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Review Article
REVIEW OF SUPERCRITICAL CO2 POWER CYCLE TECHNOLOGY
AND CURRENT STATUS OF RESEARCH AND DEVELOPMENT
YOONHAN AHN a, SEONG JUN BAE a, MINSEOK KIM a, SEONG KUK CHO a,
SEUNGJOON BAIK a, JEONG IK LEE a,*, and JAE EUN CHA b
a
Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu,
Daejeon, 305-701, South Korea
b
Fast Reactor Technology Development Division, Korean Atomic Energy Research Institute, 305-353, DukJin-Dong 150, Yuseong-gu, Daejeon,
South Korea
article info
abstract
Article history:
The supercritical CO2 (S-CO2) Brayton cycle has recently been gaining a lot of attention for
Received 16 January 2015
application to next generation nuclear reactors. The advantages of the S-CO2 cycle are high
Received in revised form
efficiency in the mild turbine inlet temperature region and a small physical footprint with a
19 May 2015
simple layout, compact turbomachinery, and heat exchangers. Several heat sources
Accepted 6 June 2015
including nuclear, fossil fuel, waste heat, and renewable heat sources such as solar ther-
Available online 13 August 2015
mal or fuel cells are potential application areas of the S-CO2 cycle. In this paper, the current
development progress of the S-CO2 cycle is introduced. Moreover, a quick comparison of
Keywords:
Advanced power system
Integral test loop
Layout study
Research review
Supercritical carbon dioxide cycle
1.
various S-CO2 layouts is presented in terms of cycle performance.
Copyright © 2015, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society.
Introduction
Since the early 2000s, numerous countries have cooperated to
develop Generation IV (Gen IV) nuclear reactors. Sodiumcooled Fast Reactor (SFR), Lead-cooled Fast Reactor (LFR),
Gas-cooled Fast Reactor (GFR), Super-Critical Water-cooled
Reactor (SCWR), Very High Temperature gas-cooled Reactor
(VHTR), and Molten Salt Reactor (MSR) were selected as Gen IV
reactor candidates. Characteristics of Gen IV reactors vary in
neutron energy spectrum and the fluid type used for coolant.
The operating temperatures of Gen IV reactors are commonly
higher, which is ~500e900 C, compared with conventional
water-cooled reactors, which operates ~300 C.
The main reason why Gen IV reactors have high operating
temperatures is to increase the nuclear power plant efficiency
which is currently lower than fossil fuel power plants.
Increasing the reactor outlet temperature typically leads to a
higher turbine inlet temperature in the power conversion
* Corresponding author.
E-mail address: jeongiklee@kaist.ac.kr (J.I. Lee).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://
creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
http://dx.doi.org/10.1016/j.net.2015.06.009
1738-5733/Copyright © 2015, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society.
648
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systems and potentially improves thermal efficiency according to the second law of thermodynamics.
Therefore, the Gen IV reactor thermal efficiency can be
improved with an increase of reactor outlet temperature.
Furthermore, several related issues due to the low efficiency
of current nuclear power plants can be solved as well. For
example, the cooling water requirement for existing nuclear
power plants is distinctively higher compared with those of
other power plants and it is usually criticized not only from
the economic point of view but also from the view point of
environmental protection. Therefore, Gen IV reactors should
not only enhance thermal efficiency but also minimize the
influence on the environment.
To successfully utilize the high reactor outlet temperature,
interest in alternative power conversion systems is also
increasing. The steam Rankine cycle and gas turbine systems
have been utilized by large size power plants for several decades. When the turbine inlet temperature is > 550 C, the
ultra-supercritical (USC) steam cycle is required to further
improve the efficiency of a steam Rankine cycle. However, the
USC steam cycle inevitably suffers from material degradation
due to high temperature and pressure operating conditions.
Therefore, when the USC steam Rankine cycle is coupled to a
nuclear power plant, the plant reliability can be a significant
issue if the system is composed only of existing materials. As a
result, an alternative power conversion system which can
operate in the mild turbine inlet temperature region
(500e900 C) is essential to improve the next generation nuclear power plant performance and safety at the same time.
Among various candidates, the S-CO2 power cycle is considered as one of the promising alternatives to potentially provide high efficiency in the Gen IV reactor operating
temperature region, better stability with conventional structure materials, and eventually improved safety and reliability
of the power conversion system.
2.
S-CO2 power cycle
2.1.
The characteristics and benefits of the S-CO2 cycle
Fig. 1 shows the thermal efficiencies of various power conversion systems and heat sources with respect to the turbine
inlet temperature range. The representative heat sources in
Fig. 1 are geothermal energy, solar thermal energy, nuclear
energy, coal, waste heat recovery, and liquefied natural gas
(LNG). The power conversion systems in Fig. 1 are organic
Rankine cycle (ORC), steam Rankine cycle (steam turbine), air
Brayton cycle (gas turbine), combined cycle gas turbine
(CCGT), and S-CO2 direct and indirect cycles. As shown in
Fig. 2, the steam Rankine cycle can achieve high efficiency
under low turbine inlet temperature conditions because the
working fluid is compressed at a liquid state. In other words,
liquid water is incompressible and requires less work for
compression. In contrast, the gas turbine utilizes air,
compressible fluid, and a large amount of work is consumed
for the compression process. Therefore, the thermal efficiency
of gas turbines is not significantly higher than that of a steam
Rankine cycle although the turbine inlet temperature is much
higher because the compressor requires a large amount of
work. However, the material issue becomes significant at
higher turbine inlet temperatures with gas turbines.
As schematically shown in Fig. 2, the S-CO2 Brayton cycle is
the power conversion system which combines the advantages
of both steam Rankine cycle and gas turbine system. In other
words, the fluid is compressed in the incompressible region
and the higher turbine inlet temperature can be utilized with
less material issues compared with the steam Rankine cycle.
The CO2 critical condition is 30.98 C and 7.38 MPa; the fluid
becomes more incompressible near the critical point.
Z¼
P$M
r$R$T
Fig. 1 e Thermal efficiencies of power conversion systems and applications. CCGT, combined cycle gas turbine.
(1)
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Fig. 2 e Principles of power conversion system. S-CO2, supercritical CO2.
The compressibility factor, Z, is defined as the molecular
volumetric ratio of a fluid compared with ideal gas. It describes how much the fluid behaves like ideal gas. The fluid
behaves very close to an ideal gas when this factor is unity and
is considered to be an incompressible fluid when it is zero. For
CO2 near the critical point, the compressibility factor decreases to 0.2e0.5 as shown in Fig. 3, and the compression
work can be substantially decreased. In addition, as S-CO2 is
less corrosive compared with steam at the same temperature,
the S-CO2 cycle can potentially increase the turbine inlet
temperature [1,2].
One of the main advantages of the S-CO2 Brayton cycle is
its compact turbomachinery. As the system operates beyond
the critical point, the minimum pressure is higher (~7,400 kPa)
Fig. 3 e CO2 compressibility factor near the critical point
(30.98 C, 7.38 MPa).
than any existing steam Rankine cycle (a few kPa) or gas
Brayton cycle (~100 kPa), and thus the fluid remains dense
throughout the entire power system. Therefore, the volumetric flow rate decreases as the fluid density is higher,
resulting in 10 times smaller turbomachinery compared with
the turbomachinery of a steam Rankine cycle.
However, the cycle pressure ratio of the S-CO2 Brayton
cycle is much smaller compared with the steam Rankine cycle
and the turbine outlet temperature is relatively high. Therefore, a large amount of heat must be recuperated to increase
the thermal efficiency. In other words, the recuperation process in the S-CO2 Brayton cycle greatly influences the thermal
efficiency.
The most efficient layout of the S-CO2 cycle is generally
agreed to be the recompressing layout until now, which was
suggested by Feher [3] and Angelino [4] and later revitalized by
Dostal et al [5] for the next generation reactor application.
However, according to recent studies, various optimized layouts can be utilized for the S-CO2 power cycle depending on
the application [6]. This is because the S-CO2 cycle is similar to
a steam Rankine cycle in terms of layout while the S-CO2 cycle
is similar to a gas turbine system from the main component
design point of view.
One of the S-CO2 Brayton cycle characteristics is that the
specific heat of the cold side flow is two to three times
higher than that of the hot side flow in recuperators. It is
especially important for the S-CO2 cycle layout design and
also explains why the recompressing layout can have high
efficiency as shown in Figs. 4 and 5. In other words, the CO2
flow is split to compensate for the specific heat difference in
the low temperature recuperator and to maximize the heat
recuperation in the recompressing layout. Therefore, the
waste heat is reduced and thermal efficiency can be
improved.
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Fig. 4 e S-CO2 recompressing cycle layout. S-CO2,
supercritical CO2.
The operating condition is important in the S-CO2 heat
exchangers. As a large amount of heat is recovered in recuperators to increase the thermal efficiency, high effectiveness
is required and therefore the capital cost increases when
conventional Shell and Tube Heat Exchangers (STHE) are utilized. However, various compact heat exchangers with high
compactness (up to 10 times compared with STHE), such as a
Printed Circuit Heat Exchanger (PCHE), have been commercialized and can be applied to the S-CO2 cycle directly.
The benefits of the S-CO2 cycle can be summarized as the
following. (1) The thermal efficiency can be increased up to 5%
point compared with the steam Rankine cycle. (2) The turbomachinery can be much smaller and the overall system size
can be reduced up to four times compared with the conventional steam Rankine cycle. (3) The competitiveness of the dry
air cooled S-CO2 cycle has been investigated by multiple researchers [7,8]. Especially for a concentrated solar power (CSP)
application, several research programs including SunShot
(Washington, DOE, USA) and Australian Solar Thermal
Research Initiative (ASTRI, CSRISO, Action, Australia) emphasized the reduction of water consumption and competitive
performance when the complete or hybrid air cooling option is
utilized [7,9]. On the contrary, Moisseytsev and Sienicki [10]
claimed that an air cooling S-CO2 cycle would not be competitive with a water cooling S-CO2 cycle for a 400 MWe SFR
application. Moisseytsev and Sienicki [10] claimed that the size
of the air cooling heat exchanger and the overall capital cost
excessively increase. Overall, the feasibility of the air cooled SCO2 cycle is not agreed upon by researchers. Yet, the positive
potential of the air cooled S-CO2 cycle can grow as the system
design becomes more sophisticated and the component level
technology becomes more advanced. (4) As the minimum
pressure is higher than the CO2 critical pressure (7.38 MPa), the
purification system requirements are lower than those of the
steam Rankine cycle to prevent air ingress. Thus, the power
conversion system can be much simpler. In the steam cycle
case, the low pressure in the condenser causes gas ingression
and complex purification systems are required. (5) Among
various fluids, CO2 is relatively cheaper and less harmful when
an appropriate ventilation system is installed to prepare for a
sudden large release of CO2 from the power conversion system.
2.2.
S-CO2 power cycle application
As discussed above, many potential advantages exist for the
S-CO2 power cycle and it can be applied to various heat
Fig. 5 e T-s diagram of S-CO2 recompressing cycle. T, temperature; s, entropy; S-CO2, supercritical CO2.
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Fig. 6 e The comparison of steam, air, S-CO2 power conversion systems [11,12]. S-CO2, supercritical CO2.
sources. For instance, since the S-CO2 cycle can be considered
as an alternative to the steam Rankine cycle, it can be applied
to nuclear energy ranging from pressurized water reactors
(both large and small modular reactors) to the next generation
nuclear reactors and fusion reactor applications as well. Other
than nuclear energy applications, the S-CO2 power cycle can
be utilized as a topping cycle for fossil fuel powered plants and
a bottoming cycle of gas combined cycle plants. There are also
promising heat sources soon to be developed, which include
several renewable energy sources such as high temperature
fuel cells, concentrated solar power, and geothermal power. A
brief comparison of air, steam, and S-CO2 power conversion
systems and the potential application areas of the S-CO2 cycle
are shown in Figs. 6 and 7, respectively.
2.2.1.
Nuclear application
The S-CO2 power cycle is being researched for application to
sodium-cooled fast reactors [13,14]. The S-CO2 cycle can
replace a violent sodiumewater reaction with a mild
sodiumeCO2 reaction and potentially increase the safety of
the nuclear system as well as thermal efficiency. Related to
the sodium reaction, the safety of the S-CO2 cycle has been
Fig. 7 e The potential application of S-CO2 cycle. S-CO2, supercritical CO2.
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investigated in KAERI. The ignition temperature of the
NaeCO2 reaction was verified to be 595 C in terms of sodium
temperature [15]. In addition, numerical modeling is being
studied to predict the impact of a CO2 leak in a sodiumeCO2
heat exchanger and a related experiment is being designed for
the validation [16]. In contrast, the nitrogen Brayton cycle is
being studied as an alternative of the power conversion system for a sodium-cooled fast reactor to inherently eliminate
the chemical reaction between sodium and the power conversion fluid in France. At high pressure conditions, the nitrogen Brayton cycle can also achieve performance
competitive to the superheated steam Rankine cycle. The
economics of the nitrogen Brayton cycle for SFR application is
being investigated by the French Alternative Energies and
Atomic Energy Commission (CEA, Paris, France) because it can
eliminate the expensive safety systems needed to detect and
mitigate the sodium water reaction [17]. However, the nitrogen Brayton cycle can only be utilized in the sodium-cooled
fast reactor application and unfortunately other than the
nuclear application no immediate application area can be
found. This limitation in the application area may become a
substantial obstacle to establish a firm supply chain and gain
support from a wide spectrum of energy industries. In
contrast, the S-CO2 power cycle can potentially be utilized for
Small and Medium sized Reactors (SMR) such as SMART, large
size conventional water-cooled reactors, and fusion reactor
applications as well as other energy sources such as coal,
natural gas, and renewable energies [18e21].
2.2.2.
Coal power application
The S-CO2 cycle is also considered to be a promising candidate
for the coal-fired power plant topping cycle to improve thermal efficiency. Various power plant vendors and operators
including Pratt Whitney & Rocketdyne (PWR, California, USA)
and Electricite De France (EDF, Paris, France) are studying SCO2 cycle design for application to coal power plants [22,23].
This innovative layout can achieve competitive efficiency
compared with the conventional power conversion system as
well as capturing and storing CO2. In other words, the innovative S-CO2 topping cycle can produce the same amount of
net electricity as a nonCO2 capturing steam power plant while
reducing the CO2 emission significantly.
2.2.3.
Exhaust/waste heat recovery application
The S-CO2 power cycle is expected to first be utilized and
commercialized for the exhaust/waste heat recovery application. The patents related to this application belong to Echogen
(Ohio, USA) and General Electric (New York, USA) [24,25]. The
exhaust gas temperature from a gas turbine or general topping
cycle is usually > 450 C and the conventional steam Rankine
cycle utilizes this exhaust gas to improve the thermal efficiency. The S-CO2 cycle can potentially replace the steam
Rankine cycle to further improve the thermal efficiency and it
can be utilized to recover waste heat from a small gas turbine
as well, which it is not practically feasible with the steam
Rankine cycle [26].
2.2.4.
Renewable energy application
The S-CO2 cycle can be utilized for various heat sources
including solar thermal power, waste heat from high
temperature fuel cells, and geothermal energy. The S-CO2
cycle can potentially improve the economics of renewable
energy systems significantly [27e30].
3.
Various layouts of S-CO2 cycle
3.1.
S-CO2 cycle literature review
Several layouts of S-CO2 cycle were suggested and compared
by Angelino [4]. His original work focused on the condensation
cycle but some layouts such as the recompression cycle,
partial cooling cycle, and precompression cycle were also
suggested in his work and they are still being investigated in
the S-CO2 cycle research field. He showed that the efficiency of
the recompression cycle with 650 C turbine inlet temperature
is competitive to the reheat steam Rankine cycle. He summarized his work on the CO2 condensation cycle for two
temperature range applications; one is for the mild temperature range (450e550 C) with the benefits of simple layout and
compactness, the other is for the high temperature range
(650e800 C) with high efficiency as well as simplicity and
compactness. Dostal et al [5] revitalized the S-CO2 cycle for
nuclear applications and designed the recompression cycle
with a turbine inlet temperature of 550e750 C. For the S-CO2
heat exchangers, he assumed the heat exchanger to be a
printed circuit heat exchanger (PCHE) and estimated the
physical size of a S-CO2 cycle [3].
After Dostal et al's [5] work, S-CO2 cycle research on various
heat sources including concentrated solar power (CSP), fuel
cells, gas turbine exhaust heat recovery systems, and alternative power conversion systems of current power plants
were conducted [27e30]. Most studies adopted the recompression cycle, which is known as the most efficient layout for
the S-CO2 cycle. However, relatively small specific work of the
recompression cycle can limit the system performance,
especially in the waste heat recovery systems. Kimzey [31]
compared performances of various S-CO2 bottoming cycle
layouts, which can potentially maximize the output power
from the exhaust gas of current gas turbines. Bae et al [20]
designed the cascade CO2 system that consists of a topping
S-CO2 recuperation cycle and a bottoming CO2 Rankine cycle
for the bottoming cycle application of fuel cells. Several S-CO2
cycle layouts from Angelino's [4] work were compared by
Martin and Dostal [32]. This study reviewed the wide spectrum
of S-CO2 layouts including the topping and bottoming cycle
applications and suggests a S-CO2 layout classification for
further development of more innovative power systems with
S-CO2.
3.2.
S-CO2 cycle layout classification
Several S-CO2 cycle layouts have been analyzed in the previous studies [3,4,24,33,34]. However, the general classification
of S-CO2 cycles has not been discussed thoroughly. Although
some advanced S-CO2 layouts were suggested in the literature, these suggested layouts are simply a combination of
several commonly utilized processes in power plant engineering such as intercooling, reheating, and recuperation.
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Fig. 8 e S-CO2 cycle single flow layouts. S-CO2, supercritical CO2.
Fig. 9 e S-CO2 cycle split flow layouts. S-CO2, supercritical CO2.
Table 1 e The reference SFR system.
Reactor sodium inlet temperature ( C)
Reactor sodium outlet temperature ( C)
Reactor sodium mass flow (kg/sec)
545
390
508.0
IHX, intermediate heat exchanger; SFR, sodium-cooled fast reactor.
(Na-CO2) IHX sodium inlet temperature ( C)
(Na-CO2) IHX sodium outlet temperature ( C)
Intermediate loop sodium mass flow (kg/sec)
526
364
484.5
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Table 2 e S-CO2 single flow layout design conditions.
Layout
Recuperation
Intercooling
Reheating
Interrecuperation
500
275.9
354.4
32
249.7
315.5
334.5
339.0
314.6
430.7
Turbine inlet temperature ( C)
IHX inlet temperature ( C)
CO2 mass flow rate (kg/sec)
Compressor inlet
temperature ( C)
Compressor inlet & outlet
pressure (MPa)
Turbine & compressor
isentropic efficiency (%)
HT/LT recuperator
effectiveness (%)
Precompression
7.5/25
Split-expansion
281.1
363.1
270.0
364.2
6.16/7.5
7.5/25
7.5/25
92/88
95/95
HT, high temperature; LT, low temperature; IHX, intermediate heat exchanger; S-CO2, supercritical CO2.
Table 3 e S-CO2 split flow layout design conditions.
Layout
Turbine inlet temperature ( C)
IHX inlet temperature ( C)
CO2 mass flow rate (kg/sec)
Compressor inlet temperature ( C)
Compressor inlet & outlet pressure (MPa)
Turbine & compressor isentropic
efficiency (%)
HT/LT recuperator effectiveness (%)
Flow split ratio (mH/mT)
Recompression
Modified
recompression
335.5
486.1
32
7.5/25
500
283.3
367.1
5.0/7.5
7.5/25
Preheating
Turbine
split flow 1
Turbine
split flow 2
Turbine
split flow 3
98.7
262.6
150.4
377.1
275.9
708.9
98.7
383.0
0.43
0.5
0.46
7.5/25
92/88
95/95
0.31
0.4
0.5
HT, high temperature; LT, low temperature; IHX, intermediate heat exchanger; S-CO2, supercritical CO2.
Therefore, this study provides a general layout classification
and compares various S-CO2 cycle layouts in a fair way.
In the closed Brayton cycle design, the recuperation process is generally required to improve the cycle efficiency by
minimizing the waste heat. Therefore the recuperation layout
can be considered as the reference layout in S-CO2 cycle
design. Other layouts are compared with the recuperation
layout.
The S-CO2 cycle layouts considered in this study are shown
in Figs. 8 and 9. The CO2 flow can be separated depending on
the application. Therefore, the cycle can be categorized
depending on whether the flow is split or not. Single (nonsplit)
flow layouts are composed of intercooling, reheating, precompression, interrecuperation, and split expansion cycles as
shown in Fig. 8. The intercooling and reheating layouts are
adopted to minimize or maximize the compression or
expansion work, respectively. One of the major characteristics
of the S-CO2 cycle is its low pressure ratio because the limit of
minimum pressure in the system is influenced by the critical
pressure (7.38 MPa), which is relatively high compared with
the steam Rankine cycle (~0.07 MPa) or air Brayton cycle
(~0.1 MPa). As the exhaust CO2 temperature in the turbine is
still high due to the low cycle pressure ratio, the heat can be
recuperated in several ways. In the single flow layouts, the
interrecuperation, precompression, and split expansion layouts are suggested depending on the position where the
recuperation process occurs.
The split flow layouts are composed of recompression,
modified recompression, preheating, and turbine split flow 1,
2, and 3 as shown in Fig. 9. The difference between the
recompression layout and the others is the recuperation
process. In the recompression layout, the flow is split and high
specific heat in the cold side flow is matched with the hot side
large flow with lower specific heat in the low temperature
recuperator (LTR) to maximize the cycle efficiency. In the
Fig. 10 e The efficiencies of S-CO2 split flow layouts for
various flow split ratio. S-CO2, supercritical CO2.
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large. The transferred heat is limited by the CO2 temperature
change in the intermediate heat exchanger.
3.3.
Fig. 11 e Performance comparison of S-CO2 Cycle layout.
S-CO2, supercritical CO2.
modified recompression layout, the turbine expands below
the critical pressure to produce more work. Compressor 1
compresses CO2 near the critical point and the other processes
are similar to the original recompression layout.
In contrast, the other layouts, such as the preheating and
turbine split flow 1, 2, and 3 layouts, maximize the temperature difference in the intermediate heat exchanger. For
example, in the waste heat recovery or cogeneration power
plant systems, large temperature change in the heat source is
proportional to the heat flowing into the power conversion
system. In this case, more power can be produced even with a
lower thermodynamic efficiency when the absorbed heat is
Performance comparison of S-CO2 cycle layouts
Twelve layouts are analyzed to investigate and compare S-CO2
cycles' performance. The boundary conditions to design the SCO2 cycle for SFR application is shown in Table 1 [35]. The
design conditions of each layout are listed in Tables 2 and 3. It
should be noted that pressure drop in the heat exchangers and
pipes is ignored for simplicity. As the S-CO2 cycle in this study
is for the SFR application, the turbine inlet temperature is
maintained at 500 C. Previous studies showed that S-CO2
cycle efficiency is sensitive not only to the temperature ratio
but also to the pressure ratio [3,36,37]. However, the
maximum pressure is limited due to the capital cost related to
the piping and measurement systems. The minimum pressure of the S-CO2 cycle significantly influences the cycle efficiency and operational stability of the S-CO2 cycle. As the inlet
condition approaches the critical point, the cycle efficiency is
improved [38,39]. In this study, the minimum and maximum
pressures are fixed at 7.5 MPa and 25 MPa, respectively, for the
simple comparison. Since the flow split ratio influences the
cycle efficiency, a sensitivity study was performed and the
result is shown in Fig. 10. The optimum flow split ratio of each
S-CO2 split flow layout is determined when the cycle efficiency
is maximized.
The cycle efficiency and recuperator UA (overall heat
transfer rate times heat transfer area) ratio (compared with
the recuperation cycle) of the S-CO2 layouts are compared in
Fig. 11. To assess the recuperator size, the LMTD (Log Mean
Temperature Difference) method was used and the UA of each
layout is compared. The recompression layout shows the best
efficiency but requires the largest recuperator size.
Fig. 12 e The layout of SNL experiment loop. HTR, high temperature recuperator; LTR, low temperature recuperator; M/G,
motor generator alternator; MC, main compressor; RC, recompression compressor; SNL, Sandia National Laboratory; TB,
turbine [40].
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Fig. 13 e The layout of KAPL experiment loop. CP, compressor; G, generator; KAPL, Knolls Atomic Power Laboratory; M/G,
motor generator alternator; RCP, recuperator; TB, turbine [41].
4.
S-CO2 power system development status
4.1.
International development
While the research on the S-CO2 Bryton cycle is conducted at
lab scale, the development of the S-CO2 Rankine cycle is
relatively mature for commercialization. However, in this
study, the discussion focuses on the S-CO2 Brayton cycle
which can be applied to the next generation nuclear reactor
systems. The current development of the S-CO2 Brayton cycle
will be introduced.
S-CO2 integral systems tests, which are composed of main
components such as turbomachinery and heat exchangers,
were designed and constructed in several research institutes
Fig. 14 e The layout of IAE experiment loop. CP, compressor; HTR, high temperature recuperator; IAE, Institute of Applied
Energy; LTR, low temperature recuperator; M/G, motor generator alternator; TB, turbine [42].
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Table 4 e The design comparison of existing S-CO2 integral system loops.
Sandia National Lab (US)
Turbomachinery type
Cycle layout
Heat (kW)
Efficiency (%)
Mass flow rate (kg/sec)
T.I.T ( C)
Pressure ratio
Rotating speed (1,000 rpm)
Turbine efficiency (%)
2-TAC
Recompressing
780
31.5
3.5 (target)
2.7 (achieved)
537 (target)
342 (achieved)
1.8 (target)
1.65 (achieved)
75 (target)
52 (achieved)
86 (turbine 1)
87 (turbine 2)
Knolls Atomic Power Lab (US)
Institute of Applied Energy (Japan)
1-TAC, 1-turbine
Simple recuperated
834.9
14.7
5.35 (target)
3.54 (achieved)
300 (target)
1-TAC (2-recuperaters)
Simple recuperated
160
7
1.4 (achieved)
1.8 (target)
1.44 (achieved)
75 (target)
55-60 (achieved)
79.8
(power turbine)
79.7
(compressor driving turbine)
1.4 (achieved)
277 (achieved)
69 (achieved)
65
S-CO2, supercritical CO2.
such as Sandia National Lab (SNL, New Mexico, USA), Knolls
Atomic Power Lab (KAPL, New York, USA), and Institute of
Applied Energy (IAE, Kyoto, Japan) (Figs. 12e14). They
commonly utilize similar turbomachinery features and PCHE
for the S-CO2 power cycle. SNL demonstrated the recompressing cycle with two TAC (turbineealternatorecompressor)
type turbomachineries for the next generation reactor application and KAPL demonstrated the simple recuperated cycle
with two turbines (a power turbine and compressor-driven
turbine) for the water-cooled reactor application. IAE constructed a small scale S-CO2 cycle test facility to investigate the
small size turbomachinery and assess the cycle performance.
The design parameters of S-CO2 test facilities are shown in
Table 4. Based on the experimental results from the S-CO2
integral system tests, several conclusions can be drawn. (1)
The thermal efficiency of the S-CO2 power cycle increases
when the compressor inlet temperature approaches the critical point. When the operating condition is slightly below the
critical point, no noticeable noise or vibration is observed. (2)
The design of bearings to balance the thrust loads is challenging for small scale S-CO2 turbomachinery (both for turbines and compressors). (3) When the pressure of S-CO2
turbomachinery is maintained at high level, the windage loss
in the rotor increases and influences the compressor performance. Therefore, the cavity pressure must be lowered to
2 MPa to decrease the CO2 density; (4) when a recompressing
cycle is designed for the SNL integrated system test, the
operation strategy must be strictly established as the outlet
flow of two S-CO2 compressors must balance to prevent flow
reversal. (5) To maintain the operational stability of integral
system operations, the CO2 inventory for the system loops
must be precisely measured and controlled. In addition,
caution is required for depressurization process because
Teflon type pipe joints can be damaged by the abrupt CO2
decompression process. (6) Heat exchanger, including PCHE,
performances are generally satisfactory. (7) The thermal efficiencies of most power conversion systems improve as the
size increases and the S-CO2 cycle follows the same trend. In
other words, high efficiency is hardly demonstrated in a small
scale S-CO2 cycle test facility. However, a large size S-CO2 cycle
test facility of > 10 MW electric power can demonstrate high
efficiency with the utilization of conventional bearing and
sealing technology, which can minimize the performance
degradation of S-CO2 turbomachinery. Therefore, a facility of >
10 MW power is required to demonstrate the high efficiency of
the S-CO2 cycle.
4.2.
Development progress in Korea
In contrast to the cycle efficiency of a conventional power
conversion system, which highly depends on the turbine inlet
temperature and heat source, S-CO2 cycle efficiency is influenced by the low temperature regions such as a precooler and
a compressor. Therefore, the Korea Advanced Institute of
Science and Technology (KAIST) constructed a low pressure
ratio compressor test loop, S-CO2 Pressurizing Experiment
Fig. 15 e The layout of S-CO2 pressurizing experiment
(SCO2PE). S-CO2, supercritical CO2.
658
N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 6 4 7 e6 6 1
Fig. 16 e S-CO2 pressurizing experiment (SCO2PE). S-CO2,
supercritical CO2.
(SCO2PE), to demonstrate the S-CO2 compressor performance,
as shown in Figs. 15 and 16.
The KAIST research team demonstrated and observed
similar phenomena as the international research institutions
[43]. While operating the compressor in various conditions
including the phase change, no perceivable noise or vibration
was observed. In addition, experiments were conducted at
various phases (gas, liquid, and supercritical state) and
experimental data to compare with the transient analysis of
the S-CO2 cycle was obtained as well.
Korean Atomic Energy Research Institute (KAERI) constructed a S-CO2 Integral Experiment Loop (SCIEL) with the
cooperation of KAIST and POTECH. The schematic layout and
figure of SCIEL are shown in Figs. 17 and 18, respectively. The
SCIEL design parameters are listed in Table 5. The main difference of SCIEL compared with other S-CO2 cycle experiment
facilities is its high pressure ratio with two compression and
expansion stages. The efficiency of the Brayton cycle usually
depends not only on the turbine inlet temperature but also on
the cycle pressure ratio. However, the existing turbomachinery
in integral system test (IST) facilities was designed for a pressure ratio < 2 due to the technical limits. However, as the S-CO2
power cycle can achieve high efficiency at a higher pressure
ratio, 2.7, SCIEL is designed to achieve a higher pressure ratio
with two stages of compression and expansion. The turbine
inlet temperature was determined to be 500 C for the sodiumcooled fast reactor application, which is similar to SNL IST.
The major characteristic of SCIEL turbomachinery design is
to control the thrust loads caused by the pressure difference of
the compressor and the turbine impellers with two approaches. The first unique approach is to separate the shaft of
turbine and compressor and the second unique approach is to
utilize a twin impeller compressor, which has two identical
impellers to minimize the thrust load acting on the shaft. In
addition, while the compressor impellers of existing ISTs are
unshrouded, a shrouded impeller is utilized to balance the
Fig. 17 e The layout of S-CO2 integral experiment loop (SCIEL). HPC, high pressure compressor; HPT, high pressure turbine;
HTR, high temperature recuperator, LPC, low pressure compressor; LPT, low pressure turnine; LTR, low temperature
recuperator; M/G, motor generator alternator; S-CO2, supercritical CO2.
N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 6 4 7 e6 6 1
659
Fig. 18 e S-CO2 integral experiment loop (SCIEL). DAS, data acquisition system; S-CO2, supercritical CO2.
Table 5 e The design parameters of S-CO2 Integral Experiment Loop (SCIEL).
Design variable
Maximum pressure (MPa)
TIT ( C)
Pressure ratio
Compressor efficiency (%)
Heater power (kW)
CO2 flow rate (kg/sec)
Value
Design variable
Value
20
500
1.8 (LPC), 1.5 (HPC)
65
1,300
4.8
Turbine/compressor efficiency (%)
HTR & LTR effectiveness (%)
Cycle efficiency (%)
LPT (rpm)
TAC (rpm)
LPC (rpm)
85/65
74/54
19.6
83,000
100,000
70,000
HTR, high temperature recuperator; LPC, low pressure compressor; LPT, low pressure turbine; LTR, low temperature recuperator; S-CO2, supercritical CO2; TAC, turbineealternatorecompressor; TIT, turbine inlet temperature.
pressure distribution of the inner and outer shroud at the
same time.
Currently, the compressor test loop of SCIEL has been
constructed and the preliminary experiment in the supercritical phase was performed. The heat exchangers of SCIEL
are PCHE type, which is similar to the existing ISTs around the
world. The turbine and the heater are planned to be added to
generate electricity in 2015 and the high pressure compressor
and high pressure turbine will be installed for the final stage of
SCIEL construction.
Korea Institute of Energy Research (KIER) is constructing a
separate experiment loop for low temperature heat source
applications such as a waste heat recovery system. The design
power capacity is on the magnitude of tens of kW and the
layout is a simple cycle without recuperation process. The
heater capacity is 647 kW and the turbine inlet temperature is
< 200 C.
5.
Summary
The S-CO2 cycle can achieve relatively high efficiency within
the mild turbine inlet temperature range (450e600 C)
compared with other power conversion systems. The main
benefit of the S-CO2 cycle is the small size of the overall system and its application includes not only the next generation
nuclear reactors but also conventional water-cooled reactors,
coal power plants, and several renewable energy sources.
Various layouts were compared and the recompression
cycle shows the best efficiency. The layout is suitable for
application to advanced nuclear reactor systems. However,
for the bottoming cycle applications, the specific work should
be compared because other layouts might be more favorable.
As S-CO2 cycle performance can vary depending on the layout
configuration, further studies on the layouts are required to
design a better performing cycle.
To evaluate the S-CO2 cycle performance, various countries constructed and demonstrated S-CO2 integral system
test loops and similar research works are ongoing in Korea as
well. However, to evaluate the commercial S-CO2 power systems, development of a large scale (> 10 MW) prototype S-CO2
system is necessary. The research activities are focused on a
large scale S-CO2 power system and various foreign research
institutions and Korean researchers are attempting to realize
the future power system that can significantly transform the
energy industry around the world.
660
N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 6 4 7 e6 6 1
Conflicts of interest
All authors have no conflicts of interest to declare.
Acknowledgments
This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT,
and Future Planning.
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