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

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

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

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)