Journal of Power Sources 132 (2004) 77–85

Analysis and optimization of a solid oxide fuel cell and intercooled

gas turbine (SOFC–ICGT) hybrid cycle
Yaofan Yi, Ashok D. Rao, Jacob Brouwer, G. Scott Samuelsen∗
Advanced Power and Energy Program/National Fuel Cell Research Center, University of California, Irvine, CA 92697-3550, USA
Received 1 August 2003; accepted 20 August 2003

Abstract

The power generation community faces a major challenge: to protect the environment while producing a plentiful supply of clean low-cost
energy. “21st Century Energy Plants” (Vision 21 Plants) have been proposed and conceptualized to meet the energy and environmental
challenges. The solid oxide fuel cell and intercooled gas turbine (SOFC–ICGT) hybrid cycle introduced in this work is one example of
a Vision 21 Plant. The system includes an internal-reforming tubular-SOFC, an intercooled gas turbine, a humidifier, and other auxiliary
components. A recently developed thermodynamic analysis computer code entitled advanced power systems analyses tools (APSAT) was
applied to analyze the system performance of the SOFC–ICGT cycle. Sensitivity analyses of several major system parameters were studied
to identify the key development needs and design and operating improvements for this hybrid cycle. A novel optimization strategy including
a design of experiments (DOEx) approach is proposed and applied to the hybrid system. Using this optimization strategy, a system electrical
efficiency higher than 75% (net ac/lower heating value (LHV)) could be achieved when the system was designed to operate under a high
operating pressure (50 bara) and with a low percent excess air (EA) (55%) in the SOFC.
© 2003 Elsevier B.V. All rights reserved.

Keywords: DOEx (design of experiments); SOFC (solid oxide fuel cell); Hybrid cycle; Humidifier; ICGT (intercooled gas turbine); Vision 21

1. Introduction and acid rain-forming pollutants, and reduce greenhouse gas

emissions (CO2 ) by 40–50% by efficiency improvements
The Vision 21 program of the US Department of Energy (with further reductions in greenhouse gas emissions if cou-
proposes a new approach to produce energy that addresses pled with carbon sequestration) [4].
pollution control as an integral part of high-efficiency energy Previous work by Rao and Samuelsen [10] investigated
production in a “Vision 21 Energy Plant”. Integral pollution various advanced cycles including gas turbine combined cy-
control, ultra-high efficiency, and potential for carbon diox- cles, humidified air turbine cycles, oxygen–hydrogen-fired
ide capture and sequestration are the salient features of a direct Rankine cycles, and hybrid fuel cell gas turbine cy-
Vision 21 energy plant concept. Vision 21 energy plants are cles. These analyses determined that fuel cell gas turbine
also expected to produce value added products other than hybrid cycle technology is key to reaching the Vision 21
electricity such as hydrogen or liquid transportation fuels. efficiency and emissions goals.
The fuel-to-electricity conversion efficiency goal of a Vision Recently, several fuel cell gas turbine hybrid system con-
21 energy plant is greater than 60% (higher heating value figurations have been suggested by various research groups
(HHV) basis) using coal as fuel source and greater than around the world.
75% (lower heating value (LHV) basis) using natural gas. The Department of Heat and Power Engineering at Lund
Currently, the corresponding efficiencies for most operating University in Sweden has conducted theoretical studies of
coal-fueled power plants are between 33 and 35%, and for hybrid SOFC/GT systems. Five parameters were studied in a
operating natural gas-fueled plants typical efficiencies range reference system, including turbine inlet temperature (TIT),
from 45 to 55%. A Vision 21 plant is also required to have cell voltage, compressor pressure ratio, air flow rate, and air
near zero emissions of criteria pollutants, including smog- inlet temperature. A maximum electrical efficiency of 65%
(LHV) was found at a pressure ratio of two [8].
Chan et al. [2] of the Nanyang Technological University,
∗ Corresponding author. Tel.: +1-949-824-5468; fax: +1-949-824-7423. Singapore conducted two case studies on the SOFC/GT hy-
E-mail address: gss@nfcrc.uci.edu (G.S. Samuelsen). brid system with particular attention to the effects of op-

0378-7753/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jpowsour.2003.08.035
78 Y. Yi et al. / Journal of Power Sources 132 (2004) 77–85

erating pressure and fuel flow-rate on the performance of 2. SOFC–ICGT system description
the components and overall system. Results showed that an
internal-reforming hybrid SOFC/GT system could achieve The Vision 21 hybrid cycle studied in this work is a solid
an electrical efficiency of more than 60% (LHV). oxide fuel cell integrated with an intercooled gas turbine
A fossil fuel based hybrid SOFC/GT system with lique- system (SOFC–ICGT) as shown in Fig. 1. The system op-
faction recovery of carbon dioxide was proposed by Inui erating pressure can be designed as high as 50 bara. In the
et al. [5]. Results showed that the total thermal efficiency SOFC–ICGT cycle, two compressors (a low pressure and a
of the system using natural gas as the fuel reached 70.64% high pressure compressor (HPC)) combined with an inter-
(LHV). cooler are utilized, to limit the operating temperature in the
Some research groups have investigated and presented the high-pressure stages of the compressor as well as reduce the
part-load performance of hybrid SOFC/GT systems in ad- compression power required. The compressed air provides
dition to design point performance. These research groups the oxygen required by the SOFC stacks. Natural gas fuel
have produced analyses such as those presented by Campa- is supplied at a pressure of 20.7 bara, is desulfurized and
nari [1], Costamagna et al. [3], and Kimijima and Kasagi then humidified and preheated in a counter-current humidi-
[6]. These studies suggest that expected electrical efficien- fier. The fuel gas leaving the humidifier at its dew point is
cies of hybrid systems are limited to 65% at design-point or then superheated by the turbine exhaust in the recuperator
part-load operation. before entering the internal-reforming tubular-SOFC. The
Previous research work conducted at the National Fuel high temperature and pressure effluent from the SOFC is
Cell Research Center of the University of California, Irvine expanded through turbines to drive the compressors and an
(NFCRC/UCI), which was limited to hybrid systems of less ac generator. Turbine exhaust, after superheating the fuel in
than 20 MW output, showed that the efficiency goal of the the recuperator, provides heat to the humidifier circulating
Vision 21 program was achievable with small hybrids. This water in an economizer before leaving the system.
was found to be the case despite having turbomachinery A humidifier is applied in the current cycle to improve
components with limited efficiencies (due to the smaller the performance of the system, facilitate steam reformation
size). However, achieving an efficiency of 75% at this scale of the natural gas, and prevent cooking in downstream ele-
required a complex cycle including a reheat gas turbine with ments. The humidifier also allows the recovery of low tem-
two SOFCs, one upstream of the high pressure expander and perature heat from the turbine exhaust, preheats the fuel,
one upstream of the low pressure expander when limiting and increases the power output from the turbine. The water
the operating pressure of the SOFC to 15 bara [9]. exiting from the bottom of the humidifier, together with the
Although many researchers have analyzed hybrid fuel cell make-up water, is circulated to recover heat from the turbine
gas turbine systems as noted above, no optimization strategy exhaust. The make-up water accounts for the water evap-
has been proposed, which may be a reason for predicted orated within the humidifier into the fuel stream while the
limitations in the achievable system efficiency. blow-down from the humidifier limits the build up of solids
A multi-disciplinary team led by the NFCRC/UCI pro- within the humidifier.
poses herein to define the system engineering issues asso- Note that the solution strategy used in the internal re-
ciated with the integration of key components and subsys- forming tubular SOFC includes the recycle of a portion of
tems into large size (300 MW central station) power plant the anode exhaust gas (∼60%), which is combined with the
systems that meet the performance and emission goals of superheated fuel gas entering the SOFC. The internal re-
Vision 21. formers are interspersed between tubular SOFC cell bundles
The NFCRC team is conducting a detailed study of Vi- with the anode exhaust gas recycle providing enough water
sion 21 Energy Plants that includes: (1) developing a consis- vapor and heat to facilitate partial steam reformation in the
tent analysis strategy to identify those combinations of fu- internal reformer, avoid carbon deposition in the reforming
els and fuel handling/processing systems, power generation catalyst, and effectively remove heat from the SOFC stack.
components, and emission control systems that meet Vision This strategy is typical of the design of Siemens Westing-
21 goals; (2) conceptualization of cycle configurations that house Power Corporation.
have the potential to meet Vision 21 goals; (3) accomplish-
ing detailed analyses of the resulting energy plants identi-
fying key technical, operability, and economic factors that 3. Modeling
would affect the integration of the components and subsys-
tems into a viable Vision 21 energy plant; and (4) making A special steady-state simulation tool—advanced power
recommendations for R&D that address the issues that need system analysis tool (APSAT), developed at the National
to be resolved to assure successful system integration into Fuel Cell Research Center, was applied to simulate the
a viable Vision 21 power plant [7]. As a milestone in the SOFC–ICGT cycle. The detailed description of this simu-
third step, the current SOFC–ICGT hybrid cycle is presented lation tool can be found in [9,10]. Fundamental thermody-
and optimized as the first Vision 21 power plant capable of namics, heat transfer, and equations of state appropriate for
meeting Vision 21 goals. solving the complex and coupled chemical, electrochemi-
 Y. Yi et al. / Journal of Power Sources 132 (2004) 77–85 79

Intercooled Gas Turbine (8)

(1)
Air
LPC HPC HPT LPT
1 4 16 17
Generator
(7) (21)
(5) Intercooler T (22)

2 (18) SOFC
6 Contr.1 C
3 System
(19) Eductor 14
F (20)
15 A (17)
(6) (4) Combustor Reformer F
Condensate (25)
Cooling Water
13 20 Controller2
21 Recycler1 Pipe
F
(16)
12
18 Recuperator

(23)

Recycler2 (9) Economizer

19
(12) 22 (13) 11
(3)
5 10
From Water (14) (27) (24) 1,2,3 ... Module Number
Treatment Stack Gas (1), (2), (3) ... Stream Number
Humidifier
(26) 8 GT Working Fluid
(11) Water
(2) 7 Natural Gas
Natural Gas 9
(28) (10) Controll route

Fig. 1. SOFC–ICGT hybrid cycle: system configuration with identified modules and streams.

cal, heat, and mass transfer processes extant in high tem- Table 1
perature fuel cell systems are included in APSAT. APSAT Setting values of various parameters
was verified to be a capable and accurate simulation tool in Parameter Setting value
previous simulation work and comparisons to the Siemens
Gas turbine (GT)
Westinghouse 220 kW hybrid SOFC/GT system [12,13]. Compressor isentropic efficiency (LPC) (%) 90
Compressor isentropic efficiency (HPC) (%) 88
3.1. Basic parameter settings Recuperator effectiveness (%) 90
Recuperator pressure drop (%) 2
In this study, it is assumed that all system components Turbine isentropic efficiency (LPT) (%) 94
Turbine isentropic efficiency (HPT) (%) 92
are working at their respective design conditions under GT firing temperature (◦ C) <1700
steady-state operation. A set of operating parameters and Generator efficiency (%) 98.5
the assumed efficiencies/effectiveness of these system com- Solid oxide fuel cell (SOFC)
ponents are given in Table 1. Note that the efficiencies Cell length (cm) 150
and materials specifications presented in Table 1 require Cell outside diameter (cm) 2.2
state-of-the-art components, but do not depend on advances Cell voltage (V) 0.7
beyond those possible today. Cell operating temperature (◦ C) 1000
Fuel utilization (%) 85
Stack heat loss (%) 2
3.2. Simulation building Stack pressure loss (%) 2
dc–ac Converter efficiency (%) 95
The SOFC–ICGT system with identified modules and Limiting current density (mA/cm2 ) 350
streams is shown in Fig. 1. In this system, two controllers Others
and two recyclers are applied to facilitate iterative solution Fuel inlet pressure (bara) 20.7 (300 psia)
Fuel compressor isentropic efficiency (%) 82
of the complete system to satisfy all mass, momentum and
Motor efficiency (for pumps) (%) 92
energy balances within the design constraints. Controller
80 Y. Yi et al. / Journal of Power Sources 132 (2004) 77–85

1 is used to control the inlet temperature of high-pressure 68.90

compressor by adjusting the mass flow rate of cooling wa-
System Electrical Efficiency (%)

ter into the intercooler. Controller 2 can adjust the fuel flow
68.85
from the source to meet the SOFC fuel requirement. Recy-
cler 1 is used to maintain the effectiveness of recuperator
at the specified value. Recycler 2 iteratively determines the 68.80
required water flow to the humidifier.

68.75
4. Parametric analysis
68.70
Four basic parameters are studied in these analyses: (1) 24 26 28 30 32
moisture content (MC) of the gas out of the humidifier Moisture Content in the Gas Outlet of Humidifier (%)
(changed by controlling the water temperature out of the
economizer); (2) excess air (EA) level in the SOFC (changed Fig. 2. Sensitivity of moisture content to the system efficiency, πLPC = 4,
πoverall = 15.2, EA = 80%.
by varying the air flow rate when fuel flow rate is fixed);
(3) overall compressor pressure ratio (πOverall ); and (4) in-
tercooler location or pressure ratio of lower pressure com-
pressor (πLPC ). The only response presented in this paper varying the temperature of the water into the humidifier (or
is the overall fuel-to-ac-electricity efficiency. The sensitiv- out of the economizer). When the water temperature out of
ity of the efficiency to each of these parameters was studied. the economizer is required to be higher, so as to achieve a
The definitions of the system efficiency and the net system higher moisture content in the gas exiting the humidifier, the
power output in this study are given as follows: required effectiveness of the economizer must be increased,
System efficiency or overall fuel-to-ac-electricity effi- leading to a higher economizer cost in practice. This eco-
ciency nomic tradeoff must be considered in practical design of
such systems.
net system power output
ηsystem = (1)
lower heating value of fuel 4.2. Excess air
Net system power output is equal to SOFC ac power after ac-
counting for the inverter loss (but without transformer/busbar The amount of excess air in this work is defined as the
losses) plus gas turbine power at the generator terminals percentage of the excess air supplied that is greater than the
(but without transformer/busbar losses) minus all auxiliary stoichiometric amount divided by the stoichiometric amount.
power (for pumps, fuel compressor, etc.). The effects of excess air were studied while maintaining
LPC pressure ratio at 4, overall pressure ratio at 15.2, and
4.1. Moisture content of the gas out of humidifier moisture content of the gas out of the humidifier at 31%.
Fig. 3 shows that with a decrease in excess air, the effi-
The effect of moisture content on the system efficiency ciency increases for a given fuel utilization within the SOFC.
was studied while setting the other three parameters as fol- This is because the temperature of the working fluid en-
lows: LPC pressure ratio, 4; overall pressure ratio, 15.2; and tering the expander of the GT increases as the excess air
excess air, 80%. The results presented in Fig. 2 show that decreases (note that no additional fuel is fired between the
increasing the content of moisture introduced in the fuel in- SOFC and the GT expander and only the unutilized fuel is
creases efficiency and net output initially. When the content combusted). Thus, for the same fuel utilization and electro-
of moisture is further increased, however, the efficiency and chemical production, the requirement to heat less air results
net output decrease. This behavior is caused by increasing in higher turbine inlet temperature and higher overall system
moisture content that leads to both increased mass flow of efficiency.
the working fluid in the expanders and concurrent reduc-
tions in expander inlet temperatures for a given amount of 4.3. Overall compression ratio
excess oxygen in the SOFC. Initially the effect of increasing
flow rate of the working fluid dominates and a net increase The effects of overall compression ratio on system ef-
in the efficiency and net output is realized. When the mois- ficiency are presented in Fig. 4. The system efficiency in-
ture content of the fuel gas is further increased, the effect creases monotonically with the overall compression ratio in
of lowering the inlet temperature to the expanders begins the range of compression ratios studied. In this case, LPC
to dominate decreasing the efficiency and net output of the pressure ratio was set to 4; excess air was kept at 80%, and
system. moisture content was set to 29%. Higher efficiencies are re-
A concern with increasing moisture content is the sys- alized at higher pressure ratios due to increased enthalpy in
tem cost. Notice that the moisture content is changed via the expander inlet streams allowing more power production
 Y. Yi et al. / Journal of Power Sources 132 (2004) 77–85 81

69.5
System Electrical Efficiency (%)

69.0

68.5

68.0

67.5

67.0
50 70 90 110 130 150
Excess Air (%)

Fig. 3. Sensitivity of system efficiency to excess air, πLPC = 4, πoverall = 15.2, MC = 31%.

in the gas turbine portion of the cycle. In the typical gas tur- overall compression ratio is held constant (35 in this case).
bine cycle, this is usually balanced by increased compressor The reasons for this behavior are:
power demands. However, when the current hybrid system
compression ratio increases, the fuel cell portion of the cy- • With a higher LPC pressure ratio, the temperature of LPC
cle increases in both output and efficiency, due to enhanced outlet increases, and thus the temperature of air inlet to
electrochemical kinetics. Therefore, although the compres- the intercooler increases. In this case, the temperature of
sors consume more power when the pressure ratio increases, air leaving the intercooler (or air inlet to HPC) is held
the increase of the power produced in the expanders and the constant (30 ◦ C), so the increase of LPC pressure ratio
fuel cell is more than the increased compressor power de- increases the heat removed by the cooling water, which is
mand within the range of pressures ratios investigated. Note an overall system loss. Thus, the heat rejection from the
that the current monotonic dependence and magnitude of in- cycle or loss to the environment increases when increasing
creased performance with increasing pressure ratio depends LPC pressure ratio.
not only on the overall system design, but particularly on • Increasing the LPC pressure ratio decreases the HPC pres-
the use of an intercooled compressor design as specified in sure ratio, since the overall pressure ratio is fixed. Since
Table 1. the temperature of air inlet to the HPC (or air outlet from
intercooler) is held constant, the temperature of the HPC
4.4. Intercooler location outlet decreases with increasing PLC pressure ratio, and
thus reduces the expander inlet temperature. Less power
The results presented in Fig. 5 show that increasing LPC is developed by the expanders as a result causing the sys-
pressure ratio beyond three decreases efficiency when the tem efficiency to decrease.

72 73
System Electrical Efficiency (%)

System Electrical Efficiency (%)

72
71

71
70
70

69
69

68 68
12 17 22 27 32 37 2 3 4 5 6 7 8
Pressure Ratio Pressure Ratio of LPC

Fig. 4. Sensitivity of system efficiency to overall pressure ratio, πLPC = 4, Fig. 5. Sensitivity of system efficiency to pressure ratio of LPC,
MC = 29%, EA = 80%. πoverall = 35, MC = 27.6%, EA = 80%.
82 Y. Yi et al. / Journal of Power Sources 132 (2004) 77–85

5. System optimization

Results presented in the parametric analyses of Figs. 2–5

suggest that reasonable efficiencies can be achieved with the
SOFC–ICGT system configuration and that it is a promis-
ing cycle for meeting Vision 21 goals. To further understand
SOFC–ICGT system performance and to reach the Vision
21 goals, the concurrent effects of parameters and of interac-
tions amongst these parameters on the overall system perfor-
mance were studied using a novel optimization process. A
special analysis tool using a design of experiments (DOEx)
approach was used to analyze the interactions of parame-
ters, which are challenging to examine in a basic parametric
analysis. The DOEx approach can also help optimize the
results to choose the system operation design points [11].
The basic premise of DOEx is one of determining sta-
tistical significance of effects, removing of ineffectual pa-
rameters, and determining the interactions amongst a large
number of parameters that may be important to overall per-
formance using detailed statistical analyses of variance. De- Fig. 6. Interactive effects of excess air and moisture content on efficiency,
tails of this approach can be found at Stat-Ease [11]. These where πLPC = 4.5, πoverall = 37.5.
approaches are typically applied to experimental parametric
investigations. In the current optimization strategy, we pro-
pose to apply similar statistical analyses to simulation results overall pressure ratio and LPC pressure ratio are fixed. The
where complex interactions and parametric effects must be system efficiency increases with the decrease in excess air,
determined to increase overall system efficiency. which is consistent with the results from one factor effect
analysis on excess air (Fig. 3). Moreover, from the contours
5.1. Design list of Fig. 6, one can deduce that at higher excess air levels,
the impact of moisture content on system efficiency is more
The four design parameters listed in Table 2 were stud- significant than at lower excess air levels.
ied in the basic parametric analyses utilizing the DOEx ap- Fig. 7 shows that when overall pressure ratio and moisture
proach. The electrical efficiency of the overall system was content are kept constant, the highest system efficiency can
the only response that was considered in the design. More be achieved when both excess air and LPC pressure ratio are
details about the DOEx approach in this work can be found
in [12].

5.2. Parameter interaction analysis

Compared to the one-factor effect plot shown in basic

parametric analysis, interaction plots can provide more de-
tails regarding the joint impacts of certain parametric varia-
tions (factor) on the system performance.
Fig. 6 illustrates the interactive effects of excess air and
moisture content on system efficiency in the case where

Table 2
Design list
Range

Factors
Moisture content (%) 22–28
Excess air (%) 72–170
Overall pressure ratio 15–60
LPC pressure ratio 3–6
Response
Efficiency (LHV) (%) To be maximized Fig. 7. Interactive effects of excess air and LPC pressure ratio on efficiency,
where πoverall = 37.5, MC = 25%.
 Y. Yi et al. / Journal of Power Sources 132 (2004) 77–85 83

in compressors compared to the increase of the power pro-

duced in the expanders. As a result, increasing overall pres-
sure ratio does not improve the system performance when
excess air is relatively high.

5.3. Optimization

The interactions analyses and parameter modifications in-

vestigated using the DOEx approach can be used to de-
termine optimum system operation [11]. The optimization
analyses performed using Stat-Ease show that the highest
system efficiency can be achieved when the overall pressure
ratio is high, excess air is low and the pressure ratio of the
LPC is low.
Considering the complicated effect of moisture content on
the system performance (see Fig. 2), some additional sys-
tems simulation work is conducted in order to determine the
optimum design points. The range of moisture content inves-
tigated was set to from 25 to 34%. Excess air is set to 55%,
Fig. 8. Interactive effects of excess air and overall pressure ratio on the lowest value chosen for this study since the efficiency in-
efficiency, where πLPC = 4.5, MC = 25%. creases with decreased excess air; overall pressure ratio is set
to 50 since the efficiency increases with increasing overall
pressure ratio, and the LPC pressure ratio is set to three, the
at their lowest levels. Also, when excess air is higher, the lowest value studied since the efficiency increases lower LPC
effect of LPC pressure ratio on system efficiency is much pressure ratio. The results from simulation of the overall sys-
more significant. As discussed in the one factor effect analy- tem of Fig. 1 under these conditions for a series of variations
sis, higher LPC pressure ratios result in higher heat removal in moisture content are shown in Fig. 9. These results show
from the cycle by cooling water in the intercooler. When that the highest efficiency can be achieved when the moisture
excess air increases, the air flow rate in the compressor in- content is around 32%. This is determined for conditions
creases (fuel flow rate is kept constant). As a result, the heat when the temperature difference between hot gas into the
removed in the intercooler is higher, and therefore the de- economizer and the water out of the economizer is held close
crease of the system efficiency is more significant. Notice to 35 ◦ C.
that when the LPC pressure ratio is lower, then the operat- Utilizing this optimum design, where moisture content is
ing temperature in the high pressure compressor is higher. set at 32%, the system efficiency is calculated by APSAT to
HPC discharge temperature is about 480 ◦ C when overall be 75.8%. The optimum design, thus determined, is summa-
pressure ratio is 60 and LPC pressure ratio is 3. Therefore, rized in Table 3. As stated previously, the supply pressure
the material requirements for the HPC as well as its cost of the fuel is assumed to be 300 psia in this study. Con-
are higher when lowering the LPC pressure ratio to achieve sidering that many of the pipelines transporting the large
a higher overall system efficiency. Thus, the optimum pres- quantities of natural gas required by central station power
sure ratio for the LPC may be dictated by the design of the plants operate at a pressure of 600–1000 psia (which is sig-
HPC materials and the HPC costs that the overall system can nificantly higher than 50 bara), a fuel compressor is not nec-
tolerate. essarily required. Simulation of the system without a fuel
In Fig. 8, the overall pressure ratio is shown to have an compressor resulted in a calculated net system efficiency
obvious positive impact on the system efficiency when ex- of 76.2%.
cess air is lower. However, the effect becomes negative when
excess air becomes greater than 150% in this case. When
excess air is low, the turbine inlet temperature is high (about
Table 3
1000 ◦ C when excess air is 72% and overall pressure ratio Optimum design
is 60). As a result, the increase of overall pressure ratio im-
proves the gas turbine efficiency as well as the whole system Moisture content (%) 32
Excess air (%) 55
efficiency, because of larger power generation in the turbine Pressure ratio 50
expansion process with higher pressure ratio and higher TIT. LPC pressure ratio 3
On the contrary, if excess air is higher, TIT becomes lower SOFC ac power (MW) 450
(about 700 ◦ C when excess air is 170% and overall pressure GT power (MW) 177
ratio is 60). Thus, when excess air is too high, the gas turbine Total power (MW) 627
Electric efficiency (LHV) (%) 75.8
efficiency goes down due to more power being consumed
84 Y. Yi et al. / Journal of Power Sources 132 (2004) 77–85

75.8 628
System Electrical Efficiency

75.7 627

75.6 626 System Power Output

(AC, MW)
(%)

75.5 625

75.4 624

75.3 623

75.2 622
25 27 29 31 33 35
Moisture Content in the Fuel (%)
Efficiency Power Output

Fig. 9. Optimization: system efficiency and power output vs. moisture content.

6. Discussion and conclusions trade-off practical concerns for carbon deposition against
all of these impacts when considering the design point for
Through analyses conducted using the APSAT computer moisture content.
code and from the novel optimization strategy using design
of experiments, an optimum design for a solid oxide fuel cell
and intercooled gas turbine hybrid cycle has been achieved. Acknowledgements
The electrical efficiency was shown to be as high as 75.8%
based on natural gas lower heating value. The authors wish to recognize California Energy Com-
Some promising characteristics of the SOFC–ICGT cycle mission and US Department of Energy for their financial
from the simulation results are: support.

• Increases in operating pressure increase overall system ef-

ficiency. However, the technical challenges in developing References
an SOFC with a very high operating pressure as well as
the associated development costs will be high. So, there [1] S. Campanari, Full load and part-load performance prediction for
is a balance between the development cost and the effi- integrated sofc and microturbine systems, J. Eng. Gas Turb. Power
122 (2000) 239–246.
ciency. (In this work, the system efficiency was the only [2] S.H. Chan, H.K. Ho, Y. Tian, Modelling of simple hybrid solid oxide
factor considered.) fuel cell and gas turbine power plant, J. Power Sources 109 (1)
• For a given overall compressor pressure ratio, a decrease (2002) 111–120.
in the LPC pressure ratio increases the overall system ef- [3] P. Costamagna, L. Magistri, A.F. Massardo, Design and part-load
ficiency. At the same time, the operating temperature in performance of a hybrid system based on a solid oxide fuel cell
reactor and a micro gas turbine, J. Power Sources 96 (2) (2001)
the HPC and also the requirements for more exotic ma-
352–368.
terials for HPC construction increase. A tradeoff between [4] DOE, Vision 21 Program Plan—Clean Energy Plants for the 21st
cost and efficiency is again required. Century, US Department of Energy, 1999.
• Decreasing excess air in the SOFC has a positive effect [5] Y. Inui, S. Yanagisawa, T. Ishida, Proposal of High Performance
on the overall efficiency. SOFC Combined Power Generation System With Carbon Dioxide
Recovery, Energy Conversion and Management, 2002.
• Moisture content in the fuel has a bimodal effect on the
[6] S. Kimijima, N. Kasagi, in: Proceedings of the ASME Turbo Expo,
system performance. When increasing the moisture con- Performance Evaluation of Gas Turbine-Fuel Cell Hybrid Micro
tent of the fuel, the mass flow rate of working fluid in the Generation System, Amsterdam, The Netherlands, 2002.
expanders increases, while the inlet temperatures of the [7] NFCRC, Vision 21 Systems Analysis Methodologies, National Fuel
expanders decrease. Additionally, the moisture content in Cell Research Center of the University of California, Irvine, 2000.
[8] J. Palsson, A. Selimovic, L. Sjunnesson, Combined solid oxide fuel
the fuel also affects the partial pressure of the reactants
cell and gas turbine systems for efficient power and heat generation,
in the anode gas: the anode concentration and activation J. Power Sources 86 (1–2) (2000) 442–448.
polarizations as well as the reforming process are all af- [9] A.D. Rao, A Thermodynamic Analysis of Tubular SOFC Based
fected. As in previous parametric considerations, one must Hybrid Systems. Engineering, University of California, Irvine, 2000.
 Y. Yi et al. / Journal of Power Sources 132 (2004) 77–85 85

[10] A.D. Rao, G.S. Samuelsen, Analysis strategies for tubular solid oxide and Aerospace Engineering, vol. 113, University of California, Irvine,
fuel cell based hybrid systems, J. Eng. Gas Turb. Power 124 (2002) 2002.
503–509. [13] Y. Yi, T.P. Smith, J. Brouwer, G.S. Samuelsen, et al., Simulation of
[11] Stat-Ease, Design-Expert, Minneapolis, MN, Stat-Ease, 2000. a 220 kW Hybrid SOFC gas turbine system and data comparison, in:
[12] Y. Yi, Application of APSAT Code to the Simulation of Hybrid Solid Proceedings of the Eighth International Symposium on Solid Oxide
Oxide Fuel Cell/Gas Turbine Systems, Master Thesis in Mechanical Fuel Cell (SOFC-VIII), Paris, France, 2002.