation of the virgin mixture parameters.
Although the van der Waals parameters are slightly different after the thermal stress
tests at 350°C and 400°C, the mixture can be considered thermally stable up to 400°C:
this behaviour is also confirmed by the parameter k T . Decomposition phenomena occur
from 450°C where not only the isothermal compressibility increases by more than 50%
with respect to the virgin mixture but also a strong deviation of the van der Waals
parameters from initial values can be observed

 

 

Figure 1 Results of P-T measurements for the mixture CO 2 +C6F 14 .

Table 1 Parameters a, b and MM of the van der Waals equation of state of the mixture carbon dioxide and perfluorohexane.

a

(MPam6kmol-2)

 b

(m3kmol-1)

 MM

(kgkmol-1)
Virgin mixture 0.818 0.086 102.4
350°C 0.901 0.087 102.9
400°C 0.924 0.088 101.0
450°C 0.016 0.360 158.7
500°C 0.004 0.584 176.1

 

Table 2 The estimated isothermal compressibility kT of carbon dioxide and perfluorohexane at 120°C for the virgin and the decomposed mixture using the van der Waals coefficients and MM of Table 4.

1/kT

(MPa)

 u(kT)

(MPa)
Virgin mixture 2.246 0.052
350°C 2.084 0.054
400°C 2.072 0.057
450°C 3.435 0.122
500°C 4.149 0.247

 

PS: For more information, follow this link to the online article: link.

Two Open Access publications by partners of the SCARABEUS consortium just published in Applied Sciences’ special issue on Recent Advancement of Thermal Fluid Engineering in the Supercritical CO2 Power Cycle

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This Special Issue is comprised of a selection of technical papers dealing with various
aspects of supercritical Carbon Dioxide technology for power generation. The
publication is published by MDPI, with Prof. Jeong Ik Lee (Korea Advanced Institute of
Science and Technology, Korea) and Prof. David Sánchez (University of Seville, Spain)
serving as Guest Editors.

University of Seville sets the stage for the economic assessment of the SCARABEUS
technology

The paper by University of Seville aims to estimate the cost of electricity that should be
expected from Concentrating Solar Power plants making use of either steam turbines
(state-of-the-art technology) or standard supercritical CO2 power cycles (so-called next
generation CSP plants). The objective of this is twofold. First, to assess the actual
potential of sCO2 technology to reduce the cost of electricity produced by
contemporary solar tower plants with large scale thermal energy storage. Second, to
check whether or not such reduction suffices to achieve the very ambitious objectives
set forth by the solar community (currently achieved by large-scale photovoltaics).

Figure 1 Goals and current costs for solar electricity as set forth by the SunShot programme.

The results confirm that sCO2 technology has the potential to reduce the cost of
producing solar electricity with respect to the current technology based on steam
turbines. Unfortunately, whilst these results serve to confirm the claims that sCO2 is
more cost effective than steam, they also suggest that the foreseen cost reduction will
not enable hitting the long-term 5¢/kWh goal.

PS: For more information, follow this link to the online article: link.

City University looks into feasible turbine designs for distributed power generation
systems based on sCO 2

Ms. Salma Salah, a PhD student from the SCARABEUS research team at City, University
of London, has published her first paper entitled “Mean-Line Design of a Supercritical
CO2 Micro Axial Turbine” in a Special Issue on sCO2 technology in Applied Sciences.
This paper examines the effect of various design parameters on the performance and
feasibility of a micro-scale axial sCO2 turbine. The design methodology developed will
later be used to design the turbine for the SCARABEUS plant.
According to the abstract of the paper, the aim of this study is to investigate the design
of a single-stage 100 kW sCO 2 axial turbine through the identification of optimal
turbine design parameters from both mechanical and aerodynamic performance
perspectives. For this purpose, a preliminary design tool has been developed and
refined by accounting for passage losses using loss models that are widely used for the
design of turbomachinery operating with fluids such as air or steam. The designs were
assessed for a turbine that runs at inlet conditions of 923 K, 170 bar, expansion ratio of
3 and shaft speeds of 150k, 200k and 250k RPM respectively. It was found that feasible
single-stage designs could be achieved if the turbine is designed with a high loading
coefficient and low flow coefficient. Moreover, a turbine with the lowest degree of
reaction, over a specified range from 0 to 0.5, was found to achieve the highest
efficiency and highest inlet rotor angles
PS: For more information, follow this link to the online article: link.

September 2019, Message from the Guest Editors of the Special Issue on Recent Advancement of Thermal Fluid Engineering in the Supercritical CO2 Power Cycle, Applied Sciences (MDPI)

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