Applied Thermal Engineering 185 (2021) 116447

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Applied Thermal Engineering

journal homepage: www.elsevier.com/locate/ate

Review of supercritical CO2 technologies and systems for power generation

Martin T. White a ,∗, Giuseppe Bianchi b , Lei Chai b , Savvas A. Tassou b , Abdulnaser I. Sayma a
a
Thermo-Fluids Research Centre, School of Mathematics, Computer Science and Engineering, City, University of London, Northampton Square, London, EC1V
0HB, UK
b
Centre for Sustainable Energy Use in Food Chains, Institute of Energy Futures, Brunel University London, Kingston Lane, Uxbridge, Middlesex, UB8 3PH, UK

ARTICLE INFO ABSTRACT

Keywords: Thermal-power cycles operating with supercritical carbon dioxide (sCO2 ) could have a significant role in
Supercritical carbon dioxide future power generation systems with applications including fossil fuel, nuclear power, concentrated-solar
sCO2 power, and waste-heat recovery. The use of sCO2 as a working fluid offers potential benefits including high
Power generation
thermal efficiencies using heat-source temperatures ranging between approximately 350 ◦ C and 800 ◦ C, a simple
Turbomachinery
and compact physical footprint, and good operational flexibility, which could realise lower levelised costs of
Heat exchangers
Control systems
electricity compared to existing technologies. However, there remain technical challenges to overcome that
Applications relate to the design and operation of the turbomachinery components and heat exchangers, material selection
considering the high operating temperatures and pressures, in addition to characterising the behaviour of
supercritical CO2 . Moreover, the sensitivity of the cycle to the ambient conditions, alongside the variable
nature of heat availability in target applications, introduce challenges related to the optimal operation and
control. The aim of this paper is to provide a review of the current state-of-the-art of sCO2 power generation
systems, with a focus on technical and operational issues. Following an overview of the historical background
and thermodynamic aspects, emphasis is placed on discussing the current research and development status
in the areas of turbomachinery, heat exchangers, materials and control system design, with priority given to
experimental prototypes. Developments and current challenges within the key application areas are summarised
and future research trends are identified.

1. Introduction and motivation cycles will likely remain a pivotal component within future energy
networks.
Since the industrial revolution, societies throughout the world have Existing thermodynamic power cycles, such as the Rankine cycle
remained reliant on fossil fuels to provide heat, which is subsequently and the Joule–Brayton cycle, typically operate with water or air respec-
converted into electricity through thermodynamic power cycles. Unfor- tively. However, in the drive towards higher cycle thermal efficiencies,
tunately, this reliance on fossil-fuelled power generation to sustain eco- and to overcome some of the technical challenges related to existing
nomic growth has taken its toll on the environment through greenhouse cycles, attention has turned to the use of alternative working fluids.
gas emissions, leading to global warming, alongside environmental As such, supercritical carbon dioxide (sCO2 ) power cycles have been
pollution. As such, over the past few decades there has been a rapid put forward as a promising candidate with the main advantages being
growth in the deployment of renewable energy technologies, such as high thermal efficiencies from heat sources ranging between 350 ◦ C and
solar photovoltaics and wind, which no longer rely on thermodynamic 800 ◦ C, a simple and compact physical footprint, and good operational
power cycles. However, to meet the need for secure, reliable, clean and
flexibility to cope with the uncertain availability of renewable energy
sustainable energy, it is widely acknowledged that a broad portfolio of
sources. The potential of sCO2 is confirmed by the significant growth in
energy conversion and storage technologies will be required. This is
research within the last decade, alongside the financial support that has
likely to include nuclear power generation, concentrated-solar power
been made available internationally to aid technological advancement.
plants, and the use of blue and green hydrogen, alongside the imple-
This rapid growth in research, and the potential of sCO2 power
mentation of technologies to improve overall energy efficiency, such as
waste-heat recovery, and the continued use of fossil fuels, ultimately cycles, warrant a detailed review of current research activities, along-
with carbon capture and storage [1,2]. Thus, thermodynamic power side the most promising applications and future research trends. A

∗ Corresponding author.
E-mail addresses: martin.white@city.ac.uk (M.T. White), giuseppe.bianchi@brunel.ac.uk (G. Bianchi), lei.chai@brunel.ac.uk (L. Chai),
savvas.tassou@brunel.ac.uk (S.A. Tassou), a.sayma@city.ac.uk (A.I. Sayma).

https://doi.org/10.1016/j.applthermaleng.2020.116447
Received 30 July 2020; Received in revised form 11 November 2020; Accepted 6 December 2020
Available online 10 December 2020
1359-4311/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

number of review papers have been published prior to 2018 focus-

ing on: cycle layouts, thermodynamics and an overview of early test
loops [3]; transcritical CO2 cycles for low-grade heat conversion [4];
and sCO2 technology with a focus on concentrated-solar power and nu-
clear power applications [5]. More recently, in 2019 Liu et al. [6] pro-
vided a review of sCO2 technology, with a detailed treatment of cycle
layouts, thermodynamic modelling and optimisation, alongside a dis-
cussion of applications and some component level aspects. Yin et al. [7]
provided a review of sCO2 cycles specifically for concentrated-solar
power applications, primarily covering cycle design, system thermody-
namic and economic modelling, and materials. Finally, Yu et al. [8]
provided a bibliometric analysis, in addition to briefly discussing ap-
plications, cycle configurations and modelling, CO2 -based mixtures,
system components and experiments.
The motivation and novelty of the current review can be sum-
marised as follows. Firstly, the rapid growth in sCO2 technology means
most published work has been conducted within the last few years
since the early review papers [3–5]; this is substantiated by Fig. 1.
Secondly, a significant emphasis within previous studies has been on Fig. 1. Historical evolution and geographical distribution of intellectual property
outputs in the field of sCO2 power systems. Elaboration from Scopus and Espacenet
cycle layout design, alongside thermodynamic modelling and opti-
world databases between January 1988 and March 2020. The doughnut charts refer
misation. This review differentiates from these previous studies by to the total number of outputs in the period surveyed while their legend shows the
focusing on practical technological challenges related to sCO2 systems shares in the People’s Republic of China (CN), the United States of America (US), the
and equipment, alongside a summary of key application areas. Follow- Republic of Korea (KR) and the rest of the world (OTHER).
ing a brief overview of the historical background of sCO2 power cycles
in Section 2, an overview of thermodynamic aspects is presented in
Section 3 to contextualise the technical and operational issues within offers the highest thermal efficiency, whilst retaining a relatively simple
the thermodynamic cycle. A review of technical issues is then presented cycle arrangement.
in Section 4, with emphasis on turbomachinery, heat exchangers, mate- Since Dostal’s work, there have been numerous studies investigating
rial options and system control. This is followed by a discussion of the the merits of various cycle architectures for different applications. This
key application areas in Section 5, namely fossil-fuelled and waste-heat is demonstrated in Fig. 1 by the steep increase in the number of
to power generation, concentrated-solar power and nuclear, alongside academic documents (journal papers, conference proceedings, books
a number of other notable applications. The paper concludes with a etc.) and patents indexed in well-known databases. Research outputs
summary of the challenges facing sCO2 power generation systems, and have been increasing since 2006, while the number of patents has
an outlook to the future. risen significantly from 2016. China (CN), United States (US) and
South Korea (KR) are the top countries. This leadership results from
2. Historical background the research and innovation programmes on sCO2 power technology
carried out in these and other countries like Japan and Australia. A
The real gas effects of CO2 were investigated in the 19th century detailed summary of the sCO2 power activities with focus on the US is
available in the book edited by Brun, Friedman and Dennis [17]. This
through the experimental work by Andrews who tested the validity of
is currently the most thorough reference on sCO2 power technology.
Boyle’s law over a wide range of pressures [9]. However, it was in the
Supercritical CO2 is a topic that recently gained interest and ap-
mid-20th century that CO2 began to be considered as a working fluid
peal within many scientific conferences. Besides the tracks allocated
for power generation systems. In that period, research activities aimed
within turbomachinery and waste-heat recovery conferences, the most
at overcoming the limitations of the open Joule–Brayton and steam
established events are the International sCO2 Power Cycles Symposium
Rankine cycles through the use of alternative working fluids in closed
taking place in the US and the European sCO2 conference for energy
loop cycles. Most of the published literature attributes the paternity
systems organised by the European sCO2 alliance [18,19].
of the concept of closed loop CO2 power cycles to a 1950 Swiss
patent granted to Sulzer that focused on partially-cooled condensation
3. Thermodynamic aspects
cycles [10].
Much of the early research on sCO2 power systems was completed The primary aim of this paper is to cover technical and operational
in the 1960s. Dekhtiarev suggested CO2 as a suitable working fluid, issues, rather than covering the topic of thermodynamic cycle analysis
proposing a reheated, precompression transcritical cycle operating with in detail. However, it is important to contextualise the technical and
CO2 [11], although the works of Feher and Angelino are more regularly operational issues within the overall thermodynamic cycle, and thus
cited. Feher [12] investigated a simple recuperated supercritical cycle the purpose of this section is to provide only a short overview of
and evaluated the sensitivity of the cycle to the operating parameters thermodynamic aspects. For a more detailed overview, readers are
and the component performance. Within his work, the mismatch in the referred to Friedman & Anderson [20].
heat capacities between the hot and cold fluids within the recuperator
was discussed, which introduces irreversibility within the cycle. An- 3.1. Thermodynamic behaviour of supercritical CO2
gelino [13–15] further considered both supercritical and transcritical
cycles and investigated a range of different cycle layouts with a view The operation of a closed-loop power cycle with a carefully selected
to minimising this irreversibility. working fluid could overcome some of the limitations of power plants
Following the 1960s, interest in sCO2 power cycles diminished until operating with air or water. Specifically, for certain fluids, operating
it was reignited by Dostal [16] who proposed the cycle for nuclear the cycle compression process close to the critical point allows the com-
power applications. Much like the earlier works of Angelino, Dostal pression work to be significantly reduced, whilst the non-isothermal
evaluated a range of cycles, including intercooling, reheating, recom- heat-transfer processes facilitate a high degree of internal heat recuper-
pression and precompression, concluding that the recompression cycle ation [12,13]. The choice of working fluid is not limited to CO2 , and a

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M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

Fig. 2. Thermodynamic behaviour of sCO2 in the vicinity of the critical point (𝑇cr , 𝑝cr ): (a) density 𝜌; (b) specific-heat capacity at constant pressure 𝑐𝑝 ; (c) Prandtl number
𝑃 𝑟 = 𝑐𝑝 𝜇∕𝑘; (d) specific work for an isentropic compression for a compression ratio of 2, 𝑤𝑐 .

range of fluids have been studied [16,21,22]. However, the interest in 3.2. Classification of thermodynamic power cycles
CO2 can be attributed to the temperature of its critical point, which is
defined by a critical temperature and critical pressure of 𝑇cr = 31.1 ◦ C A general thermodynamic power cycle is composed of four funda-
and 𝑝cr = 73.8 bar respectively, being close to ambient conditions. This mental processes, namely compression, heat addition at high pressure
allows the low work compression process to be achieved following heat (𝑝2 ), expansion, and heat rejection at low pressure (𝑝1 ), and can be
rejection down to close to ambient temperatures. Moreover, CO2 is categorised according to whether phase change occurs within the cycle.
abundant, low cost, non-toxic, non-flammable and thermally stable at In the Joule–Brayton cycle the cycle remains within the vapour region,
high temperatures. whilst in the Rankine cycle the working fluid undergoes phase change
The thermodynamic behaviour of CO2 in the vicinity of critical
in the heat-addition and heat-rejection processes.1 Thermodynamic cy-
point, and the motivation for operating the compression process close
cles can also be classified according to whether the operating pressures
to the vicinity of the critical point, is explored in Fig. 2. These results
are below or above the critical pressure of the working fluid (𝑝cr ). This
were generated using NIST REFPROP [23] to compute the thermody-
allows three classifications, which include the subcritical (𝑝1 < 𝑝cr and
namic properties of CO2 , which is the most widely employed method;
a detailed assessment of different property prediction methods is pro- 𝑝2 < 𝑝cr ), supercritical (𝑝1 > 𝑝cr and 𝑝2 > 𝑝cr ), and transcritical cycles
vided by White & Weiland [24]. Referring to Fig. 2, the right-hand plot (𝑝1 < 𝑝cr and 𝑝2 > 𝑝cr ). The term supercritical Rankine cycle is often
reports the specific work for an isentropic compression process with used in the context of steam and organic Rankine cycles to describe
a compression ratio of two. This compression ratio is representative cycles in which the heat-addition takes place above 𝑝cr , whilst heat-
of existing experimental sCO2 systems (see Table 1), although similar rejection occurs below 𝑝cr [26,27]. However, the terms supercritical
trends are observed at other compression ratios. Noting that a low and transcritical are used here to distinguish between cycles operating
compression work increases cycle efficiency (i.e., 𝜂 = (𝑤t − 𝑤c )∕𝑞h , with or without condensation, as used within the CO2 research commu-
where 𝜂, 𝑤t and 𝑞h are the thermal efficiency, specific expansion nity [28]. Combining these definitions enables a general classification
work and heat addition respectively), the advantages of sCO2 become of thermodynamic cycles as reported in Fig. 3.
apparent. The two sCO2 cycles of primary interest are the supercritical cycle
Operating under supercritical pressures, however, has implications and the transcritical cycle, which are shown by the blue and green
on both cycle operation and component design. Firstly, operation pres- cycles in Fig. 3 respectively. In both cycles the high pressure of the
sures that are within the range of 50 to 250 bar require all parts to be system exceeds the critical pressure. In the supercritical cycle, the low
designed to safely operate under both high pressures and high pressure pressure of the system is also above 73.8 bar, and there is no distinction
differentials, and may also require specialist materials that can with- between the fluid being in a liquid or a vapour state, whilst in the
stand harsh operating conditions, i.e., high pressure, high temperature transcritical cycle, the low pressure of the system is below 73.8 bar,
and the occurrence of corrosion (see Section 4.3). The high pressures
and condensation is possible within the low-pressure heat-rejection
also lead to high densities, and consequently low volumetric-flow rates
process. Referring to the right-hand plot in Fig. 2, it is observed that the
through the system. This enables compact pipework and plants with
lower the compressor inlet temperature and the closer the compression
a small physical footprint, but introduces challenges in designing tur-
process is to the saturated liquid line, the lower the compression work.
bomachinery components with a high power density (see Section 4.1).
There are also challenges related to starting the compression process This motivates the use of a transcritical cycle to maximise thermal
close to the critical point. Specifically, there are significant variations in efficiency. Consequently, this means a transcritical cycle can only be
the thermodynamic properties of CO2 in the vicinity of the critical point considered where it is possible to cool the CO2 below 31.1 ◦ C. If this
(see Fig. 2). The sharp drop in density from around 700 to 200 kg/m3 is not possible, a supercritical cycle should be considered with the
around 𝑇 ∕𝑇cr ≈ 1 and 1 < 𝑝cr < 1.2, and the sharp spike in the specific- compression process starting close to the critical point to maximise
heat capacity around 0.98 < 𝑇 ∕𝑇cr < 1.05, will affect compressor thermal efficiency.
operation, particularly at off-design conditions, introducing challenges Alongside pure CO2 , CO2 mixtures have been proposed for closed-
related to system control to ensure steady and efficient operation (see loop power cycles. By doping CO2 with another fluid the thermody-
Section 4.4). These variations also introduce challenges in compressor namic properties can be altered, and the critical point of the working
design and performance assessment, in addition to managing the possi- fluid shifted. This could include the use of noble gases, alongside
bility of condensation (see Section 4.1). Finally, the property variations
also influence heat-exchanger design and operation. This is both from
a more fundamental perspective point, where the sudden change in 1
The cycle devised by Rankine considers an expansion from saturated
specific-heat capacity at constant pressure influences the effectiveness vapour and while Hirn [25] proposed the superheated Rankine cycle, the term
of internal heat-exchange processes, to more practical aspects relating Rankine cycle is used here to refer more generally to a cycle in which phase
to off-design performance (see Section 4.2). change occurs.

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M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

can be extended to more complex layouts, which may include reheat,

recompression, precompression, or any combination thereof; a few
notable cycle arrangements are also reported in Fig. 4.
The reheat cycle (Fig. 4b) divides the expansion process (4 → 5
and 6 → 7), and introduces an intermediate reheating process. This
increases the average temperature of heat addition, and hence cycle
efficiency, and also increases the turbine exhaust temperature, which
increases the potential internal heat recovery within the recuperator.
Within Dostal’s work [16], the recompression cycle (Fig. 4c) was
identified as a promising cycle, with the characteristics of high thermal
efficiency whilst retaining a simple cycle arrangement. Within the re-
compression cycle, the recuperation is divided into a high-temperature
recuperator (3 → 4 and 6 → 7) and a low-temperature recuperator
(2 → 3 and 7 → 8), with a secondary compressor installed that operates
between points 3 and 8. The purpose of this compressor is to bypass
a fraction of the main flow from the heat-rejection process and main
compressor, which reduces the heat-capacity rate (i.e., 𝑚𝑐 ̇ 𝑝 ) within the
high-pressure side of the low-temperature recuperator (2 → 3). This
helps offset the mismatch in the heat capacities of the hot and cold
fluids within the recuperator, reducing the irreversibility compared
to the simple recuperated cycle [12–15]. Today, the recompression
cycle is widely considered to be a promising cycle for sCO2 power
cycles, and the improvement in thermal efficiency is expected to offset
the additional cost associated with the additional components [20].
A combination of reheat and recuperation has been proposed for a
large-scale sCO2 cycle [40].
Since the individual works of Feher, Angelino and Dostal, there
have been numerous studies investigating the merits of various cycle
architectures for different applications. Extensive assessments of cycle
layouts have been carried out by Crespi et al. [28,35]. Whilst the
Fig. 3. Classification of thermodynamic power cycles. (For interpretation of the authors make some general observations about optimal cycle, they also
references to colour in this figure legend, the reader is referred to the web version note that when reporting an optimal cycle it is important to provide
of this article.)
an indication of the operating and boundary conditions, alongside
any technological limits. To this end, thermodynamic modelling and
optimisation is likely to remain an important step when evaluating the
nitrogen and oxygen, to lower the critical temperature and pressure, performance of sCO2 cycles for a given application.
which could enhance thermal efficiency by allowing higher pressure
ratios [29]. Alternatively, doping could increase the critical temper- 4. Component aspects
ature, allowing condensation cycles to be realised at elevated heat
sink temperatures. For this purpose, neon, SF6 and butane have been 4.1. Turbomachinery
studied for low-temperature applications (160 ◦ C) [30], hydrocarbons
for temperatures up to around 400 ◦ C [31], and TiCl4 , N2 O4 and NO2 The properties of sCO2 introduce opportunities and challenges in
for applications up to 700 ◦ C [32]. An important step in realising such turbomachinery design. The high operating pressures and densities
cycles is to characterise the behaviour and thermal stability of CO2 allow compact turbomachinery components, which could allow a sig-
mixtures [33]. nificant reduction in size, and hence cost, of the overall plant, but also
lead to high power density turbomachinery, and, for power outputs
3.3. Thermodynamic modelling studies below a few MWe , turbomachines that must rotate at high speed to
maintain high isentropic efficiency. Small diameters and higher rota-
The purpose of thermodynamic modelling is to investigate how the tional speeds are associated with increased aerodynamic losses, such
cycle parameters, such as the operating temperatures and pressures, as increased tip-clearance and secondary-flow losses, and introduce
affect thermodynamic cycle performance. This is done by coupling a challenges in the design of the shaft, bearings and seals to ensure stable
suitable equation of state for CO2 with a thermodynamic model of rotordynamic behaviour across the range of expected operating speeds.
each component within the cycle and running a parametric or optimi- The high operating pressures and large absolute pressure difference
sation study to identify optimal cycles. This may be from the first-law across a single turbine stage also introduce challenges related to the
perspective of maximising thermal efficiency [34,35], the second-law mechanical design of the turbine housing and the turbine blades, and
perspective of reducing internal irreversibility [36,37], or from the in managing axial thrust and leakage to the surroundings. This creates
perspective of minimising the specific-investment cost or the levelised a complex design space in which suitable designs that meet the trade-
cost of electricity (LCOE) [38,39]. off between aerodynamic, rotordynamic and mechanical performance
The baseline cycle for most studies is the simple recuperated cycle, need to be identified.
be that either a supercritical or transcritical cycle, where a recuperator Within any sCO2 system there are a minimum of two turboma-
is used to transfer heat from the hot exhaust leaving the turbine to the chines, namely the compressor or pump and the turbine. Within more
cold fluid leaving the compressor (see points 2 → 3 and 5 → 6 in Fig. 4a, complex cycles, such as the recompression cycle, this is supplemented
4d). Within the literature the terms recuperation and regeneration with an additional recompressor. Whilst the turbine, and recompressor,
are used interchangeably to describe the simple cycle; here the term operate sufficiently far from the critical point that they operate with a
recuperation is used since the internal heat recovery occurs with the gas-like substance, the proximity of the main compressor to the critical
use of a recuperative heat exchanger. The simple recuperated cycle point and its operation with supercritical pressures, mean the main

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M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

Fig. 4. Potential sCO2 power cycles: (a) supercritical recuperated; (b) supercritical reheated–recuperated (c) supercritical recompression; (d) transcritical recuperated. The red and
blue lines represent the heat-addition and heat-rejection processes respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the
web version of this article.)

compressor can be referred to as a pump or compressor. For generality, ratios in the range of 2 to 5 [45]. These values are somewhat higher
the term compressor will be used here, and the term pump is reserved than existing prototypes, which were designed to minimise cost and
for condensing transcritical cycles. risk [43].
Both compressors and turbines take the form of either an axial or Another useful chart for sCO2 turbomachinery selection was pro-
a centrifugal/radial-inflow architecture. In the former, the flow passes posed by Sienicki et al. [46] (Fig. 5). Alongside identifying the power
through the machine parallel to the axis of rotation, whilst in the ranges where different turbomachinery architectures are expected to
latter the flow turns through 90◦ . The axial design allows multiple be most suitable, it provides an overview of the available options
expansion stages to be readily mounted onto the same shaft, whilst for the bearings, seals, alternators and shaft arrangement. The main
in a radial or centrifugal design the change in radius between the conclusion from this chart is that below around 10 MWe , radial and
rotor inlet and outlet facilitates a larger enthalpy change over a single centrifugal machines are preferred, with rotational speeds in excess of
stage. For large-scale applications multi-stage axial turbomachines are 30,000 RPM, which requires a permanent magnet rotor, and potentially
favoured, while for smaller-scale applications the radial-inflow turbine multiple shafts. For large-scale sCO2 power systems (> 100 MWe ) axial
and centrifugal compressor designs are preferred as they can accom- turbomachinery is preferred.
modate the required pressure change over a single stage. Preliminary
sizing and selection is typically completed using the maps developed 4.1.1. Turbomachinery design and simulation
by Balje [41], which relate the achievable design-point efficiency for Alongside experimental demonstrations, there has been a concerted
the different turbomachinery architectures to the specific speed 𝑁s and effort in the design, simulation and optimisation of sCO2 turbomachin-
specific diameter 𝐷s : ery. The use of meanline design tools and optimisation methods to
√ identify optimal geometries that can achieve the desired aerodynamic
𝜔 𝑉̇ performance is widely applied within the field of turbomachinery,
𝑁s = ; (1)
3∕4
𝛥ℎs and in theory these tools can be readily applied to sCO2 turboma-
chinery. These employ loss models to account for various losses, such
1∕4 as passage, incidence, clearance and windage losses, which are typi-
𝐷𝛥ℎs
𝐷s = √ , (2) cally empirically-derived for air and steam turbomachinery. There are
𝑉̇ many examples of such studies, although a few examples include those
which relate rotational speed 𝜔 and diameter 𝐷 to the isentropic relating to sCO2 compressor design [47,48], turbine design [49,50],
enthalpy change across the machine 𝛥ℎs , and volumetric-flow rate 𝑉̇ off-design turbomachinery prediction [51], and the integration of tur-
at either the inlet (compressor) or outlet (turbine) of the machine. bomachinery design models with thermodynamic cycle design [44,45].
From a simple thermodynamic cycle analysis it can be shown that However, a major issue facing these models is that they have not been
increasing the pressure ratio of the cycle (to a point) will increase experimentally validated for sCO2 applications, and as demonstrated
the cycle thermal efficiency. This will increase the enthalpy change by Lee & Gurgenci [52], the choice of model can affect the results
across the compressor and turbine and for a specified net power output generated.
reduce the mass- and volumetric-flow rates for both machines. Thus, A similar statement can be made with regards to the use of
more efficient cycles are likely to require smaller diameters and higher computational-fluid dynamic (CFD) tools to assess sCO2 turbomachin-
rotational speeds, which further exacerbates the issues mentioned pre- ery performance. Arguably the turbine, and possibly a recompressor,
viously [42]. Considering that the cycle thermal efficiency is sensitive operate sufficiently far away from the critical point such that the CO2
to the turbomachinery efficiencies, with Allison et al. [43] suggesting behaves like an ideal gas. Thus, existing meanline models and CFD
a drop in compressor efficiency from 90% to 80% could lead to a solvers may provide an adequate means to generate suitable turbo-
drop in overall cycle efficiency of 2.0% and the same drop in turbine machinery designs. However, for sCO2 turbomachinery the trade-off
efficiency could lead to a drop in overall cycle efficiency of 4.4%, it is between aerodynamic, rotordynamic and mechanical considerations,
clear that turbomachinery and cycle performance are closely coupled. particularly for small-scale applications, may shift the optimal design
Thus, turbomachinery and cycle design should be tackled in unison, away from the conventional design space. Thus, the use of CFD to assess
although even then, such studies point to optimal cycles with turbine novel designs prior to experimentation is a useful approach. A good
inlet pressures between 200 and 400 bar [44], or optimal pressure example of this is provided by Keep [53,54], who investigated low

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M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

Fig. 5. Turbomachinery options for sCO2 power cycles. Source: Reproduced with permission from Sienicki et al. [46].

specific-speed turbine designs and found an isentropic efficiency of 81% main compressor and the other containing the recompressor of a re-
for a 300 kWe radial-inflow turbine could be possible, suggesting that compression Brayton cycle [63]. The use of a TAC unit allows all
efficiency could be improved by modifying the rotor–stator interspace, of the rotating machinery to be mounted on a single shaft. This has
minimising the clearance gap or using a shrouded rotor, or adding the advantage of simplicity, particularly for a simple cycle where
splitter blades to the rotor. there is only a single compressor and turbine, but does require the
Another area that represents a challenge for scientific theory and compressor and turbine to rotate at the same speed and be matched
CFD simulation is the simulation and performance prediction of sCO2 appropriately. Although in the reported test the heat input was limited,
compressors operating near the critical point. The non-linear thermo- the results suggested that the turbomachinery behaved as expected, in
dynamics of CO2 around the critical point lead to possible real-gas addition to verifying the speed control algorithms, stable operation of
effects, whilst condensation near the leading-edge of the compressor the single-shafted TAC unit and cold startup methods [63]. However,
may occur. Studies have investigated real-gas models to predict the be- a significant challenge identified for small-scale sCO2 turbomachin-
haviour of CO2 near the critical point [55], real-gas effects [56,57], the ery during the initial SNL tests arose from the need to use gas-foil
performance of centrifugal compressor diffusers [58], and condensation bearings, owing to the high power density of the turbomachinery,
effects both numerically and experimentally in a converging–diverging which results in large shaft diameters and high shaft surface velocities.
nozzle [59,60]. The results suggest the time required for stable liquid Whilst the journal bearings behaved adequately, thrust bearing failures
droplets to form during expansion significantly reduces as the crit- were observed [85]. It was suggested to use smaller diameter thrust
ical point is approached. Operation near the critical point also has bearings, and employ additional cooling for the bearings [63]. Failure
implications for compressor stability and the prediction of off-design was also attributed to the Teflon coating which was unsuitable for high
performance, since conventional similitude laws cannot be applied [61] temperature operation. Later, this lead to low-friction coatings for gas-
and uncertainty in determining efficiency is introduced [62]. foil bearings being investigated to facilitate rotation during start-up
and shut-down where the shaft typically rides along the foil bearing
4.1.2. Existing sCO2 turbomachinery surface [85].
Given the challenging operating conditions and design space, it is The turbomachinery installed within a demonstration sCO2 test loop
critical to demonstrate that the desired turbomachinery performance installed at Carleton University is also based on the SNL design [86].
can be realised in practice. To this end, a number of sCO2 test loops More recently, the SNL test loop has been upgraded to test a turbocom-
have been constructed, or are currently underway. A summary of the pressor designed by Peregrine Turbine Technologies, which consists of
turbomachinery designs for these test loops is provided in Table 1. two centrifugal compressor stages and a single radial-inflow turbine
Before discussing these in detail, it is worth emphasising two points. on the shaft [64]. Although the test capabilities could not match the
Firstly, most of the turbomachinery that has been tested to date is design point of the machine, the tests demonstrated the use of a blow-
not representative of what would be planned for larger systems. This down procedure to start the compressor in the absence of a starting
introduces specific challenges and considerations that might eventually motor and enabled the team to resolve issues relating to the thrust
be unnecessary for larger-scale plants. Nonetheless, initial testing at and radial bearings [65]. Initial testing reported issues with both the
the laboratory-scale is a necessity given the cost and complexity of radial and thrust bearings experiencing rubs or failure. Issues with the
designing and constructing MW-scale test loops. Secondly, it is noted thrust bearing were resolved by adjusting the turbine back pressure to
that unlike other components within the cycle, it is difficult to decouple adjust the thrust, whilst it was hypothesised that the radial bearing
the turbomachinery into separate compression, re-compression and failure arose from a non-uniform turbine inlet temperature and/or
expansion machines, since in many existing prototypes these machines flow rate. To overcome this issue, the length to diameter ratio of the
share a common shaft, and have the same challenges. For this rea- radial bearing was increased to increase the load capacity, resulting in
son, the discussion around compression and expansion machines is successful operation.
combined here into a single section. Alongside developments at SNL, a collaboration between Bechtel
Sandia National Laboratories (SNL), in partnership with the De- Marine Propulsion Corporation and the Bettis Atomic Power Labora-
partment of Energy and Barber Nichols, developed and tested two tory developed a 100 kWe integrated system test (IST), for which the
turbine–alternator–compressor (TAC) units, with one containing the turbomachinery comprised of a variable speed turbine-compressor, and

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M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

Table 1
Existing sCO2 turbomachinery designs.
Name Power Cyclea 𝑇max 𝑃max PR Typeb Seals Bearings Architecturec 𝑁 𝐷 𝑚̇
[kWe ] [◦ C] [bar] [kRPM] [mm] [kg/s]
SNL [63] 125 RC 537 170 1.8 TAC Labyrinth Gas foil IFR 75 68.1 2.7
CC (main) 75 37.3 3.5
TAC Labyrinth Gas foil IFR 75 68.3 3.08
CC (recomp) 75 57.9 2.4
PTT – RC 750 423 TC Leaf Gas IFR, CC (x2) 118 – 5.5
[64,65]
IST 100 RE 299 1.8 TC, TG Labyrinth Gas foil IFR (TC, TG) 75 53 –
[66,67] CC (TC) 75 38 –
Echogen 8000 RE 485 TC – – IFR, CC 24–36 – –
[68] TG Dry-gas Tilting pad IFR 30 – –
SWRI/GE 1000 RC 715 251 2.9 T Dry gas, Tilting pad 4-stage AT 27 – 8.41
[69,70] (10,000) Labyrinth
STEP [71] 10,000 RC 715 250 2.7 T – – 3-stage AT – – 103
NET Power 200,000 AL 1150 300 T – – 7-stage AT – – –
[72–74] (kWth )
TIT [75] 10 RE 277 119 1.45 TAC – Gas IFR 100 35 1.1
CC 100 30 1.1
KAIST 300 RE 500 200 2.67 MC – – CC (twin, shrouded) 70 – 3.2
(SCIEL) TG – – IFR (shrouded) 80 – 5.05
[76,77] TAC – – – 68 – –
KIER 1 RE 200 130 2.27 TG – Angular ball IFR (PA) 200 22.6 –
[78–80] 10 S 180 130 1.65 TAC Labyrinth Gas foil CC, IFR (shrouded) 70 50 –
60 RE 392 135 1.75 TG – Tilting pad 1-stage AT (PA) 45 73 1.74
sCO2 -HeRo 7 S 200 117.5 1.5 TAC Labyrinth Angular ball IFR (shrouded) 50 66 0.65
[81,82] CC (shrouded) 50 40 0.65
I-ThERM 50 RE 435 127 1.7 TAC – Angular ball IFR turbine 60 72 2.1
[83,84] CC 60 55 2.1
a
Cycle layout: simple (S), recuperated (RE), recompression (RC), Allam (AL).
b
Letters refer to components mounted on the same shaft: turbine (T); alternator (A); compressor (C); generator (G); motor (M).
c
Types of turbine: inward-flow radial turbine (IFR); centrifugal compressor (CC); axial turbine (AT); partial admission (PA).

a constant speed turbine-generator [66]. However, it was noted that constructed from a single-stage centrifugal compressor and single-stage
since the turbomachinery tested is not representative of what would radial turbine. Tests reported an isentropic efficiency that exceeds 80%
be planned for larger systems, the use of gas foil bearings introduced for the turbine-compressor, and in the range of 20 to 75% for the
specific startup and operational procedures that would be unnecessary turbine-generator, although the turbine-generator was not tested at its
for larger-scale plants. A later study indicated that at peak operating design point [68].
power the generator-turbine and compressor-turbine operated above Other significant developments in turbomachinery for sCO2 systems
their predicted performance, with isentropic efficiencies of 83.6% and within the US can be related to work under the SunShot, APOLLO and
85.2% at power outputs of 56.8 and 52.6 kW respectively [67]. The STEP projects. Under the SunShot programme a 1 MWe -scale sCO2 test
compressor operated with an isentropic efficiency of 72.4%, which loop has been constructed to test a multi-stage axial turbine with a
was well above the predicted value of 58%; although the accuracy net power output of 10 MWe [70,90,91]. To minimise development
of compressor maps developed using ideal-gas assumptions in close costs, the tests employ a reduced mass-flow rate, through reduced area
proximity to the critical point should be considered [61]. Ultimately, nozzle and blade passages, so the design velocities can be maintained
the IST demonstrates that at this small-scale feasible turbomachinery without requiring to operate the turbine at full capacity. The design
components can be developed, although high windage losses were of the turbine is reported by Kalra et al. [69] and comprises of a four-
observed due to the high pressure, and subsequent high density, of the stage shrouded axial turbine with a rotational speed of 27,000 RPM and
fluid within the cavity that contains the motor-generator. Moreover, an isentropic efficiency in excess of 85%, as predicted from meanline
much like the SNL tests, the IST tests also experienced issues relating to and CFD analysis. Compared to the small-scale systems, issues around
heating generated from the gas-foil bearings. As a result, the rotational high bearing and windage losses, in addition to motor control issues,
speed was limited to 60,000 rpm to maintain safe bearing temperatures are expected to be less critical in large-scale systems since existing
using the available cooling without exceeding the thrust load capacity technologies such as shaft-end seals and oil-film bearings should be
of gas-foil thrust bearings [67,70]. suitable [70]. Having said this, there have been developments in hy-
Echogen power systems [87] have developed sCO2 technology for drostatic bearings, which require an external sCO2 supply but facilitate
waste-heat recovery applications, which has been licenced to Siemens hermetic turbomachinery designs removing the need for sCO2 shaft
for the oil and gas sector [88] and to General Electric for marine seals [92], hydrostatically-assisted gas foil bearing designs [93], and
applications [89]. The Echogen EPS100 unit has a net power out- foil bearings for large MW-scale turbomachinery where the use of
put of 8 MWe , and, similar to the IST, the turbomachinery consists foil bearings is quoted to eliminate speed and temperature limitations
of a constant speed turbine-generator and variable speed turbine- and the need for liquid lubrication [94]. Other challenges related to
compressor. The turbine-generator rotates at 30,000 RPM and utilises the design of the SunShot turbine include the mechanical design of
a four-pole synchronous generator and epicyclic gearbox, whilst the the shaft and the casing. Whilst turbine inlet conditions are similar
turbine-compressor has a nominal shaft power of 2.7 MW and is to those of existing steam turbines, sCO2 turbines experience higher

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M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

outlet temperatures and pressures and require a steeper temperature Outside of the US, a number of notable sCO2 turbomachinery proto-
gradient within the shaft to allow the use of dry-gas seals [43]. Thus, types have been developed. In Japan, Utamura et al. [75] developed a
the thermal management of the shaft is important to ensure an axial single-shaft TAC unit with a target net power of 10 kW and nominal
temperature gradient in the shaft to minimise stress in the shaft and rotational speed of 100,000 RPM. Compressor isentropic efficiencies
casing [70,95]. Moore et al. [95] reported initial testing of the SunShot between 30% and 70% are reported, although high windage losses were
turbine, reaching a rotational speed of 21,000 RPM and turbine inlet reported. Within the Korea Institute of Energy (KIER), three experimen-
conditions of 550 ◦ C and 180 bar. The primary aim of these tests tal loops have been developed, as described by Cho et al. [78]. The first
was to break in the turbine and to monitor vibrations, critical speeds employed a 10 kWe TAC unit, with shrouded centrifugal compressor
and bearing temperatures, which was considered a success since sta- and radial turbine rotors to overcome thrust balancing issues, although
ble operation was observed. Future tests expect to move towards the issues with the gas foil bearings are reported. The second was used to
design point of 715 ◦ C, 250 bar and 27,000 RPM. Under the APOLLO test a 1 kWe turbine-generator constructed from a radial turbine with
programme, work has been initiated on the design of the compressor partial admission and utilised commercial ball bearings. The final loop
for the same recompression Brayton cycle. Cich et al. [96] describe the was designed for a power output of 60 kWe . Further developments are
design of the compressor assembly considering all rotordynamic and described by Shin et al. [79]. For the 60 kWe system, commercially
mechanical design considerations. The authors propose an arrangement available tilting-pad bearings were employed to overcome high axial
where the main compressor and recompressor, which are both single- and radial thrusts. However, to employ these bearings it was necessary
stage centrifugal machines, are positioned back-to-back and variable to reduce the rotational speed, which lead to the selection of a partial-
inlet guide vanes are utilised. Within the APOLLO programme, Hanwha admission, single-stage axial impulse turbine with a rotational speed
Techwin and Southwest Research Institute have also been developing of 45,000 RPM. In subsequent tests, an isentropic turbine efficiency of
an integrally geared compressor–expander system, in which the turbine 51% has been reported, with the authors emphasising being able to
and compressor stages are all of the radial design and are mounted resolve bearing failure issues through the use of an axial machine as a
within a single integrally-geared unit [97]. The unit is designed as success [80].
a 5–25 MWe modular power block, and comprises of a two-stage A collaboration between the Korea Advanced Institute of Science
main compressor, two-stage recompressor, and a four-stage turbine and Technology (KAIST) and Korean Atomic Energy Research Institute
with reheat [43]. The aim of the STEP (Supercritical Transformational (KAERI) has led to the development of the Supercritical CO2 Integral
Electric Power) project is to construct and commission a 10 MWe sCO2 Experiment Loop (SCIEL). For the initial low pressure ratio tests a
simple cycle was constructed with a separate motor-driven compressor
plant [71]. The turbine is based on the SunShot turbine, but the stage
and turbine-generator set, rather than using a single TAC unit, and
count is reduced from four to three, facilitating a more compact design,
a twin impeller centrifugal design was selected for the compressor
whilst the volute area is optimised and thermal management within the
to control thrust loads [76]. At the time, future tests were planned
turbine enhanced. The compressor within the STEP facility is provided
for higher pressure ratio tests, which would utilise an additional TAC
by Baker Hughes, and leverages both existing commercial product lines
unit constructed from a high-pressure turbine and high-pressure com-
and work undertaken under the APOLLO programme [71].
pressor [77]. Alongside the SCIEL, a compressor test facility, named
The ultimate goal of the SunShot, APOLLO and STEP projects is to
the SCO2PE (Supercritical CO2 Pressurising Experiment) has been con-
realise large-scale sCO2 power plants, and thus the turbomachinery for
structed to test compressor operation near the critical point, which
both a 50 and a 450 MWe plant has been proposed [40,98]. Since a
utilises a 26 kWe canned motor pump, with a centrifugal shrouded
limitation on the maximum shaft diameter would restrict the SunShot
impeller and rotational speed of 4620 RPM [62,102]. Compression
turbine being directly up-scaled to 50 MWe , a six-stage axial turbine
efficiencies between 10 and 60% are reported for varying inlet con-
has been proposed with a 406 cm tip diameter, rotational speed of
ditions [102], although the uncertainty of calculating efficiency close
9500 RPM and an estimated efficiency of 90.3%; for the 450 MWe
to the critical point is noted [62]. Wang et al. [103] report the design
plant, a reheat recompression cycle is proposed with a dual-flow four-
of an integral test loop with a power output of 1 MWe at the Nuclear
stage high-pressure turbine and three-stage low-pressure turbine, with Power Institute of China. Initially, the system will employ a TAC unit
predicted efficiencies of 90.6% and 91.6% respectively, all mounted on with a design speed below 30,000 RPM, although further details have
a single shaft [40]. However, the availability of large-diameter film- not been reported.
riding end seals was seen as a limitation, leading to dedicated tests to Arguably, within Europe the development of sCO2 turbomachinery
develop a new seal design [99]. In terms of compressor design for the has lagged behind developments elsewhere, although there are a few
450 MWe plant, a back-to-back arrangement was proposed consisting exceptions. Hacks et al. [81,82] designed a TAC unit for the H2020
of a two-stage centrifugal design and a four-stage centrifugal design for sCO2 -HeRo project, which comprises of a single-stage centrifugal com-
the main compressor and recompressor respectively, all mounted on a pressor and radial turbine, both with 2D shrouded blades; the use of
single shaft [98]. shrouded impellers allows the use of seals to reduce clearance losses,
NET Power are developing an oxy-fuel thermodynamic power cycle whilst a 2D blade more easily permits the installation of the seals com-
for which Toshiba are providing a turbine designed to operate with pared to a 3D blade. Whilst the optimal rotational speed for a high stage
inlet conditions of 300 bar and 1150 ◦ C [100]. The concept for the efficiency would be 200,000 RPM, the rotational speed was limited to
turbine design is to use proven technology as much as possible, whilst 50,000 RPM to minimise windage losses [82]. Within the H2020 I-
testing a scaled turbine that is representative of the final turbine ThERM project, the current authors from Brunel University London, in
used within a commercial plant. As such, Toshiba have developed a collaboration with Enogia, have developed the High Temperature Heat
preliminary turbine design for a 500 MWth system, which has subse- To Power Conversion facility (HT2C), which utilises a TAC unit with
quently been scaled for a thermal input of around 200 MWth and then unshrouded single-stage radial compression and expansion stages [83,
operated with partial admission to reduce the required thermal input 84], which is currently undergoing testing. Other developments within
to 50 MWth [73]. The design comprises of a single seven-stage axial Europe include the H2020 sCO2 -Flex project which aims to adapt
turbine [72,74], which combines proven technology for high-pressure fossil-fuel power plants through the use of sCO2 and will involve the
steam turbines, namely the use of an inner and outer pressure casing, design of the turbomachinery for a 25 MWe cycle [104], and the
and proven gas turbine technology, such as coatings and internal H2020 SCARABEUS project, within which the current authors from
cooling of the turbine blades [73]. Freed et al. [101] report on the City, University of London are leading the conceptual design of the
development of a gas-turbine driven integrally-geared compressor for turbomachinery for a 100 MWe sCO2 CSP plant operating with CO2
the plant. blends [105].

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M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

Table 2
High pressure heat exchangers [106,107].
Type 𝑃max 𝑇max Maximum surface area
[bar] [◦ C] density [m2 ∕m3 ]
Plate and shell 100 900 200
Brazed plate-fin 120 650 1500
Diffusion-bonded plate-fin 200 400 800
Packinox plate 300 700 300
Microtube 400 650 2000
Printed circuit 500 900 5000
Shell and tube 1400 600 100

4.2. Heat exchangers

In sCO2 systems, the heat exchangers greatly influence the overall

cycle efficiency and system size and must adhere to specific operating
conditions and design requirements. The primary challenge is to main-
tain cycle compactness and endure the high temperature and pressures.
The other important challenge is to find a compromise between the
heat exchanger type, cost, durability and performance. Three heat
exchangers are generally involved in sCO2 systems; the heater which
absorbs heat from the heat source; the recuperator which transfers heat
from the turbine exhaust to the compressor exhaust; and the cooler
which rejects heat to the environment. A summary of candidate types
of high pressure heat exchangers, along with an appraisal of maximum
pressure, maximum temperature and maximum surface area density, is
provided in Table 2. Fig. 6. Designed boiler adapted to an sCO2 Brayton cycle [108].

4.2.1. Heaters
Two types of heat exchanger are typically employed as CO2 heater, a conventional shell-and-tube heat exchanger, including the ability
depending on the heat transfer process and heat source temperature; of microtubes to withstand very high pressures. As shown in Fig. 7,
one is a radiant heating section combining radiation and convection microtube heat exchangers can be designed with the heat carrier fluid
processes, used in fuel-fired applications, and the other uses only crossing the tube bundle, which can greatly reduce the pressure drop
convection heating, for example in waste-heat recovery applications. in the shell side. Plate-fin heat exchangers are a matrix of alternate
For the radiant heating section, the CO2 heater geometry is similar flat plates consisting of enclosed channels and fin corrugations. The
to that in steam cycles, but the lower turbine pressure ratio and fins, such as the plain triangular, louver, perforated, wavy fin, or with
the different thermophysical properties make the CO2 heater design vortex generators, enhance the heat transfer of the lower pressure
significantly different from that found in steam cycles; relatively higher heat carrier fluid. This sandwich construction has a naturally strain-
mass flow rates are required for the same level of heat input, and compliant design, leading to the potential to achieve a high-cycle
shorter piping length to minimise the pressure drop. A representation fatigue life. However, smaller channel dimensions are associated with
of the heating section, proposed by Moullec [108], is shown in Fig. 6. some disadvantages, including higher pressure drop per unit length,
This coal-fired boiler employed eight heat exchangers to heat the CO2 propensity to fouling and the difficulty in repair in case of leakage
to the maximum temperature and cool the flue gas down to about inside the heat exchanger core [111].
540 ◦ C. The main challenges in the design were to reduce the flue
gas temperature to meet the capabilities of modern flue gas preheaters 4.2.2. Recuperators
(typically around 370 ◦ C), to decrease the CO2 pressure drop to a Printed circuit heat exchangers (PCHEs) are the most widely
commercially feasible level, and to maintain adequate safety margins adopted sCO2 recuperators, due to their compactness and structural
to avoid tube overheating or unacceptably high temperatures. rigidity and reliable performance under conditions of extreme pressure
For convection heat transfer only, the shell-and-tube heat exchanger and temperature [111,112]. The diffusion bonding process creates an
is the most common type with various configuration options. The sCO2 exceptionally strong heat exchanger core as shown in Fig. 8, which
flows along the tubes, and the heat carrier fluid flows across the consists of stacks of flat metal plates with fluid flow channels either
tubes from the shell side to transfer heat between the two fluids. The chemically etched or pressed into them. The diffusion bonding process
tubes should have good thermal conductivity to achieve the desired allows the plates to be joined together with the same strength as the
heat transfer rates and withstand the operating temperature and pres- parent metal. The chemically etched process allows the mechanical
sure [109]. The shell and tubes should withstand the thermal stresses design to be flexible so that etching patterns can be adjusted to match
between them and should be designed for high-cycle fatigue life. A the required operating temperature and pressure-drop constraints. The
major problem with shell-and-tube heat exchangers used as CO2 heaters etched flow passages as shown in Fig. 9 are mainly categorised into
are their large physical size, leading to high capital cost due to the four types: straight channel, zigzag (or wavy) channel, channel with
significant amount of material needed to contain the high-temperature, S-shaped fins, and channel with airfoil fins. Since the 2000s, sCO2 test
high-pressure sCO2 environment. Therefore, compact heat exchangers facilities have been developed in the US, Japan, South Korea, China
are more suitable for sCO2 heater applications, due to their large and the UK and the thermohydraulic performance of PCHEs has been
surface area to volume ratio and high heat transfer coefficient [106]. extensively investigated. Huang et al. [113] and Kwon et al. [111]
Considering that the heat carrier fluid usually operates under high reviewed the flow and heat transfer characteristics of sCO2 as well as
temperature but low pressure, the microtube architecture and plate-fin available correlations for the design of PCHEs. Chai and Tassou [112]
structure are suitable for CO2 heater applications [110]. The microtube detailed characteristics and challenges relevant to PCHEs in sCO2 Bray-
heat exchanger can provide significantly improved performance over ton cycles, including material selection, manufacture and assembly,

9
M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

Fig. 9. Etched flow passages: (a) straight channel, (b) zigzag (or wavy) channel, (c)
channel with S-shaped fins, and (d) channel with airfoil fins [112].

difference in the fluid properties of the two fluids, leads to a minimum

heat transfer rate. Lastly, cleaning PCHEs is complicated due to a
Fig. 7. Microtube CO2 heater for exhaust waste heat recovery: the heat exchanger welded body from the core to the header. Hence, it is advisable to
schematic at the top of the figure consists of four modules; each module, displayed in employ PCHEs within a limited fouling environment, or to at least use
the picture at the bottom of the figure, is made up of microtubes whose arrangement strainers.
is reported in the A–A cross-section view (courtesy of Reaction Engines Ltd.).
According to [114], up to 90% of the cost of the sCO2 Brayton cycle
could be associated to heat exchangers assuming the use of PCHEs. As
such, to facilitate techno-economic feasibility and the market uptake
of sCO2 power systems, alternatives to PCHE have been developed.
Fourspring et al. [115] described two compact heat-transfer surfaces
in the form of a recuperator. One surface employed a wire mesh as
the extended heat-transfer surface, and the other surface employed a
folded-wavy-fin as the extended heat-transfer surface. The recuperator
employing the traditional, folded-wavy-fin heat-transfer surface can
achieve the design heat-transfer rate with less than half of the allowable
pressure drop. Carlson et al. [116] suggested that cast-metal heat
exchangers may offer performance similar to or better than PCHEs at
less than a fifth of the cost while allowing for greater flexibility in ma-
terial selection and channel geometry. Chordia et al. [107] developed
a microtube heat exchanger bundled together with axial separation
Fig. 8. (a) Typical PCHE: (b) flow paths and details of the diffusion-bonded core (c) sheets that direct flow in a strictly counter-current direction, which
and of an etched plate (courtesy of Heatric Meggitt UK). has the potential to meet the high temperature and high differential
pressure criteria but with much lower capital cost.

thermohydraulic performance, geometric optimisation and provided 4.2.3. Coolers

suggestions for further research and development. The operating temperatures and pressures in the cooler are lower
A summary of heat transfer and friction factor correlations for than in the heater or recuperator, leading to less concern about struc-
PCHEs is listed in Table 3. These correlations were proposed based tural integrity and material selection. The cooling fluid is either air or
on CO2 , water and helium data. It is important to note that some of water, leading to widely different heat exchanger configurations. Their
the performance data used was far from the critical point so many differing thermophysical properties significantly affect the heat flux in
published correlations may be subject to a high degree of uncertainty. the cooler and influence the operating parameters and hence the heat
Moreover, most of the correlations were developed for specific flow transfer performance.
passages and using thermophysical properties corresponding to the In air-coupled coolers, the sCO2 operates at much higher pressure
average temperature. Therefore, they are not universal but for specific and has a significantly higher heat transfer coefficient than the cooling
PCHE configurations. air, so the finned-tube heat design is appropriate for this applica-
Despite the superior heat transfer performance, a major problem tion [109,110]. The sCO2 flows in the array of circular or flat tubes,
caused by the non-straight channels in PCHEs is their large pressure and the cooling air crosses the finned tubes where flat or continuous
drop, due to longer flow passages and complicated channel geometry. (plain, wavy, or interrupted) external fins are employed to increase
Another important issue is related to the pinch point, particularly in the the heat transfer surface area. Since the cooling sCO2 is close to the
low temperature recuperator where two heat exchangers are employed critical point, the large variations of the thermodynamic properties
to optimise the capital and operating costs. The rapid decrease in the can significantly affect predictions of the heat transfer and pressure
temperature difference between the exchanging fluids, due the large drop. Therefore, the traditional Dittus–Boelter correlation [131] and

10
M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

Table 3
Summary of heat-transfer correlations for printed circuit heat exchangers.
Type Ref. Correlation Applicability range Fluid
( )2
(𝑓 ∕2)(𝑅𝑒−1000)𝑃 𝑟 1 1
Straight [117,118] 𝑁𝑢 = √ , 𝑓= 2300 ≤ 𝑅𝑒 ≤ 5 × 106 , Helium
1+12.7(𝑃 𝑟2∕3 −1) 𝑓 ∕2 4 1.72 log 𝑅𝑒−1.64
channel 0.5 ≤ 𝑃 𝑟 ≤ 2000
( )4 ( )3
𝑅𝑒 𝑅𝑒
[118,119] 𝑁𝑢 = 3.5239 1000 − 45.148 1000 ... 2300 ≤ 𝑅𝑒 ≤ 3100 Helium
( )2 ( )
𝑅𝑒 𝑅𝑒
+212.13 1000 − 427.45 1000 + 316.08

[120] 𝑁𝑢 = (0.01352 ± 0.0094)𝑅𝑒(0.80058±0.0921) 1200 ≤ 𝑅𝑒 ≤ 1850 Helium

𝑁𝑢 = (3.6361 × 10−4 ± 7.855 × 10−5 )𝑅𝑒(1.2804±0.0273) 1850 < 𝑅𝑒 ≤ 2900
𝑁𝑢 = (0.047516 ± 0.015662)𝑅𝑒(0.633151±0.044606) 1200 ≤ 𝑅𝑒 ≤ 1850
𝑁𝑢 = (3.680123 × 10−4 ± 1.184389 × 10−4 )𝑅𝑒(1.282182±0.042068) 1850 < 𝑅𝑒 ≤ 2900
( )0.14
[121] 𝑁𝑢 = 0.7203𝑅𝑒0.1775 𝑃 𝑟1∕3 𝜇∕𝜇w , 𝑓 = 1.3383𝑅𝑒−0.5003 100 < 𝑅𝑒 ≤ 850 Water
Zigzag [122] ℎhot = 2.52𝑅𝑒0.681 2800 < 𝑅𝑒 < 5800 CO2
channel ℎcold = 5.49𝑅𝑒0.625 6200 < 𝑅𝑒 < 12, 100
( )
𝑓hot = (−1.402 × ±0.087) × 10−6 𝑅𝑒+(0.04495±0.00038) 2800 < 𝑅𝑒 < 5800
( )
𝑓cold = (−1.545 ± 0.099) × 10−6 𝑅𝑒+(0.09318±0.0009) 6200 < 𝑅𝑒 < 12, 100
[123] 𝑁𝑢 = (0.1696 ± 0.0144) 𝑅𝑒0.629±0.009 𝑃 𝑟0.317±0.014 2500 < 𝑅𝑒 < 2.2 × 104 , 0.8 < 𝑃 𝑟 < 2.2 CO2
𝑓 = (0.1924 ± 0.0299) 𝑅𝑒−0.091±0.016 3500 < 𝑅𝑒 < 2.2 × 104
[124] 𝑁𝑢 = 3.255 + 0.00729(𝑅𝑒 − 350) 350 < 𝑅𝑒 < 800, 𝑃 𝑟 = 0.66 Helium
𝑓 𝑅𝑒 = 16.51 + 0.1627𝑅𝑒 350 < 𝑅𝑒 < 1200
𝑁𝑢 = 4.089 + 0.00365𝑅𝑒𝑃 𝑟0.58 𝑅𝑒 < 2500
𝑓 𝑅𝑒 = 15.78 + 0.004868𝑅𝑒0.8416 − (10.939 − 11.014𝜈𝑠 ∕𝜈) 𝑅𝑒 < 2500
[125] 𝑁𝑢 = (0.0291 ± 0.0015)𝑅𝑒0.8138±0.005 , 𝜃 = 32.5◦ , 0.7 < 𝑃 𝑟 < 1, CO2
𝑓 = (0.2515 ± 0.0097)𝑅𝑒−0.2013±0.0041 2000 < 𝑅𝑒 < 58, 000
𝑁𝑢 = (0.0188 ± 0.0032)𝑅𝑒0.8742±0.0162 , 𝜃 = 40◦ , 0.7 < 𝑃 𝑟 < 1,
𝑓 = (0.2881 ± 0.0212)𝑅𝑒−0.1322±0.0079 2000 < 𝑅𝑒 < 55, 000
[126] 𝑁𝑢 = (0.05515 ± 0.00160)𝑅𝑒0.69195±0.00559 1400 ≤ 𝑅𝑒 ≤ 2200 Helium
𝑁𝑢 = (0.09221 ± 0.01397)𝑅𝑒0.62507±0.01949 2200 < 𝑅𝑒 ≤ 3558
𝑓 = 17.639𝑅𝑒−(0.8861±0.0017) 1400 ≤ 𝑅𝑒 ≤ 2200
𝑓 = 0.019044 ± 0.001692 2200 < 𝑅𝑒 ≤ 3558
[127] 𝑁𝑢 = 5.05(0.02𝜃 + 0.003)𝑅𝑒𝑃 𝑟0.6 5◦ ≤ 𝜃 ≤ 15◦ , 𝑃 𝑟 ≤ 1, Helium
200 ≤ 𝑅𝑒 ≤ 550, 4.1 ≤ 𝑙R ∕𝐷h ≤ 12.3
𝑁𝑢h = (0.71𝜃 + 0.289)(𝑙R ∕𝐷)−0.087 ... 15◦ < 𝜃 ≤ 45◦ ,
2
×𝑅𝑒(−0.11(𝜃−0.55) −0.004(𝑙R ∕𝐷)𝜃+0.54) 𝑃 𝑟0.56 , 550 < 𝑅𝑒 ≤ 2000,
𝑁𝑢c = (0.18𝜃 + 0.457)(𝑙R ∕𝐷)−0.038 ... 𝑃 𝑟 ≤ 1,
2
×𝑅𝑒(−0.23(𝜃−0.74) −0.004(𝑙R ∕𝐷)𝜃+0.56) 𝑃 𝑟0.58 4.1 ≤ 𝑙R ∕𝐷h ≤ 12.3
15.78
𝑓app = 𝑅𝑒
+ 0.0067268 exp(6.6705𝜃)(𝑙R ∕𝐷)−2.3833𝜃+0.26648 ... 5◦ ≤ 𝜃 ≤ 45◦ ,
+0.043551𝜃 − 0.010814 50 ≤ 𝑅𝑒 ≤ 2000,
(sharp-edged zigzag channels) 4.09 ≤ 𝑙R ∕𝐷h ≤ 32.73
𝑓app = 15.78
𝑅𝑒
+ 0.029311 exp(1.9216𝜃)(𝑙R ∕𝐷)−0.8261𝜃+0.031254 ... 5◦ ≤ 𝜃 ≤ 45◦ ,
+0.047659𝜃 − 0.028674 50 ≤ 𝑅𝑒 ≤ 2000,
(round-edged zigzag channels) 4.09 ≤ 𝑙R ∕𝐷h ≤ 32.73
S-shape [123] 𝑁𝑢 = (0.1740 ± 0.0118)𝑅𝑒(0.593±0.007) 𝑃 𝑟(0.430±0.014) , 3500 < 𝑅𝑒 < 2.3 × 104 , CO2
fins 𝑓 = (0.4545 ± 0.0405)𝑅𝑒(−0.340±0.009) 0.75 < 𝑃 𝑟 < 2.2
[128] 𝑁𝑢h = 0.207𝑅𝑒0.627 𝑃 𝑟0.340 1500 < 𝑅𝑒 < 1.5 × 104 , 1 < 𝑃 𝑟 < 3 CO2 ,
𝑁𝑢c = 0.253𝑅𝑒0.597 𝑃 𝑟0.349 100 < 𝑅𝑒 < 1500, 2 < 𝑃 𝑟 < 11 Water
Airfoil [129] 𝑁𝑢 = 3.7 + 0.0013𝑅𝑒0.78 𝑃 𝑟0.38 𝑅𝑒 < 2500, 0.6 < 𝑃 𝑟 < 0.8 CO2
fins 𝑁𝑢 = 0.027𝑅𝑒0 .78𝑃 𝑟0.4 3 × 103 < 𝑅𝑒 < 1.5 × 105 ,
0.6 < 𝑃 𝑟 < 0.8
𝑓 𝑅𝑒 = 0.931 + 0.028𝑅𝑒0.86 𝑅𝑒 < 1.5 × 105
( )−0.17 ( )−0.248 (
𝑤
)−0.187
𝑝 𝑤f −0.19 𝑙 f
[130] 𝑗 = 0.026 𝑙 f 𝑙
𝑅𝑒 v 8000 < 𝑅𝑒 < 105 CO2
( f )−0.252 (v )−0.255 (
𝑤
)−0.274
𝑝 𝑤f −0.173 𝑙 f
𝑓 = 0.357 𝑙 f 𝑙
𝑅𝑒 v 8000 < 𝑅𝑒 < 105
f v

𝐷h : hydraulic diameter, m; 𝑓 : friction factor; ℎ: heat transfer coefficient, W∕(m2 K); 𝑗: Colburn factor; 𝑙: length, m; 𝑙R : relative length, m; 𝑙v : transverse pitch, m; 𝑅𝑒: Reynolds
number; 𝑁𝑢: Nusselt number; 𝑝: pitch, m; 𝑝f : longitudinal pitch, m; 𝑃 𝑟: Prandtl number; 𝑤: width, m; 𝑤f : width of internal channel, m; 𝜃: fin angle; 𝜈: kinematic viscosity, m2 ∕s.
Subscripts: in, inlet; c, cold; h, hot; f: fin; in, inlet; min: minimum.

Gnielinski correlation [117], which are not able to account for the two orders of magnitude lower heat transfer coefficient of the cooling
property difference between the wall and bulk fluid temperatures, are air than the sCO2 . The temperature drop of the sCO2 mostly takes
not accurate enough to predict the heat transfer coefficient for the place at the very small part of the heat exchanger [135]. The much
sCO2 . Jackson [132] summarised convective heat transfer correlations lower density and specific heat capacity of the cooling air compared
developed for fluids at supercritical pressure and compared them with to the sCO2 also mean the cooler requires extremely high air mass
approximately 2000 different experimental conditions. The correlation flow rates and larger heat transfer surfaces. To address these problems,
of Krasnoshchekov [133] showed the best performance with 98% of the microchannel-plate fin heat exchanger as shown in Fig. 10 can be
the sCO2 data within 25% difference. Chai and Tassou [134] also considered for the CO2 cooler design. The microchannel-plate fin cooler
compared their sCO2 results with six heat transfer correlations and the has the potential to achieve better heat transfer performance than the
Krasnoshchekov correlation again showed the best prediction. A major finned-tube one and significantly reduce the overall heat exchanger
problem with air-coupled coolers is the pinch point, due to the one or size.

11
M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

of the power plant [160]. The sCO2 environment strongly influences

the evolution of ionic solubility and dissociation processes and further
affects the performance of corrosion, oxidation and creep resistance of
the material [161].
A brief summary of studies investigating material performance with
moderate to high temperature sCO2 is provided in Table 4. Within this
the selected materials, test conditions, test characteristics and main
findings are presented. However, most of these studies are limited to
low sCO2 velocity laboratory conditions, and are not the representative
of the high-velocity operating conditions expected within practical
sCO2 power systems.
Key components concerning material selection are the heater, the
recuperator, and the turbine. The heater involves either direct heating
through the mixing of the heat carrier fluid with the sCO2 or indirect
Fig. 10. Microchannel heat exchanger (courtesy of Danfoss). heat transfer process between the different fluids, all of which operate
at high temperature and high pressure and expose a large surface area
to the heat source. A major consideration in heater design is material
In water-coupled CO2 coolers, the pinch point issue is not as pro- strength and durability. The thick-walled corrosion-resistant tubing
nounced due to comparable density and specific heat capacity between should withstand rapid start-ups and transients, and should guarantee
water and sCO2 . Plate heat exchangers have been employed in water- long thermal cycling and fatigue life under temperature and pressure
coupled cooler applications. The corrugated plates create channels containments of both sCO2 and the heat source.
with increased flow turbulence and extended heat transfer area, which The recuperator operates under high temperature, high pressure
improves heat transfer performance and minimises the fouling risk. and significant pressure differentials between the exchanging fluids
Some review papers have been published to underline this application. which requires considerable material resistance to creep and corrosion
Ayub [136] summarised the single-phase correlations available for for long service duration without any structural degradation [112].
the design and analysis of plate heat exchangers. Abu-Khader [137] To satisfy these requirements, most sCO2 test facilities have employed
presented advances in plate heat exchanger design while Elmaaty PCHEs for heat recuperation (see Section 4.2.2). PCHEs are manu-
et al. [138] reviewed plate structures and heat transfer mechanisms. factured by photochemically machining the flow passages into flat
It should be noted that there is no generally accepted heat trans- plates followed by stacking the plates together which are then diffusion
fer correlation for sCO2 flowing between corrugated plates. Chai and bonded. As noted by Li et al. [167], the material challenge for these
Tassou [110] modified the correlation proposed by Wanniarachchi heat exchangers is to withstand exposure to high temperatures and
et al. [139] with a function developed by Krasnoshchekov [133] to pressures for up to 60 years within a very corrosive environment
account for the sCO2 working close to the near-critical region in plate involving possible oxidation, carburisation or decarburisation. Material
heat exchangers. Alongside plate heat exchangers, some newly pro- selection also demands a balance between heat conduction performance
posed technologies employed in CO2 heat-pump water heaters can be and capital cost.
considered for sCO2 cooler applications. These include fluted tube-in- The operating conditions within the turbine bring about both high
tube [140], serpentine microchannel [141] and multi-twisted-tube heat temperature and pressure drops from inlet to outlet. The concern for
exchangers [142]. They have been shown to be capable of operating at material selection relates to the thermal expansion during operation,
high pressures, have a high overall heat transfer coefficient and good the mechanical interference during startup and shutdown, the tempera-
temperature matching between water and the sCO2 in counter-flow
ture limits of the seals and bearings, corrosion and other design issues.
arrangements.
As mentioned by Wright et al. [183], sCO2 pressures up to 300 bar
The significant variation of sCO2 thermophysical properties in the
and the higher density (compared to steam) can significantly influence
near-critical-point region differentiates the heat transfer and flow mech-
the loading on the turbine rotor blades and necessitate higher strength
anism from conventional fluids. Particularly, the abrupt increase of spe-
alloys. The alloy choice is determined by the required strength at peak
cific heat greatly increases the heat-transfer coefficient which reaches
temperature, which depends on the structural design of the turbine
its peak at the pseudocritical temperature [143]. The heat transfer and
blades and whether they are cooled. Additionally, flexible materials are
flow mechanism are also influenced by geometrical and operational
usually used for seals in the turbomachinery. The sCO2 can dissolve into
parameters, including channel shape and dimension, flow direction,
these materials at high pressure and cause rapid gas decompression,
mass flux, heat flux, inlet temperature and pressure, and heating or
leading to unique challenges for seal design.
cooling conditions etc. [143–151]. Due to the density change, buoyancy
To keep material costs low, various classes of structural alloys, in-
also significantly affects the heat transfer and flow mechanism for all
flow orientations at Reynolds numbers up to 105 [144,145,152–155]. cluding low-alloyed steels, austenitic steels, Nickel alloys, and Titanium
Heat transfer can also be significantly impaired by flow acceleration, es- alloys, can be employed in a power cycle for different equipment and
pecially for high-heat-flux conditions and in mini/micro channels [156, component design. Generally, at operating temperatures lower than
157]. Several authors have developed empirical correlations for specific 650 ◦ C, traditional stainless steel can be employed, while for operating
geometries; however, most have been developed for a given range of temperatures higher than 650 ◦ C, nickel-based or even titanium-based
temperature, pressure, heat flux, and flow characteristics. Comparisons alloys would be required to reduce oxidation from the sCO2 envi-
of various correlations for sCO2 heat transfer showed that several ronment and achieve pressure containment without excessive material
correlations can be used for preliminary estimation of heat transfer in thicknesses, but at much higher capital cost [164].
tubes, but no single correlation is able to accurately describe local heat
transfer for different channel geometries [158,159]. 4.4. Control systems

4.3. Material considerations Power plants based on sCO2 technology are widely regarded as
flexible systems thanks to their capability to operate efficiently both
Material selection for sCO2 systems is dictated by the mechanical at full and part load, and to quickly adapt to large variations in the
and thermal properties of the material, compatibility with the high- operating conditions and at high ramp rates [184]. In the short to
temperature and pressure CO2 environment, and the fabrication cost medium term, this is a key requirement for base-load power systems

12
M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

Table 4
Representative studies of material selection for sCO2 power system.
Reference Materials Test conditions Test characteristics Remarks
Maziasz et al. 347SS and Alloys 120, 650–800 ◦ C Creep strength 347SS cannot be used as temperature exceeds 650 ◦ C;
[162–164] 214, 230, 625, 740, Alloys 214, 625, HR120 and AL20-25+Nb can have very
803, HR120 and good properties for high temperatures.
AL20-25+Nb
Osman et al. [165] 347SS 700 ◦ C/54 and 221 Creep rupture Thin foil specimens of 347SS had higher creep rates and
MPa rupture ductility than their bulk specimen counterparts.
Evans et al. [166] Alloy 625 750 ◦ C/100 MPa Creep rupture Alloy 625 is an attractive potential alloy for use in the
high temperature heat exchanger.
Li et al. [167] Alloys 800H, HX, 230 900 ◦ C Creep strength Alloy 617 is the leading candidate material for the high
and 617 temperature heat exchanger.
Klower et al. [168] Alloy 617 700 ◦ C Creep strength Alloy 617 can be a candidate material for 700 ◦ C power
plants.
Anderson et al. 347SS, NF616, 650 ◦ C/3925 psi Corrosion Cr and Al had profound influence on imparting corrosion
[169,170] HCM12A and Alloy resistance.
800H
Cao et al. [171] 316SS, 310SS and Alloy 650 ◦ C/200 bar Corrosion Alloy 800H exhibited the best corrosion resistance,
800H followed by 310SS and 316SS.
Firouzdor et al. AL-6XN, Alloy PE-16, 650 ◦ C/200 bar Corrosion Cr2 O3 oxide layers protect the Haynes 230 and Alloy 625
[172] Haynes 230 and Alloy from further corrosion.
625
Lee et al. [173,174] Alloys 800HT, 600 and 550, 600 and Corrosion and carburisation The 𝛼-alumina layer results in superior carburisation
690 650 ◦ C/200 bar resistance.
Rouillard et al. T91, 316L, 253MA® 550 ◦ C/250 bar Corrosion Alloy 800 were much more corrosion-resistant than T91.
[175] and Alloy 800
Holcomb et al. 347H, Alloys 625 and 730 ◦ C/207 bar Oxidation and corrosion Little effect of pressure on the oxidation behaviour of
[176] 282 Alloys 625 and 282; Austenitic stainless steels would be
more cost effective for long term use in sCO2 power
system.
Adam et al. [177] Alloy 800H 650 and 750 ◦ C/200 Corrosion sCO2 resulted in a higher density of carbides beneath the
bar oxide scale than the air.
Gui et al. [178] T91, VM12, Super 650 ◦ C/150 bar Oxidation Super 304H and Sanicro 25 showed enhanced corrosion
304H, and Sanicro 25 resistance due to the chromia-rich oxide scales formed on
them.
Liang et al. [179] T91, TP347HFG and 650 ◦ C/150 bar Oxidation TP347HFG and 617 showed enhanced their corrosion
Alloy 617 resistance due to the chromia-rich oxide scales formed on
them.
Pint et al. [180] Fe- and Ni-based alloys 750 ◦ C/300 bar Oxidation Pressure had a limited effect on oxide thickness and
internal oxidation and reaction products.
Bidabadi et al. Alloy F91 550 ◦ C/100 bar Oxidation Pressure affects the increase rate of carbon concentration
[181] at the oxide–alloy interface.
Kim et al. [182] 316H and Alloy 800HT 600 ◦ C/200 bar Corrosion 316H and Alloy 800HT exhibited reduced elongation at
fracture after sCO2 exposure; Alloy 800HT shows much
greater ductility reduction and brittle failure at the
bond-line.

given the increasing penetration of renewable, yet intermittent, energy approach is justified by the timescale of any dynamics within turboma-
sources in the global energy mix. As such, a large body of research chinery, which is significantly shorter than that of heat exchangers. As
has focused on the development of control strategies to address the such, performance maps merely act as look-up tables to provide the
part-load operation and the transient behaviour of sCO2 systems during boundary conditions to the equipment upstream and downstream. To
startup and shutdown. In this framework, the use of dynamic modelling account for the time variation of the revolution speed imposed by a
approaches have facilitated the understanding of the transient perfor- change of load at the generator, some studies further considered the
mance of sCO2 power equipment and systems prior to the development turbomachinery rotordynamic aspects [200]. Unlike the conventional
of the physical controls. The dynamic modelling of sCO2 power systems map-based approaches which rely on a single non-dimensional map to
describe the whole operation of the turbomachine, the strong real gas
has been carried out through low order models (zero and one di-
effects of CO2 in close proximity to the critical point of CO2 require
mensional) implemented either in ad-hoc tools (ANL’s Plant Dynamics
multi-dimensional maps at different inlet conditions [187]. This holds
Code [185,186], MIT’s SCPS and TSCYCO [187]) or in commercial ones
especially for compressors working in the critical region. Alternatively,
such as Modelica/Dymola® [188–191], GT-SUITE® [192,193], ASPEN
dimensionless turbomachinery performance maps, expressed in terms
PLUS® [194,195], and Apros® [196]. of flow and load coefficients, may be employed to overcome this
Compressors and turbines are typically modelled using a map- issue [201,202].
based approach [67] in which the inputs are either from ad-hoc Heat exchangers are typically modelled with a transient one-
models (mean-line codes [197], 3D CFD [193]) or from experimental dimensional formulation of the conservation equations underpinned
data [198]. On the other hand, the off-design performance of turboma- by semi-empirical correlations for the heat transfer coefficient and
chinery can be scaled using similarity theory [191,199]. The map-based friction factor, listed in Section 4.2 [187,203]. The fine discretisation

13
M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

in which the fluid density is less sensitive to pressure changes (e.g. from
32 ◦ C to 34 ◦ C). In this way, the CO2 inventory withdrawal required
for operation at part-load can take place without introducing significant
instabilities to the system whilst still maintaining operation outside the
two-phase region [209]. However, since with no inventory control the
CO2 mass in the cycle is constant, an isobaric change of compressor
inlet conditions is only possible if the turbine inlet temperature is
regulated at the same time as compressor inlet temperature. In the case
analysed in [209], this was achieved through a reduction of the nuclear
reactor power and the sodium mass flow rate, i.e. the heat input to the
sCO2 power block.
In the recompression cycle, the flow split ratio between the two
compressors (Fig. 11c) was considered to maintain compressor surge
Fig. 11. Summary of control strategies reported on a recompressed cycle layout: (a) margin [208]. In the simple recuperated cycle, the same control func-
heat sink flow rate, (b) cooler bypass, (c) compressor flow split, (d) compressor bypass,
tionality can be achieved through a bypass of flow from downstream
(e) turbine bypass, (f) turbine throttling, (g) turbine speed, (h) single tank inventory
control, (i) dual tank inventory control. of the compressor to upstream of the cooler (Fig. 11d) [67]. The
control of the recompressor flow showed marginal benefits when used
for load regulation compared to the inventory control. However, the
compressor flow control resulted in optimal load tracking with lower
required to resolve the non-linearities of heat transfer phenomena
control complexity [208].
in sCO2 applications results in high computational costs for dynamic
simulations which goes against the requirements for an appropriate The control of the turbine inlet temperature has been addressed
control system. To overcome this shortcoming, a correction factor for through multiple approaches: turbine flow bypass, turbine flow throt-
the logarithmic mean temperature difference (LMTD) coupled with an tling, inventory control, speed control if the power turbine is on an
iterative pressure drop calculation has been proposed to accelerate the independent shaft and/or if the generator is a synchronous permanent
off-design simulations in PCHE up to 350 times and with less than magnet (Fig. 11g) [204]. The turbine flow bypass (Fig. 11e) reduces the
5% deviation from the one-dimensional performance results [204]. In mass flow rate expanding in the turbine and was found to be a suitable
addition to this, non-model based adaptive control approaches, such as strategy for fast transients and for load variations between 90% and
the extremum seeking one, are being explored. This method considers 100% of the design value.
the sCO2 system as a black box and relies on continuous measurements Inventory control implies a change of the CO2 charge between the
of the plant performance, e.g. turbine inlet pressure and temperature power loop and the storage tanks. This approach was found to be the
as well as net power output [205]. most efficient strategy to maximise the cycle efficiency for operation
In addition to the specifics that are dictated by the heat source, between 50% and 90% of the design point as well as to increase
such as reactor core cooling in nuclear applications, in a sCO2 plant the the load by up to 110% of the nominal value [187]. The stability
purpose of the control system is to ensure an efficient operation of the implications due to the withdrawals/additions of CO2 were identified
power block without exceeding safety and operational limits. Typical as one of the major drawbacks of inventory control. Moreover, this
limitations that apply both at part-load and transient conditions are: approach usually suffers from a slow response rate compared to the
compressor stall/choking/two-phase operation, turbine choking, ex- turbine bypass control. In fact, the filling and emptying processes are
treme CO2 pressures and temperatures, extreme shaft rotational speeds, often driven by pressure gradients in the loop rather than through
cooling water exit temperature not exceeding calcification threshold the ancillaries. As such, the key limiting factor in inventory control
etc. [187]. Control systems should also aim at meeting the load de- lies in the finite capacity of the storage tanks, whose estimation can
mand and ramp rate, maintaining cycle efficiency and rejecting process be preliminary carried out through the approach proposed by Bitsch
disturbances such as heat input availability [195]. and Chaboseau in [210] and recalled in [211]. The location of the
Control strategies have mainly been investigated with reference
inventory storage tanks significantly affects the transient behaviour
to the recompression and simple recuperated cycle layouts given the
of the system due to the possible temporary mismatch between the
cost and efficiency advantages recalled in Section 3 as well as the
compressor and turbine mass flow rates. The most common layout
availability of experimental data. The controllers implemented have
considers the withdrawal point downstream of the compressor and the
mostly been proportional-integral ones (PI) since they provide zero
feeding point upstream of the cooler (Fig. 11h). This layout is suitable
error at the steady state and are insensitive to the higher-frequency
and responsive for load reductions but suffers from a response in the
terms of the inputs such as interferences [197,206]. However, the
case of increased load demand. A fast supply of CO2 from the storage
derivative term in the PID controllers set-up may also be tuned through
tanks upstream of the compressor would lead to a sudden increase in
the Cohen–Coon technique [207]. The operational stability of sCO2
power systems has mainly been addressed in terms of control of the compression power for the same turbine power which may, in the worst
compressor inlet temperature and pressure balance at the junction situations, cause a shutdown of the power system. To overcome the
points in the case of recompression cycles [208]. On the other hand, slow response rate at increasing loads, inventory control layouts with
the sCO2 performance at part-load were primarily controlled through removal and feeding points downstream of the compressor and two
the turbine inlet temperature [194]. A summary of control strategies storage systems were also considered (Fig. 11i). In this arrangement,
is reported in Fig. 11; the scheme refers to a recompressed cycle as an the turbine power is always greater than the compressor power [211].
example and does not relate to a specific application. However, an ancillary system is required to boost the inventory stored
The control of the compressor inlet temperature may be achieved by in the high-pressure cylinder beyond the compressor outlet pressure.
acting on the heat sink at the cooler. In particular, the combined control Together with inventory control, the inertia of the sCO2 power system
of the heat sink flow rate (Fig. 11a) and the cooler bypass (Fig. 11b) may also be tuned with respect to the volume ratio between hot
can result in a constant temperature at the main compressor inlet, and cold sides of the loop. A smaller hot-to-cold side volume-ratio
avoiding the two-phase region [187]. At part-load conditions, before leads to faster startup while large ratios are advisable for continuous
using inventory control, the low temperature control may be employed load requirements and fluctuating heat source and sink conditions
to isobarically increase the compressor inlet temperature, up to a region [189].

14
M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

The turbine flow throttling (Fig. 11f) was envisaged for part-load
operation between 50% and 20% of the nominal power. This approach
requires the coupling of turbine throttling with the flow split control in
recompression cycles to ensure the operation of the compressors within
their acceptable ranges. Further load reductions can be accomplished
using also the turbine bypass [187].
Besides theoretical knowledge, the outcomes of transient and con-
trol studies have been exploited to develop the controls for the existing
experimental facilities. Even though each test rig has its own unique
features, based on the operational experience reported, similar proce-
dures were identified. The filling process of sCO2 loops was carried
out with high-purity carbon dioxide (e.g. 99.995%), after vacuuming
the loop to remove moisture and non-condensable gases. Similarly to
CO2 refrigeration systems, gaseous CO2 should be employed during the
initial charging to overcome the CO2 triple point (5.17 bar, -56.60 ◦ C)
and prevent the formation of dry ice. Afterwards, liquid CO2 allows
faster charging rates until the saturation pressure at ambient conditions
is achieved [212]. From this state, a small quantity heat is supplied
to the loop, until the whole CO2 vaporises and reaches supercritical
conditions throughout the circuit (e.g. 85 bar, 38 ◦ C [66]). The heat Fig. 12. Overview of sCO2 power applications. Source: Elaboration from [217].
input during the startup phase may be supplied from different locations
of the loop and may also involve a temporary shutdown of the heat
rejection system [190]. At the same time, ancillary devices, such as 5. Applications
CO2 centrifugal pumps or air driven reciprocating gas boosters, are
used to circulate a minimal flow rate to ensure the absence of two- Supercritical CO2 technology offers a broad potential for power
phase spots. At the end of the start-up phase, the turbomachinery can generation and propulsion. An attempt to summarise the operating
be switched on and the heat load increased. While the turbomachinery ranges and sizes envisaged for the main application areas is reported
speed ramps-up fast (e.g. 20 s from 0% to 100% of design speed [213]), in Fig. 12. These application areas are elaborated on in the following
the heat is supplied gradually (e.g. 50–111 K/h [66,212]) to avoid subsections, whilst a summary of the main application areas is provided
thermal stresses in the equipment and joints. From these conditions in Table 5, in which representative cycles, operating conditions and
on-wards, the control strategies discussed above are used to reach the power ratings are presented alongside thermodynamic and economic
desired test conditions [214,215]. A cooling procedure in the reverse metrics such as thermal efficiency and levelised cost of electricity
order of the start-up procedure is employed for the shutdown of the (LCOE). A market analysis conducted by Sandia National Laboratories
turbomachinery and eventually the whole power system. projects an LCOE for an sCO2 system between 44.8 and 56.1 $/MWh
for power ratings between 100 and 300 MWe [216], but these values
Regarding the largest scale systems for which control literature
are not linked to a specific application.
has been published, it is worth acknowledging the STEP and Echogen
research contributions. In fact, besides the performance aspects pre-
5.1. Fossil fuelled and waste heat to power generation
viously discussed, the architectures of these sCO2 systems include
additional loops which are paramount to fulfil functional and opera- Supercritical CO2 power systems were originally conceived to over-
tional requirements. The controls of the STEP test facility are presented come the limitations of steam power plants. Even though sCO2 power
in [195]. The full control system consists of flow, pressure and temper- concepts regained popularity in the early 2000s for nuclear applica-
ature controllers for the natural gas fuelled heat source, temperature tions, the need for operational flexibility to accommodate the increas-
controllers for the compressor and turbine inlet temperatures, pressure- ing penetration of renewable energy sources in the global energy mix,
based inventory controller with split-range philosophy to prevent jitter- the low footprint and the possibility to integrate a carbon capture
ing (open/close), and net work controller to meet the power demand. system provided further consideration of sCO2 cycles for base-load
The control system of the Echogen’s 7.3 MWe EPS100 unit is instead power generation applications.
composed of: a compressor bypass valve to control the turbocompressor Fossil fuelled sCO2 power plants are commonly classified based on
speed and, in turn, flow rate and pressure rise; a combination of the heat addition method [218]: in direct heating, the CO2 loop is
bypass and throttle valves, and load bank resistance (simulating the an open system in which the heat input is from combustion processes
grid in the study) to control the power turbine; an inventory control involving fuels and oxidants circulating together with CO2 ; in indirect
system downstream of the cooler to control the compressor inlet pres- heating, the sCO2 power block is a closed loop system where heat addi-
sure [192]. In both applications all controllers were PI ones. However, tion and rejection take place through heat exchangers. Hence, the sCO2
the Echogen’s study additionally reports the experimental validation of unit acts as a typical bottoming heat to power system, such as steam
the model-based control system. The simulations were initialised with or organic Rankine cycles which, in turn, are the main competitors of
experimental boundary conditions at steady state and then validated sCO2 technology. The bottoming cycle approach is also applicable for
waste heat recovery.
against more than eight hours of transient data. When the measured
turbocompressor speed was used as the boundary condition instead of
5.1.1. Indirect sCO2 heating — Heat to power
the set point value, a good alignment between measurements and sim-
Heat to power generation through bottoming sCO2 cycles aims at
ulation data was reported. A key open point highlighted by Echogen,
the recovery of thermal energy from the topping process to maximise
and generally applicable to any sCO2 control research, was the need to
the power output. This goal differs from maximising cycle efficiency,
also validate such models at startup and shutdown conditions, where
which is typical of applications in which heat is generated at a cost,
different control strategies from the performance-oriented ones come
into play.

15
 M.T. White et al.
Table 5
Summary of the main application areas for sCO2 power cycles and notable thermodynamic and economic modelling studies.
Application Ref. Operating conditions Advantages Challenges
Cyclea 𝑇max 𝑝max Power LHV/HHV† LCOE/COE∗
[◦ C] [bar] [MWe ] Efficiency [$/MWh]b
Fossil fuel [108] RH 620 300 1150 50.3%
(i) increased utilisation factor due to higher operational
(coal fired) RH+CCS 620 300 1000 41.4% 73.1 (i) competition with ultra-supercritical steam power plants
flexibility than steam plants; (ii) parasitic losses due to CCS
[219] RH 620 200 635 43.9%† 75.7 with CCS; (ii) technological gap for primary heater (aka
compensated by high cycle efficiency; (iii) direct cooling
sCO2 boiler) and axial turbomachinery
reduces water consumption

(oxy-fuel) [220] AL 1150 300 846 55.1% 92 (i) integrated carbon sequestration; (ii) higher efficiency than (i) technological gap for sCO2 oxy-combustor; (ii) CO2
indirect heating cycles; (iii) optimal operation at higher cycle impurities; (iii) materials corrosion and erosion; (iv) turbine
(oxy-coal) [221] AL 1204 308 606 40.6%† 122.7∗
pressure ratios than conventional sCO2 cycles; (iv) blade film cooling; (v) alternative control strategies; (vi)
operational flexibility; (v) low footprint; (vi) very attractive competition with other oxy-fuelled power generation
LCOE concepts

Waste heat [222] PH 389 238 8.6 25.9% 40 (i) competition with conventional steam and ORC systems;
(gas turbines) [223] RCRH 572 154 146 47.73% (i) suitable for high-grade (>350 ◦ C) concentrated waste heat (ii) same techno-financial barriers as other WHR
sources (>1 MWe ); (ii) high efficiency; (iii) operational technologies; (iii) need primary heater technologies
(IC engines) [224] SR+ORC 320 200 0.2
flexibility; (iv) low footprint characterised by low pressure drop, resistance to corrosion
(industrial) [38] SR 425 200 0.9c 25.1% 8.3 and fouling, modularity etc.
[225] SR 389 256 9.5 27.9%
CSP [226] RC 580 275 82 38.4% (6350)d
RCPC 580 275 71 33.2% (6510)d
16

[227]e RC 705 273 100 48.8% 59.8 (i) CSP LCOE not yet competitive with solar PV; (ii) need
(i) suitable for next generation CSP systems (> 600 ◦ C); (ii)
[39] RCPC 900 300 50 55% (5015)d suitable heat carriers for operation at elevated temperatures;
higher efficiency should reduce size and cost of collector
[228] RCPC 630 250 115 46.2% 144 (iii) CSP plants are in regions with high ambient
field; (iii) simpler, compact power block; (iv) heat exchange
[32]f RET 550 250 35 43% 135; 164 temperatures; (iv) demonstration required at appropriate
temperature profiles allow compact thermal-energy storage
700 250 35 50% 135; 164 scale
[229] RC 550 250 50 46% 110
[230] RCICPH 468 250 10 35.6
Nuclear [231] RCRH 465 250 10 43.7% 53.6 (i) good temperature match with future Gen IV SFR and LFR
(i) technology needs to be demonstrated in other
[232] RC+ORC 550 210 250 42.5% (44.6)g reactors; (ii) increased efficiency and reduced physical
applications first; (ii) need to understand interactions
[233] RC 550 250 10 36.7-44.5% 50–55 footprint should reduce LCOE; (iii) good candidate for small
between sCO2 and reactor materials (e.g., sodium, lead)
modular reactors
Geothermal [234] S 60 119 51 5% 200 Reduced pumping work compared to indirect brine systems (i) more sensitive to ambient cooling conditions; (ii)
86.6 160 157 5% 120 and better performance at shallow depths and low reservoir operation close to the critical point means turbine designs
permeabilities differ to other sCO2 systems

Applied Thermal Engineering 185 (2021) 116447

a Allam (AL); carbon capture system (CCS); cost of electricity (COE); higher heating value (HHV); internal combustion (IC); levelised cost of electricity (LCOE); lower heating value (LHV); organic Rankine cycle (ORC); preheated (PH);
recompression (RC); recompression with intercooling and preheating (RCICPH); recompression partially-cooled (RCPC); recompression reheating (RCRH); recuperated transcritical (RET); reheated (RH); simple (S); simple recuperated
(SR).
b
Assumed conversion rate: 1 EUR = 1.1 USD.
c
Assuming 10 kg/s waste exhaust and plate heat exchangers as gas coolers.
d
Overnight capital costs, $/kWe .
e The related SunShot programme within the U.S. targets a LCOE of 60 $/MWh with a thermal efficiency in excess of 50%.

f The related SCARABEUS project targets a LCOE of 106 $/MWh (96 EURO/MWh) with a thermal efficiency in excess of 50%.

g Total product cost based on exergoeconomic assessment

M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

i.e. nuclear or fuel combustion. With the exception of coal-fired power sectors can be found in [243–246]. Common elements to these indus-
plants, in indirect heating applications the heat stream comes as a trial processes are the large size of the application, the coexistence of
by-product of a main process. As such, if not recovered, that thermal convective and radiative mechanisms in the heat source (mostly due
energy will be otherwise wasted to the environment. For this reason, to furnaces), the uninterrupted nature of the process and the need to
highly recuperative cycles are not preferred for waste heat recovery. comply to emission trading systems regulations (e.g. European ETS).
Instead, split cycles, that divert part of the CO2 flow downstream of These aspects are in favour of a waste-heat recovery retrofit with
the compressor directly to the heater rather than to the recuperators, sCO2 systems, which unlike steam or organic Rankine cycles, would be
are more suited [28,38,222]. more compact and able to fully tackle the high temperature recovery
Bottoming heat to power cycles based on steam or organic working opportunity of these energy intensive industries.
fluids have a mature technology readiness level. A number of providers
offer commercial solutions even in the kilowatt power range. In this 5.1.2. Indirect sCO2 heating — Coal power
context, even though the technology is potentially applicable to a According to the IEA’s sustainable development scenario, coal
broader spectrum of operating conditions, sCO2 technology becomes power generation is expected to drop from 10 PWh to 2 PWh by 2040.
particularly competitive for high-grade heat sources (>350 ◦ C) and Nonetheless, coal power still contributes to 37% of today’s world elec-
large-scale applications (>0.5 MWe ). These considerations result from tricity supply and is mostly used for base-load power generation [247].
the authors’ experience in sCO2 heat to power projects and business As such, a number of initiatives are being explored to retrofit or
cases developed for European industry. The aforementioned market upgrade existing coal fired power stations with sCO2 technology, with
segment allows sCO2 to tackle high temperature sources, even be- the two key drivers being decarbonisation and flexibility. The majority
yond the operating ranges of ultra-supercritical (USC) steam power of the published research has focused on cycle analysis and thermo-
cycles (600–620 ◦ C), with favourable economies of scale and lower economic assessments, including carbon capture systems [108,219,
footprint. Potential applications for sCO2 power units are gas tur- 248]. Among these works, there is strong agreement in identifying the
bines, reciprocating internal combustion engines and energy intensive sCO2 ‘boiler’ as the technological bottleneck. Compared to a steam
industries. boiler at the same duty, a coal-fired sCO2 system has larger heat
The global gas turbine market was valued at more than $6bn in transfer surfaces due to the higher mass flow rates, lower heat transfer
2019. In March 2020, pre-covid-19 projections estimated a growth at a coefficients (3–5 kW∕m2 K [249]) and lower pressure drop require-
compound annual growth rate (CAGR) of 8.2% in the 2020–2026 pe- ments [250]. Carbon dioxide also enters the heater near supercritical
riod with the additional installed capacity of 43.7 GW worldwide [235]. conditions and exit the heater at temperatures higher than the 620
The 250–500 MW segment is considered the backbone of the gas ◦ C of USC boilers [251]. This poses challenges in material selection

turbine power generation industry [236]. Market growth drivers are and availability due to high-temperature corrosion (internal because of
the global commitment towards clean energy outlooks (gas rather than CO2 and external due to the flue gas), non-stationary load profiles and
coal, integration with renewable energy sources, upgrade or retrofit of different control strategies [184], as well as cost limitations.
existing equipment etc.), the renewed interest in shale gas and liquefied
natural gas (LNG) as well as the decommissioning of nuclear power 5.1.3. Direct sCO2 heating
plants in some regions. Globally, the Asia-Pacific region has the largest Direct heating sCO2 power cycles are open-loop internal combustion
market for gas turbines, with China and India being the hot spots for engines underpinned by variants of the Joule–Brayton cycle and in
demand [237]. Unlike reciprocating internal combustion engines, in a which the heat input typically results from an oxy-fuel combustion us-
gas turbine the waste heat is almost totally concentrated in the turbine ing either natural gas or syngas from a coal gasification process. Hence,
exhaust, whose temperature ranges between 350 ◦ C and 600 ◦ C [238]. this concept is applicable and is being developed both for gaseous
Supercritical CO2 cycles could be employed as an alternative to current and solid fossil fuels. The sCO2 heater found within indirect cycles is
bottoming steam sections in greenfield or retrofitted combined cycle replaced by a combustor in which the heat of the oxy-combustion is
gas turbine (CCGT) power plants. diluted with CO2 entering the combustor after a regenerative heating
The global generator set (or genset) market was valued at over at temperatures around 750 ◦ C. The direct heating not only allows
$18bn in 2019 and, before covid-19 (April 2019), it was anticipated higher cycle temperatures than indirect sCO2 cycles but can also deliver
to grow at a CAGR of 6% by 2030. The segment above 750 kVA is optimal performance at higher pressure ratios. As a result, at the
considered the backbone of the genset industry. The main market driver turbine inlet (200+ bar and 1100+ ◦ C) the working fluid has a higher
for gensets is the need for continuous and uninterrupted power supply, power density compared to indirect heating cycles.
especially for data centres and those industries impacted by the digital Besides the theoretical efficiency advantages resulting from high
revolution [239]. Genset power units are composed of an electrical turbine inlet temperature and cycle pressure ratio, the most interesting
generator driven by a reciprocating internal combustion engine which aspect of these advanced cycle architectures is the capability to perform
can be fuelled with natural gas, diesel, gasoline, biofuels etc. Out of the a carbon sequestration together with power conversion (i.e. without
fuel energy input, nearly one third is wasted through the engine exhaust additional equipment). This is achieved thanks to the following oper-
at temperatures ranging from 350 ◦ C up to 670 ◦ C [240]. Supercritical ational features: the oxy-combustion products are primarily CO2 and
CO2 power cycles could be employed as an alternative to the current water; water can be separated downstream of the cooler; the max-
bottoming ORC sections. However, given the size of the reciprocating imum cycle pressure exceeds conventional CO2 pipelines (110–150
internal combustion engines, the power output is expected to be below bar [220]). Hence, the CO2 generated as a product of combustion can
1 MWe . This aspect, together with a possible low capacity factor, poses be separated downstream of the compressor such that the CO2 flow at
serious challenges to the economic feasibility of the sCO2 retrofit. the inlet of the high-pressure side of the recuperator is constant, even
Recent top-down estimations of the waste-heat potential in industry if the CO2 loop is open. For this reason, this family of cycles are also
indicate that the share of primary energy consumption wasted, as referred to as semi-closed.
exhausts or effluents, that is above 300 ◦ C is 11.4% (3367 TWh in A breakthrough in the field of direct heating sCO2 fossil power
absolute terms) of the total supply at world level and 8.7% (275 TWh) generation was the invention of the Allam–Fetvedt cycle, commonly
at European level [241,242]. Among the industrial sectors reviewed, referred to as the Allam cycle. The concept was patented in 2011 [252]
the ones in which high grade waste heat has the highest share of and is being up-scaled to full-scale demonstration through initiatives
primary energy consumption are iron and steel, non-ferrous metals led, partly or fully, by 8 Rivers Capital both for natural gas (50MWth
(i.e. aluminium) and non-metallic minerals (i.e. glass and cement). NET Power’s plant in La Porte, Texas (US) [253]) and coal-fired (300
Detailed assessments on waste-heat recovery opportunities in these MWe ‘Allam Cycle Zero Emission Coal Power’ project co-funded by US

17
M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

DOE [254]) power generation. The Allam cycle is a simple regenera- identify optimal cycle layouts and operating conditions for CSP ap-
tive, semi-closed sCO2 cycle in which the imbalance between residual plications. Neises & Turchi [268] compared simple recuperated, re-
enthalpy at the low pressure turbine exhaust and the heat required to compression and partially-cooled cycles for CSP applications in terms
raise the temperature of the high pressure CO2 flow prior to combustion of thermodynamic performance. Later the same authors considered
is compensated through an external heat addition at the recuperator. the economic performance and concluded that the partially-cooled
This external regenerative heat is provided either by the air separation cycle had a LCOE that was 6.2% lower than either the simple or
unit (ASU) in the gas-fired version of the cycle or by the coal gasifier recompression cycle [228]. Binotti et al. [270] compared cycles for
in the coal-fired Allam cycle [73,255]. Since the invention of the Allam a CSP application with temperatures up to 800 ◦ C and found that a
cycle, several investigations have been carried out in the field of direct recompression cycle with main compression inter-cooling outperformed
heating sCO2 power cycles. either the recompression cycle or partially-cooled cycles. Finally, Crespi
Alongside thermodynamic and techno-economic studies [221,256, et al. [39] assessed the overnight capital costs of sCO2 for CSP ap-
257], a large body or research is currently aiming to tackle the know- plications, with their conclusions suggesting that the partially-cooled
how and technological challenges introduced by the direct heating cycle is a promising candidate when both thermodynamic performance
of CO2 as well as the more ambitious operating conditions which and capital costs are considered. Their results also suggest that very
characterise these cycles: CO2 impurities due to fuel, nitrogen, water; complex cycles may be unsuitable, even though they have high thermal
corrosion aspects in coal gasification; CO2 corrosion at high and low efficiencies. Ultimately, these studies point towards the recompression
temperatures; combustion dynamics in high density flow [258–260]; and partially-cooled cycles as most promising candidates sCO2 -CSP
turbine blade film cooling and erosion due to impurities [261]; recu- systems.
perator erosion due to impurities and stress magnitude due to high Alongside thermodynamic modelling, there is a need to demonstrate
cycle pressure ratios; alternative control strategies to turbine throt- the technology at a commercial scale. Of this, the developments under
tling [262]; materials [263]; levelised costs of electricity in comparison the SunShot programme are the most notable, which targets a LCOE of
with other oxy-fuel power generation concepts [220]. 0.06 $/kWh for CSP systems, with a thermal efficiency of 50%, and
under which a 10 MWe sCO2 turbine is being tested up to 750 ◦ C
and 250 bar under reduced flow conditions [70,91]. Another notable
5.2. Concentrated-solar power
development is the Shouhang-EDF demonstration plant in China, which
involves retrofitting a 10 MWe steam power plant with a sCO2 power
A concentrated-solar power (CSP) plant uses a mirror or lens to
block by the end of 2020. The plant employs a recompression cycle
focus the sun’s rays that fall onto a given area to a much smaller
with intercooling and preheating with an estimated net efficiency of
receiver area in order to generate heat, which is subsequently converted
35.6%, and while the current maximum turbine inlet temperature
into electricity through the power block. The key components of a CSP
is limited to 468 ◦ C, the project eventually aims to achieve higher
plant are the solar collector, the solar receiver and the power block,
temperatures [230].
although thermal-energy storage is also a key component to decouple
Finally, it is worth noting that optimal locations for CSP plants are
the availability of the sun and the demand for power. Whilst all
typically in hot and arid regions where there may be a limited avail-
components are important, emphasis here will be placed on the power
ability of water. As such, it is necessary to rely on dry cooling for the
block, for which sCO2 is a promising candidate; for a more detailed
heat-rejection process, which leads to compressor inlet temperatures
review of CSP technology readers can refer to the literature [264,265].
in the region of 50 ◦ C once the approach temperature in the cooler
Cumulatively, the global installed capacity of CSP has grown five- is considered. Whilst sCO2 cycles are suitable for dry cooling [267],
fold since 2010, and in that time has seen a drop in LCOE from this increase in compressor temperature moves the compression process
0.346 $/kWh to 0.182 $/kWh [266]. However, compared to other tech- away from the critical point, somewhat negating the low compressor
nologies CSP is still in its infancy, and is associated with higher LCOE work promised by operating with sCO2 , and may also require increased
values compared to other renewable technologies, such as solar PV compressor inlet pressures to maximise efficiency [271]. To this end,
(0.068 $/kWh) [266]. Much of the drop in LCOE for CSP systems can CO2 -blends have been proposed to increase the critical point of the
be attributed to reductions in the costs of solar collector fields, which working fluid, not only moving the compression process closer to
represent around 40% of the total capital cost [267]. In contrast, there the critical point, but also facilitating the use of a transcritical cycle.
have not been significant developments in the power block technology, Manzolini et al. [32] studied CO2 -blends for CSP applications, and
which remains based on the steam Rankine cycle. suggest that for turbine inlet temperatures of 550 and 700 ◦ C CO2 -
The deployment of an sCO2 power block, in place of steam, could blends could increase thermal efficiency by 2% and reduce LCOE by
offer a number of benefits. Firstly, sCO2 power cycles are capable 10% compared to a conventional steam cycle.
of achieving higher thermal efficiencies at temperatures relevant to
CSP applications, which enables sCO2 to produce the same amount 5.3. Nuclear power
of energy using a smaller solar field. Moreover, the possibility for
more compact turbomachinery, a simpler cycle layout, and a smaller Nuclear power plants generate heat through a contained and con-
physical footprint could reduce both the thermal mass and complexity trolled nuclear reaction within a nuclear reactor core. The heat gen-
of the power block, which could improve the cycle’s response time erated by this reaction is then removed from the core through a
during intermittent operation, whilst the lack of phase-change within heat-transfer fluid and then converted into electricity by the power
the primary heat-addition process reduces the pinch point within the cycle. In the case of a direct power cycle, the heat-transfer fluid is
primary heater and should allow for more compact thermal-energy the working fluid of the power cycle, whilst in an indirect cycle the
storage systems [267,268]. Consequently, the use of sCO2 has the heat-transfer fluid leaving the core exchanges heat with the power cycle
potential to reduce capital, operational and maintenance costs of the working fluid through an additional heat-exchange process.
power block, enabling a significant reduction in the LCOE of CSP Currently, there are six Generation IV nuclear reactors that are
plants. This motivation is supported by the Gen3 CSP roadmap [269], being investigated for deployment within future nuclear power ap-
developed within the US SunShot programme, which maps out the plications [272]. These advanced reactors employ various means to
pathway towards the next-generation of CSP systems. cool the core, and subsequently provide heat to the power generation
Whilst space does not permit a detailed review of sCO2 -CSP sys- system. The goal of these reactors is to realise higher core outlet
tems (such a review is provided by Yin et al. [7]), thermodynamic temperatures (500 to 900 ◦ C) compared to water-cooled reactors (≈
modelling and optimisation remains an important research topic to 300 ◦ C) [3]. This increases the thermal efficiency and reduces the cost

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M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

of the power block. Each of the six Gen IV reactors provide different gas-cooled reactors [281]. Alongside this, Yu et al. [282] reports the
core outlet temperatures and heat across a limited range. The gas- design a micro modular reactor, named KAIST MMR, in which the core
cooled fast reactor, and very high-temperature reactor, provide core is directly cooled by the supercritical CO2 and the intermediate heat
outlet temperatures in excess of 850 ◦ C and employ a closed-loop exchanger is removed. The KAIST MMR is designed to have a thermal
helium Brayton cycle for the power generation cycle. However, the power of 36.2 MWth and a 20-year lifetime without refuelling. Within
high operating temperatures present challenges in material selection. Tokyo Institute of Technology there was also interest in sCO2 cycles for
For this reason, the work of Dostal [16] was conducted with the focus application within gas-cooled reactors, with the replacement of helium
of identifying power cycles that could provide comparable efficiencies with sCO2 [283] reducing temperatures down from 850 ◦ C to 650 ◦ C
at lower temperatures. The conclusion was that sCO2 cycles operating whilst retaining a similar thermal efficiency; thus echoing the findings
with a maximum temperature of 550 ◦ C could achieve comparable of Dostal [16]. In particular, a partial pre-cooling cycle with a turbine
efficiencies to a helium Brayton cycle operating at a temperature of inlet temperature of 650 ◦ C and cycle thermal efficiency of 45.8% was
850 ◦ C, making sCO2 a promising candidate for any reactor with core proposed. Later, Muto & Kato [284] investigated a dual expansion cycle
outlet temperatures in excess of 500 ◦ C. Of the remaining Gen IV for fast reactors and high-temperature gas-cooled reactors.
nuclear reactors, the sodium-cooled fast reactor (SFR) and lead-cooled Within China, Li et al. [233] proposed a conceptual design for
fast reactor (LFR) are the most promising applications for sCO2 cycles. a 10 MWe LFR, integrated with a recompression sCO2 cycle. Along-
Sienicki & Moisseytsev [273] note that both reactors provide core outlet side thermo-economic modelling, which predicted a thermal efficiency
temperatures in the region of 500 to 550 ◦ C, and that the temperature in the range of 36.7 and 44.5% and cost of electricity between 50
match between the temperature rise of the heat-transfer fluid in the and 55 $/MWh for a turbine inlet temperature of 550 ◦ C, the au-
primary heater and across the turbine are well matched. This enables thors presented a conceptual design for the reactor core, intermediate
sCO2 cycles to achieve higher efficiencies than an equivalent steam heat-transfer system and the auxiliary systems.
cycle, whilst having a smaller footprint, leading a potential reduction in Within Europe there have been a few studies focusing on sCO2
capital cost and LCOE. This is supported by Li et al. [5] who considered cycles for nuclear applications. In France, sCO2 was being considered
LFRs to be a new frontier for research. as a power cycle for the advanced sodium technological reactor for
Within the aforementioned references a detailed review of the industrial demonstration (ASTRID) project [285,286]. For the plant,
historical developments [273], and of thermodynamic and techno- which has a total thermal power of 1500 MWth , simulations suggested
economic modelling [5] of sCO2 cycles for nuclear applications can that a net plant efficiency of 42.2% could be obtained for turbine inlet
be found. This is further expanded on in the recent paper by Wu conditions of 25 MPa and 515 ◦ C [286]. However, as of the August
et al. [274]. For this reason, these topics are not covered in detail here. 2019 the ASTRID programme has been cancelled [287]. There have
Instead an overview of the main developments within a few notable also been developments within the Czech Republic. Specifically, the
research groups are summarised. SUSEN test loop has been developed, which was constructed with a
Within the US, most developments can be related to work at the view towards testing sCO2 components with applications focused on
Argonne National Laboratory, which considered sCO2 cycles for both gas-cooled fast reactors [288]. This test loop was also utilised within
sodium- and lead-cooled fast reactors. Moisseytsev & Sienicki [275] re- the sCO2 -HeRo project, which focused on the proof of concept for a
ported the design of a LFR, named the secure transportable autonomous small-scale heat removal safety back-up system for a light-water reac-
reactor with liquid metal coolant (STAR-LM), which coupled the reactor tor [289]. A final notable study explored the suitability of sCO2 power
to a sCO2 recompression cycle. At the design point, the turbine inlet cycles for the DEMO demonstration power plant, which investigated
temperature is 540 ◦ C, whilst the power cycle has a net power output of fusion applications within Europe [290]. Although, it seems steam is
179 MWe and estimated cycle thermal efficiency of 45%. Later, Sienicki still the power cycle of choice for that plant [291], recent simulations
et al. [276] reported on the design of a smaller reactor, referred to as suggest that sCO2 cycles may outperform steam providing that the
SSTAR, with a net power output of 20 MWe . The system employs a heat-source temperature is above 460 ◦ C [292].
direct heat exchange between the lead coolant and the CO2 , and with Finally, it is worth noting that most of experimental prototypes men-
a turbine inlet of 550 ◦ C could obtain a thermal cycle efficiency of 44%. tioned within this paper have been developed with nuclear applications
A major consideration during the development on SSTAR was to focus in mind; albeit with the removal of a nuclear heat source. To this end,
on a small reactor that is suitable for international deployment, which the challenge facing the implementation of sCO2 technologies within all
should allow non-fuel cycle states and developing nations to help meet nuclear applications is the successful demonstration of the technology
future energy demands in a sustainable manner [276]. This sentiment at an industrial scale, which must be obtained before nuclear power
is supported by the IEA’s 2015 nuclear technology roadmap [277], plants utilising sCO2 can be realised. Moreover, as summarised by Wu
which suggests that the development of small modular reactors could et al. [274], specific challenges relating to nuclear applications include:
extend the market for nuclear technology, whilst also helping to address (i) understanding the interaction between the sCO2 system and the
financing barriers. Thus, it could be argued that the small modular reactor coolant system; (ii) understanding the effect of the dynamic
reactor market could be a good proving ground for sCO2 technology. behaviour of the reactor core on the sCO2 cycle; and (iii) conducting a
Alongside studies relating to LFRs, Chang et al. [278] reports the pre- comprehensive safety analysis.
conceptual design of a SFR coupled to a recompression sCO2 cycle with
a net power of 95 MWe . Simulations estimate that for a turbine inlet 5.4. Other applications
temperature of 471.5 ◦ C a cycle thermal efficiency of 39.1% could be
obtained. A later study found that more complex power cycle layouts Supercritical CO2 power cycles could also find use in a range of
did not improve on this efficiency, but reducing the minimum temper- other applications. For example, there have been studies investigat-
ature to 20 ◦ C, and operating a condensation cycle, could improve the ing the use of sCO2 as a bottoming cycle for molten carbonate fuel
efficiency by up to 4%; although this would have implications for the cells [293–296]. Another promising area is geothermal. Within existing
required heat sink [34]. Finally, Sienicki & Moisseytsev [273] report geothermal plants, a brine solution is pumped deep underground where
the detailed design of a 100 MWe small modular SFR reactor with a it extracts heat from the surrounding rock and is heated up to around
turbine inlet temperature of 517 ◦ C with an estimated cycle thermal 100 to 200 ◦ C. The hot brine is then returned to the surface where it is
efficiency of 42.3%. converted to electricity by a power block, which is typically a closed-
Within the Korea Advanced Institute of Science and Technology loop organic Rankine cycle (ORC) with indirect heating between the
researchers have investigated the use of sCO2 power cycles with SFRs hot brine and the organic fluid. Therefore, sCO2 power cycles could
[279], in addition to water-cooled reactors [280] and high-temperature replace the ORC system [30,297], whilst the second, and perhaps more

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M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

interesting option, is the operation of a closed-loop direct cycle where unit, is most common, axial turbines and the use of separate
the brine is replaced with sCO2 and the hot, high-pressure CO2 leaving shafts have been proposed. Existing small-scale systems have
the well is expanded within a turbine. This direct cycle can lead to more also experienced significant challenges relating to bearing and
effective geothermal extraction, whilst due to a stronger thermosiphon windage losses, which need to be overcome.
effect the pumping power required to drive the cycle is reduced [234]. • Most experimental test rigs operate with conservative pressure ra-
Adams et al. [298] investigated the use of CO2 as the heat-carrier fluid, tios and some use commercial components to minimise costs and
and then later compared direct sCO2 cycles with indirect brine-based development risk. Once existing turbomachinery designs have
systems using either an ORC or sCO2 power block [297]. They con- been demonstrated, there remains an opportunity for further
cluded that direct sCO2 cycles produced more power at lower reservoir research and development to extend beyond the existing design
depths, significantly more power at higher reservoir depths, and that boundaries.
for indirect systems a transcritical sCO2 power block could outperform • To fully realise the performance benefits of the sCO2 cycle, op-
an ORC system operating with the refrigerant R245fa. Within the US, erating the compressor close to the critical point is desirable.
GreenFire Energy are developing the ECO2G, which is a direct closed- However, this introduces challenges related to droplet formation,
loop sCO2 system for geothermal applications [299]. Recently, they real-gas effects and unstable compressor operation, which require
demonstrated the technology as the Coso Geothermal Field, although further investigation to characterise compressor operation near
emphasis was placed on testing the heat exchanger technology operat- the critical point and introduce suitable design solutions.
ing with sCO2 , and hence an expansion valve was used in place of the • The design and simulation of sCO2 turbomachinery is reliant on
turbine [300]. Glos et al. [234] conducted a preliminary assessment of
empirical loss models and computational fluid dynamics (CFD)
closed-loop direct sCO2 cycles considering thermodynamic modelling,
tools that were not developed for sCO2 applications. Experimental
turbine design and a preliminary cost assessment. Their results suggest
tests are necessary to provide suitable validation data and, if
that LCOEs of 0.20 and 0.12 $/kWh could be obtained for 52 and
necessary, introduce modifications to existing tools.
157 MWe systems respectively for installation at brownfield sites where
there are existing wells. A final potential application could be as a
power block for use within the marine industry, either as part of 6.2. Heat exchangers
the ship propulsion system or for on-board power. Within the sCO2
technology roadmap prepared by Mendez & Rochau [217], the authors • For the heat transfer and flow mechanisms, the influence of
noted that the US Navy is interested in gas turbine-generator sets with buoyancy effects should be considered for the development of
a power rating between 20 and 30 MWe , which could be a promising empirical correlations and the design of sCO2 heat exchangers.
application for sCO2 technology. The US Navy has previously explored Further unique universal correlations are expected to cover a wide
the use of Echogen’s sCO2 system within marine applications, with the range of test parameters and demonstrate the local heat transfer
results suggesting fuel consumption could be reduced by 20% [301]. performance. Heat exchanger optimisation is also an important
field, which requires specific attention not only for the individual
6. Summary and future trends heat exchanger but also the system as a whole.
• Each application imposes unique constraints on heater design.
Power cycles operating with supercritical carbon dioxide (sCO2 ) The thermohydraulic challenges confront requirements of very
have advantages of high thermal efficiencies using heat-source tem- high heat flux, and high temperature and pressure differentials
peratures ranging between approximately 350 ◦ C and 800 ◦ C, a simple between the heat source and sCO2 . Further work is needed to de-
and compact physical footprint and good operational flexibility. These velop designs and manufacturing processes that lead to relatively
advantages make them promising candidates for future energy appli- low cost, high-performance heaters that can withstand thermal
cations where their adoption could lower levelised costs of electricity cycling and fatigue and can satisfy environmental constraints in
compared to existing technologies. The significant amount of research different applications.
on sCO2 power systems has led to multiple component development • Challenges facing the recuperator are the requirements to with-
campaigns and experimental test facilities aimed at demonstrating stand high-temperature pressure differentials and flow passage
technical feasibility and investigating component operational issues. design that maximises heat transfer performance and reduces
However, there remain significant hurdles to overcome, one of which pressure drop.
is the successful demonstration of the technology at an appropriate • For air-coupled coolers, reduction of the heat exchanger size
industrial scale. The following subsections summarise the current status can be achieved by reducing tube diameter and tube spacing.
within the context of the main areas discussed within this paper.
Challenges include enhancement of the heat transfer performance
on the air cooling side and optimisation of the tube circuitry to
6.1. Turbomachinery
alleviate pinch point problems, and minimise pressure drop and
footprint. For water-coupled coolers, important considerations are
• Turbomachinery for sCO2 applications has yet to be success-
to improve heat transfer performance, reduce pressure drop on
fully demonstrated at a sufficient level to allow commerciali-
the sCO2 side and the risk of leakage between the two fluids
sation. However, there are a number of on-going projects that
during operation.
aim to demonstrate turbomachinery at an industrial-scale (i.e., ≥
10 MWe ), although these are not yet fully operational.
• For large industrial-scale applications the required turbomachin- 6.3. Materials
ery may differ from the turbomachinery tested within existing
test rigs. Therefore, there is uncertainty in how much of the • Material selection for different applications requires a combina-
experimental work carried out to date can be readily transposed tion of strength and environmental compatibility under condi-
to full-scale plants. tions of high temperature and pressure in the sCO2 system and
• For small-scale applications (i.e., <1 MWe ), turbomachinery de- high temperature heat sources.
sign is challenging due to high rotational speeds, high pres- • More information is required on material strength and durability
sures, and the trade-off between aerodynamic, rotordynamic and in realistic high-velocity operating conditions and accounting for
mechanical performance. Although radial turbomachinery, often impurities in the working fluid to develop material databases and
constructed as single-shaft turbine–alternator–compressor (TAC) standards.

20
M.T. White et al. Applied Thermal Engineering 185 (2021) 116447

• More information is required on costs as well as on availability, Declaration of competing interest

manufacturability and fabrication issues, especially for welding
and diffusion bonding of metals for sCO2 applications. The authors declare that they have no known competing finan-
• Optimisation methods based on cost-performance for material cial interests or personal relationships that could have appeared to
selection and specific research to develop high-temperature ma- influence the work reported in this paper.
terials and coatings suitable for sCO2 environments are required
for the development and deployment of cost effective systems. Acknowledgements

6.4. Control systems The research presented in this paper has received funding from the
European Union’s Horizon 2020 research and innovation programme
Transient modelling and control are key to advanced design and under grant agreements: No. 680599 – I-ThERM and No. 814985 –
efficient operation of sCO2 power systems. They allow to fully exploit SCARABEUS. Aspects of the work are also funded by the Engineering
the load flexibility potential of the power generation system and assess and Physical Sciences Research Council (EPSRC) of the UK under
important operational aspects such as safety. State-of-the-art research research grants: EP/P004636/1 – OPTEMIN, EP/P009131/1 – Nex-
on sCO2 controls has focused on a limited number of cycle layouts tORC, EP/K011820/1 – RCUK CSEF. The manuscript reports all the
mostly regulated by proportional integral (PI) controllers in which the relevant data to support the understanding of the results. More detailed
power block is considered as a standalone item despite being part of information and data, if required, can be obtained by contacting the
a broader, interconnected energy system. The investigation and devel- corresponding author of the paper.
opment of overarching control architectures based on multi-variable
control approaches that integrate the power block with the heat source References
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