Joshua E. Freeh and Christopher J. Steffen, Jr.

Glenn Research Center, Cleveland, Ohio

Louis M. Larosiliere

General Electric Gas Turbines, Greenville, South Carolina

Off-Design Performance Analysis of a

Solid-Oxide Fuel Cell/Gas Turbine Hybrid

for Auxiliary Aerospace Power

NASA/TM—2005-213805

December 2005

FUELCELL2005–74099
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Joshua E. Freeh and Christopher J. Steffen, Jr.

Glenn Research Center, Cleveland, Ohio

Louis M. Larosiliere

General Electric Gas Turbines, Greenville, South Carolina

Off-Design Performance Analysis of a

Solid-Oxide Fuel Cell/Gas Turbine Hybrid

for Auxiliary Aerospace Power

NASA/TM—2005-213805

December 2005

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

Third International Conference on Fuel Cell Science, Engineering, and Technology

cosponsored by the ASME, ASME Nanotechnology Institute, Mechanical Engineering

Magazine, RIT, NFCRC, Kettering University, SME, and Fuel Cell Markets

Ypsilanti, Michigan, May 23–25, 2005

FUELCELL2005–74099
Acknowledgments

The authors would like to acknowledge Robert Tornabene and Xiao-yen Wang for their help with the heat

exchanger modeling and the system mass estimation.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov

This work was sponsored by the Low Emissions Alternative

Power Project of the Vehicle Systems Program at the

NASA Glenn Research Center.
NASA/TM—2005-213805 1

Off-Design Performance Analysis of a Solid-Oxide Fuel Cell/Gas 

Turbine Hybrid for Auxiliary Aerospace Power 

Joshua E. Freeh and Christopher J. Steffen, Jr. 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Louis M. Larosiliere 

General Electric Gas Turbines 

Greenville, South Carolina 29602 

Abstract 

A solid-oxide fuel cell/gas turbine hybrid system for 

auxiliary aerospace power is analyzed using 0–D and 1–D 

system-level models. The system is designed to produce 

440 kW of net electrical power, sized for a typical long-range 

300-passenger civil airplane, at both sea level and cruise flight 

level (12,500 m). In addition, a part power level of 250 kW is 

analyzed at the cruise condition, a requirement of the operating 

power profile. The challenge of creating a balanced system for 

the three distinct conditions is presented, along with the 

compromises necessary for each case. A parametric analysis is 

described for the cruise part power operating point, in which 

the system efficiency is maximized by varying the air flow 

rate. The system is compared to an earlier version that was 

designed solely for cruise operation. The results show that it is 

necessary to size the turbomachinery, fuel cell, and heat 

exchangers at sea level full power rather than cruise full 

power. The resulting estimated mass of the system is 1912 kg, 

which is significantly higher than the original cruise design 

point mass, 1396 kg. The net thermal efficiencies with respect 

to the fuel LHV are calculated to be 42.4 percent at sea level 

full power, 72.6 percent at cruise full power, and 72.8 percent 

at cruise part power. The cruise conditions take advantage of 

precompressed air from the on-board Environmental Control 

System, which accounts for a portion of the unusually high 

thermal efficiency at those conditions. These results show that 

it is necessary to include several operating points in the overall 

assessment of an aircraft power system due to the variations 

throughout the operating profile. 

Introduction 

The aircraft auxiliary power unit (APU) typically provides 

the majority of electrical power for on-board systems when on 

ground. It also serves as a backup power source on portions of 

certain long-distance flights, known as Extended-Range TwinEngine Operations (ETOPS) (ref. 1). Current APUs are gas 

turbine-based systems that have not been the focus of 

emissions and noise reduction due to their relative small size 

and short usage time when compared to the propulsion gas 

turbines. As a result, for ground operations, they now produce 

as much as 20 percent of the aircraft NOx emissions and a 

significant amount of noise (ref. 2). 

Future aircraft designs are projected to require a significant 

increase in auxiliary electrical loads due to increased 

application of electromechanical actuators, passenger services, 

and communications. The increase in power demand will be at 

least partly met by increasing the size of the APU and also 

migration to a full-time auxiliary power unit, where the unit is 

operational in-flight as well as on ground. The need for a 

cleaner and quieter system becomes more apparent with this 

scenario. 

There are multiple solutions to this problem. One approach 

is to improve the existing gas turbine with a cleaner and 

quieter design. For example, the overall pressure ratio and 

turbine temperatures can be increased for both higher 

efficiency and higher specific power systems. Further, more 

innovative cycles involving heat exchangers similar to those 

proposed in this hybrid system may be used for higher 

efficiency. The comparative risk with these approaches is low 

since the gas turbine is well understood; however, there is a 

limited amount of improvement possible because issues such 

as rotational machinery tip clearances and turbine blade 

cooling become more of a challenge at these smaller sizes. 

Another solution is to produce more power from the existing 

starter/generators on each of the propulsion engines. These 

units are necessary for the startup of the main engines and 

produce power efficiently and with low mass. Drawbacks 

include the adverse effect on the propulsion engine efficiency 

and potential physical interference with the rotating machinery 

as the generator power and diameter both increase. In addition, 

redundancy requirements require a power production unit 

independent of the engines, especially for ETOPS (ref. 1). For 

terrestrial power needs, a simpler approach is being used at 

some airports where the aircraft receives electrical power from 

the airport power grid itself. This may be a very positive 

solution overall, but in-flight power requirements and 

redundancy issues still exist. Finally, quieter and cleaner 

technology can be used in place of the gas turbine APU, such 

as batteries and fuel cells. This approach is being examined by 

both government and industry, and is the subject of this paper. 

Earlier efforts (refs. 2 to 4) have studied the feasibility of 

this idea from the overall aircraft perspective and at a systems 

design level. An initial requirement that guided this work is 

that only one fuel (jet fuel) is allowed due to near-term 
NASA/TM—2005-213805 2

customer and operational restrictions. Jet fuel is a complex 

mixture of hydrocarbons and impurities such as sulfur-based 

species; reforming the fuel to an acceptable level for 

automotive-based PEM fuel cells will be very difficult within 

the mass and volume restrictions. The Solid Oxide Fuel Cell 

(SOFC) is more amenable to impurities and carbon monoxide 

even acts as a fuel and indirect additional source of hydrogen 

(through the water-gas-shift reaction). Therefore, the SOFC 

has been the baseline system for much of the early analysis. A 

SOFC-based system is much heavier than a gas turbine per 

unit power, however, and one way to counteract this is to take 

advantage of the potentially higher efficiency of the SOFC. 

When the fuel mass saved from the difference in efficiency is 

accounted for, the system compares favorably with a gas 

turbine system. Furthermore, an even higher efficiency can be 

achieved by incorporating a smaller gas turbine as a bottoming 

cycle to the SOFC. Results show that such a system may 

be feasible from a mass and volume perspective and the 

estimated emissions and noise benefits merit further 

consideration (refs. 2 to 4). 

The above results have focused on the cruise portion of the 

aircraft mission; the next step is to analyze system 

performance at different points across the operating envelope. 

This paper extends the earlier analysis by including full power 

(440 kW) system performance for ground operations and part 

power (250 kW) operation at cruise. The power levels are 

typical of a 300 passenger commercial transport aircraft and 

are based on figure 1 (ref. 5), a projected power profile for 

each of the various mission segments. The 250 kW level is 

approximately the lowest operational requirement of the 

mission. 

Analysis of both sea level and cruise conditions is required 

due to the corresponding ambient temperature and pressure 

change. Based on the 1976 International Standard Atmosphere, 

the temperature changes from 288 K at sea level to 216 K at 

12,500 m (41,000 ft), a typical cruise altitude for a modern 

long-range civil transport. The corresponding pressure changes 

are even more dramatic, ranging from 1 to 0.178 bar at sea 

Figure 1.—Projected net electrical power requirements at 

various mission stages (from (ref. 5) with permission). 

level and cruise, respectively. The propulsion engines must 

perform throughout these variations and design compromises 

are made to ensure acceptable performance at all conditions. 

For the power system, similar compromises are required. 

However, this analysis uses the airplane cabin exhaust as its air 

source, greatly reducing the effect of the varying atmospheric 

pressure. For passenger comfort, the onboard Environmental 

Control System (ECS) regulates the temperature and pressure 

of the cabin to 293 K and 0.75 bar at cruise. Typically 

50 percent of the air is recirculated while the remainder is sent 

overboard. The air sent overboard is sufficient for the 

SOFC/GT power system, and is therefore used throughout the 

mission. As a result, the power system inlet pressure variation 

is only from 1 to 0.75 bar, rather than 1 to 0.178 bar, 

simplifying the compressor design and boosting the system 

thermal efficiency at cruise. Accounting for the ECS 

compressor work, which requires a significant amount of 

power, is outside the scope of this current work since the ECS 

is used for both the fuel cell and gas turbine power units. A 

broader aircraft-level study that accounts for all power 

requirements and power production methods is justified for a 

more integrated overall power system. 

Operating at different altitudes will also affect the system 

exhaust pressure. For this analysis, the exhaust pressure is the 

ambient pressure at all cases. The exhaust pressure governs the 

turbine expansion ratio and will therefore have a dramatic 

effect on this system performance. This effect will prove to be 

a major driver in this system design. 

Terrestrial power production, another potential application 

for the SOFC/GT hybrids, must successfully operate at varying 

ambient and part-power conditions as well. Some research 

groups have begun to evaluate terrestrial-based hybrid systems 

at these conditions and have suggested control strategies based 

on the model results (ref. 6). Transient analyses of system 

startup, shutdown, and power load changes have also gained 

attention due to the many related challenges and applicability 

for all hybrid systems (refs. 7 to 9). All of these conditions 

may substantially affect the design of the system. 

A final necessary note is that future SOFC technology is 

projected for this application and specifically for this analysis. 

A typical timeline for a new aircraft development, especially a 

large civil transport, is on the order of 10 to 20 years. Systems 

analyses such as the following are required at this early stage 

and projections for both technology and the application are a 

challenging part of the design process. SOFC and SOFC/GT 

hybrid technology have made progress over the past decades 

but further maturity is necessary before an application such as 

this is feasible. Systems studies may extrapolate the capability 

of a technology, but the studies also quantify the necessary 

gains that must be made for feasibility. A more thorough 

discussion for this application can be found in (ref. 3). 

Nomenclature 

ETOPS Extended-Range Twin-Engine Operations 

SOFC Solid Oxide Fuel Cell 

SOFC/GT Solid Oxide Fuel Cell/Gas Turbine Hybrid
NASA/TM—2005-213805 3

ECS Environmental Control System 

ASR Area Specific Resistance 

O/C Molar atomic oxygen-to-carbon ratio 

H2O/C Molar steam-to-carbon ratio 

PRtt Total-total pressure ratio 

W Flow rate, kg/s 

Wc Corrected flow, kg/s, δ

θ Wc = W

Nc Corrected speed, rpm, 

θ = N Nc

θ Temperature ratio, Tref

T θ =

δ Pressure ratio, Pref

P δ =

T Temperature, K 

P Pressure, psi 

Tref Reference temperature, 273 K 

Pref Reference pressure, 14.696 psi 

ηtt Total-total adiabatic efficiency 

Model Description 

The hybrid configuration, presented in further detail in 

(refs. 3 and 4), is shown in figure 2. It is a bottoming cycle in 

which the remaining fuel from the exhaust of the SOFC system 

is burned in a small combustor to produce additional heat 

energy for two heat exchangers and the turbine. The turbine 

drives a compressor that compresses the air for both the fuel 

cell and reformer. Any residual turbine power is converted to 

electrical power using a generator. The reformer is based on 

partial-oxidation chemistry though some water is added to 

prevent coking. Both the compressor and turbine are singlestage radial designs. The fuel and water pumps are simple low 

pressure, low flow rate, liquid pumps whose required work is 

negligible compared to that of the gas compressor. The mass 

Figure 2.—Schematic of the SOFC/GT hybrid system. 

and volume of each of the pumps are considered in the system 

mass estimate, however. The air heat exchanger is a compact 

gas-gas crossflow heat exchanger and the steam generator is a 

multi-pass gas-liquid design. No recycling of either the anode 

or cathode streams is considered to simplify the initial studies 

and establish a baseline fuel cell system with which to 

compare more complex versions in the future. 

As this is a system-level study, there are some simplifying 

assumptions to reduce the computational time required. All 

chemistry is assumed to be at equilibrium and the Chemical 

Equilibrium for Applications (CEA) code (ref. 10) is used for 

the calculations and related properties. The fuel cell model is 

zero-dimensional and based on the work of Hartvigsen et al. 

(ref. 11). This fuel cell model assumes that variation of 

performance with temperature and pressure is solely due to the 

Nernst equation. A constant ASR value is used and is assumed 

to be the dominant polarization term. The activation 

polarization is typically minimal for an SOFC and, for the 

operating voltages used in this application, the concentration 

polarization is small as well. The fuel cell is kept at a constant 

inlet temperature and while the exit temperature changes as a 

function of airflow and efficiency, the temperature averaged 

over the stack does not change enough to warrant a more 

detailed model. Similarly, the fuel cell inlet pressure variation 

is small enough that the dominant corresponding performance 

effect is from the Nernst equation. The turbomachinery is also 

zero-dimensional and based on non-isentropic theory. The heat 

exchangers are modeled using the compact heat exchanger 

methods from Kays and London (ref. 12). All components are 

considered adiabatic to the environment and a nominal 

pressure drop of 2 percent is included for the reformer and heat 

exchangers. The SOFC includes a 1.5 percent pressure drop. 

All models and results presented are at steady-state conditions. 

The transition from design-point modeling to off-design 

performance modeling involves different approaches to some 

of the component models. For example, at the design 

condition, the fuel cell model sizes the required total active 

area to meet an input voltage and power requirement using the 

Nernst equation and an input stack-level area-specific 

resistance. At off-design conditions, this area is now fixed and 

the input power level governs the new operating voltage and 

current density. Similarly, the heat exchanger areas are sized at 

the design point using the required temperature changes, and at 

off-design, the performance is calculated at the same area. The 

compressor and turbine off-design performance is modeled 

using performance maps that correlate flow, pressure ratio, 

rotational speed, and efficiency. The maps used in this analysis 

are based on mean-line flow-path computational models 

underpinned by traditional empiricism. Finally, the nozzle area 

is sized at the design point and kept constant at off-design. If 

necessary, a variable-area nozzle is used, where the exhaust 

area is allowed to vary at off-design to match the turbine exit 

conditions. 

The model is governed by several physical constraints that 

guide the system at both design and off-design conditions. 

First, the fuel cell temperature difference from inlet to exit is 

kept below 150 K. This value is imposed by material 

constraints where the thermal expansion coefficients of the 
NASA/TM—2005-213805 4

different cell materials may cause too much stress for the 

design. The SOFC is air-cooled and the airflow is increased if 

necessary to satisfy this requirement. The reformer chemistry 

is another important constraint that can cause coking if not met 

at all conditions. For this system, the molar atomic oxygen-tocarbon ratio (O/C) is kept at 0.95 and the molar steam-tocarbon ratio (H2O/C) is kept at 0.70. These values are based on 

prior analysis at NASA Glenn Research Center. From 

equilibrium calculations, the mixture provides a slightly 

exothermic reaction and is far from coke-producing chemistry 

that would foul the reformer and SOFC. The jet fuel is 

characterized as C12H23 with a lower heating value of 

43.03 MJ/kg, as specified in the CEA manuals (ref. 10). At the 

system level, this simplified jet fuel model compares well with 

more detailed models such as a multi-component hydrocarbon 

assay that characterizes a distillation curve (ref. 3). The O/C 

ratio is met by varying the airflow split after the compressor. 

For an actual hardware system, it is assumed that a control 

valve would be used. The H2O/C ratio is met by varying the 

water flow into the system. The combustor chemistry is also 

kept constant at stoichiometric conditions throughout the 

operating profile. This constraint is met by varying the postcathode flow split, again assuming a control valve. 

Results and Discussion 

Full Power (440 kW) Analysis 

The original system design was at the cruise operational 

point and based on a net electrical power of 440 kW. The 

configuration is shown in figure 2. The compressor pressure 

ratio and turbine expansion ratio, respectively 2.94 and 6, were 

chosen to maximize system efficiency while remaining within 

structural limits for single-stage radial designs. The fuel cell 

was sized based on this cruise condition and a cell voltage of 

0.75 V. A system thermal efficiency (LHV) of 56.1 percent 

was calculated. Further explanation of the system and all of the 

components can be found with (refs. 3 and 4). 

The changes from cruise to sea level for the electrical power 

system are comparatively simple but are still very challenging 

for such a complex system. The compressor inlet pressure 

changes from 0.75 to 1 bar and the turbine exit pressure 

changes from 0.178 to 1 bar, at cruise and sea level, 

respectively. At sea level, the turbine cannot expand the gases 

nearly as much and for a constant power level, the fuel cell 

must produce more power as a result. The increase in fuel cell 

power implies an increase in fuel cell heat produced, even at 

constant or increasing fuel cell efficiency. With an air-cooled 

fuel cell, which is the standard for SOFC designs, more air 

must be sent to the fuel cell and therefore compressed as well. 

This creates a challenging thermodynamic system, where less 

available turbine power due to the higher ambient pressure 

implies more compressor power to supply a higher power fuel 

cell, which requires more turbine power to balance the 

compressor. 

Since the sea level condition requires more airflow than 

cruise, that condition should be the 100 percent rotational 

speed design point rather than the original design in which the 

cruise condition sized the hardware. The cruise condition 

would therefore be run at a reduced speed and lower 

compressor pressure ratio, as shown in figures 3 to 6. As a 

result, the fuel cell operates at a lower inlet pressure and 

therefore slightly reduced performance when compared to the 

original cruise design point system. 

Another implication of the different air flow rates and fuel 

cell power levels between sea level and cruise is that the fuel 

cell inlet-exit temperature difference is actually less than the 

150 K requirement described earlier. At the sea level design 

point, this ∆T requirement sets the airflow. However, at cruise, 

the compressor/turbine balance sets the airflow, which is 

higher than necessary to cool the SOFC. This is a positive and 

conservative benefit in that a lower temperature gradient is 

better for the fuel cell material thermal stress profile. 

Figure 3.—Compressor map relating total-total pressure ratio, 

corrected flow, and corrected, non-dimensional shaft speed. 

The sea level full power, cruise full power, and cruise 

part power operating points are plotted. 

Figure 4.—Compressor map relating adiabatic efficiency, 

corrected flow, and corrected, non-dimensional shaft speed. 

The sea level full power, cruise full power, and cruise 

part power operating points are plotted. 
NASA/TM—2005-213805 5

The reduction in compressor pressure ratio at cruise also 

reduces the required turbine work to balance the cycle, 

counteracting the effect of the increased fuel cell cooling 

requirement described above. The turbine operation at the new 

rotational speed profile is acceptable in that the expansion ratio 

of 6 at cruise does not need to be at 100 percent speed due to 

the choked condition, seen in figure 5. The sea level point is at 

a lower expansion ratio to account for the higher ambient 

pressure and the corrected flow is lower as a result. This may 

seem counterintuitive, as the compressor corrected flow 

increases from cruise to sea level. The reason is that while the 

actual flow rate increases from cruise to sea level, the turbine 

inlet pressure increases more than the square root of the turbine 

inlet temperature increases, resulting in a lower corrected flow 

rate for the turbine. 

The fuel cell is sized at the highest required power condition 

(sea level 440 kW). At the lower fuel cell power 440 kW 

cruise condition, the voltage increases and the current density 

decreases for a more efficient and lower heat-producing fuel 

cell. The two full-power operating points, along with the 

part power operating point described below, can be seen in 

figure 7. No polarization curve is shown because the 

temperature and pressure conditions change between sea level 

and cruise conditions. The 0.75 V operating point is now set by 

the sea level condition, and the cruise point results in a cell 

voltage of about 0.84 V. 

Finally, the nozzle was initially assumed to be a constant 

area convergent design for simplicity. However, due to the 

differences in operating points, a variable area nozzle is now 

used to balance the flow requirements. This will require further 

investigation in a transient and control environment. In 

addition, the integration of the compressor, turbine and nozzle 

system requires further engineering to ensure performance 

throughout the mission profile. 

Part Power (250 kW) Analysis 

The change from 440 to 250 kW at cruise is less challenging 

when compared to a change from cruise to sea level because 

the external pressure and temperature interfaces do not change. 

The fuel cell power requirement decreases along with the 

overall power requirement and because of the favorable 

exhaust pressure, the turbine is able to provide a substantial 

amount of excess power (about 33 percent of the net total). To 

meet the lower fuel cell power requirement, the voltage 

increases while the current density decreases, as shown in 

figure 7. The higher voltage again implies an increased fuel 

cell efficiency and therefore reduced fuel cell cooling 

requirement. The airflow, as a result, is lower than the cruise 

full power level and the primary challenge is the balancing of 

the compressor and turbine at this airflow. 

To reduce the airflow in the turbomachinery, the shaft speed 

is lowered beyond the cruise full power shaft speed. This also 

reduces the available compressor pressure ratio though there is 

still some flexibility in turbine expansion ratio due to the 

choking condition shown in figure 5. By maximizing the 

Figure 5.—Turbine map relating total-total expansion ratio, 

corrected flow, and non-dimensional corrected speed. The 

sea level full power, cruise full power, and cruise 

part power operating points are plotted. 

Figure 6.—Turbine map relating total-total expansion ratio, 

adiabatic efficiency, and non-dimensional corrected speed. 

The sea level full power, cruise full power, and cruise 

part power operating points are plotted. 

Figure 7.—SOFC sea level full power, cruise full power, and 

cruise part power operating voltage vs. current density. 
NASA/TM—2005-213805 6

Figure 8.—Efficiency and voltage as a function of air flow rate 

for the cruise part power condition. The shaft speed is kept 

constant. The highest air flow rate shown is the 

point chosen for the other plots. 

expansion ratio, the net generator electrical power is 

maximized and therefore the required fuel cell power is 

minimized. This is a positive effect on the system thermal 

efficiency, despite the more efficient fuel cell stack doing less 

work. This counterintuitive logic is due to the turbine taking 

advantage of the available pressure difference with the 

atmosphere. The fuel cell continues to increase efficiency with 

less power requirement so therefore the fuel flow rate actually 

decreases with increased turbine work. 

Two independent variables are available to maximize the 

turbine expansion ratio: shaft speed and air flow rate. Figure 8 

provides an example of varying air flow rate at a constant shaft 

speed and its effect on efficiencies and the fuel cell voltage. By 

increasing the air flow rate, the turbine is slowly increasing its 

expansion ratio (based on fig. 5) until the choking point near 

0.8 kg/s air flow rate. In the choking region, the expansion 

ratio increases more quickly and the system efficiency 

increases as a result. The expansion ratio increase is limited by 

the compressor efficiency, which decreases rapidly in this 

region, shown in figure 4. An interesting effect of this 

interaction is that both the compressor and turbine adiabatic 

efficiencies decrease as system thermal efficiency increases 

within this region. Since the system mass is constant at offdesign, system thermal efficiency is a likely primary figure of 

merit. 

System Summary 

A summary of the nodal data for each operating point is 

included in table 1. Some of the parameters listed describe the 

size rather than performance and therefore are constant 

throughout the operating conditions. An example of this is the 

fuel cell active area. This is sized at the design point (sea level 

full power) and is kept constant throughout the other two 

conditions. 

The sizing of each of the components provides enough data 

to estimate a system level mass and volume. Parametric 

models were developed for this purpose and are described in 

detail in (ref. 4). The results show that the choice of design 

point has a substantial effect on the system mass. The original 

system, designed at cruise full power, has an estimated mass of 

1396 kg while the system based on a sea level full power 

design point has an estimated mass of 1912 kg. Most of the 

change is due to a larger fuel cell, which is required due to the 

larger power requirement from the fuel cell at sea level. For an 

aerospace application, this change has important implications 

for the feasibility of the overall system and its application. 

With such a complex and interconnected system, there is 

most likely a better solution for operation of the system at all 

conditions. There are several independent variables that may 

improve the system including fuel cell anode utilization, fuel 

cell design voltage, system airflow, and rotational shaft speed. 

The anode utilization regulates the amount of unspent fuel that 

exits the anode of the fuel cell. This fuel is then reacted in the 

combustor to take advantage of its heat energy. The design 

point utilization (regardless of flight condition) is chosen at a 

maximum realistic value of 0.85. Maximizing the fuel 

utilization makes the required fuel cell active area smaller, 

which can be directly correlated to fuel cell mass. From 

(ref. 4), the fuel cell mass is the single heaviest component in 

the overall system. As the utilization decreases, the combustor 

exit temperature increases, but the turbine inlet temperature 

does not increase by much due to the high amount of airflow 

required by the fuel cell. As a result, this variable is not very 

effective in balancing the system, although with a different 

configuration, it may be more useful. The fuel cell design 

voltage has a large impact of the size of the fuel cell stack as 

well. Based on an input area-specific resistance and fuel cell 

thermodynamic conditions, the Nernst voltage and resulting 

polarization curves are sized from the input design voltage. As 

the design voltage is decreased for the sea level design point, 

the required active area is reduced but the efficiency of the 

lower power cruise operating point is also reduced. The “best” 

design voltage for this system depends on the all of the 

operating points and their relative length of operation. 

The system airflow and rotational shaft speed, described 

within the part power section as available independents, may 

also be effective for other off-design operating points, 

including the full power cruise point. The models show that 

multiple balanced solutions are available based on different air 

flow rate and shaft speed combinations. The selection is 

limited to viable operating points on each of the maps, but as 

seen in the part power case, optimizing the system operation 

for efficiency is possible and effective. 

There are other approaches to balancing the system as well 

as other configurations that may reduce the size or even 

eliminate entire components. Further analysis of such a system 

would include an evaluation of the entire mission including 

ground operations. The figure of merit would be a combination 

of mass and efficiency, where in-flight efficiency is accounted 

for in terms of fuel and water mass saved. Finally, the idea of 

associating a design point with a certain operating point may 

no longer be necessary with more powerful computers and 
NASA/TM—2005-213805 7

models. For example, the optimum fuel cell size may be 

somewhere between optimum for cruise and optimum for sea 

level. This again depends on the figures of merit and the 

specific mission. The migration towards multidisciplinary 

optimization is directly applicable to these systems and may 

prove in the future to be the best approach for complex hybrid 

systems design. 

Conclusions 

A systems-level model of a solid-oxide fuel cell/gas turbine 

hybrid bottoming cycle is analyzed and designed by evaluating 

the sea level and cruise full power operating points as well as a 

cruise part power operating point for a 300 passenger 

commercial aircraft electrical power unit. Each component 

within the system is sized to meet the 440 kW input electrical 

load at sea level full power. The cruise full power and cruise 

part power points are treated as off-design conditions. The 

discussion shows the risk in designing or judging a system 

based on just one operating point for a mission in which the 

ambient conditions are variable. While the system results are 

still promising, further gains can be made through continued 

analysis, system design, and optimization. In addition, more 

detailed analysis and hardware testing are necessary to account 

for such challenges as turbomachinery integration, transient 

operation, and localized chemical, thermal, and pressure 

gradients, all of which may also affect the final conclusions. 

References 

1. FAA Advisory Circular No. 120–42A, “Extended Range 

Operation with Two-Engine Airplanes (ETOPS),” Federal 

Aviation Administration, 1988. 

2. Daggett, D., Freeh, J., Balan, C., Birmingham, D., “Fuel 

Cell APU for Commercial Aircraft,” 2003 Fuel Cell 

Seminar Abstracts, Miami, FL, USA, 2003. 

3. Steffen, C.J., Jr., Freeh, J.E., Larosiliere, L.M., “Solid 

Oxide Fuel Cell/Gas Turbine Hybrid Cycle Technology 

for Auxiliary Aerospace Power,” ASME GT2005–68619, 

ASME Turbo Expo 2005, Reno, NV, USA, 2005. 

4. Tornabene, R., Wang, X.Y., Steffen, C.J., Jr., Freeh, J.E., 

“Development of Parametric Mass and Volume Models 

for an Aerospace SOFC/Gas Turbine Hybrid System,” 

ASME GT2005–68334, ASME Turbo Expo 2005, Reno, 

NV, USA, 2005. 

5. Daggett, D., Eelman, S., and Kristiansson, G., “Fuel Cell 

APU for Commercial Aircraft,” AIAA–2003–2660, AIAA 

International Air and Space Symposium and Exposition: 

The Next 100 Years, Dayton, OH, USA, 2003. 

6. Magistri, L., Bozzolo, M., Tarnowski, O.C., Agnew, G.D., 

Massardo, A., “Design and Off-Design Analysis of a MW 

Hybrid System Based on Rolls-Royce Integrated Planar 

SOFC,” ASME GT2003–38220, ASME Turbo Expo 

2003, Atlanta, GA, USA, 2003. 

7. Magistri, L., Ferrari, M.L., Traverso, A., Costamagna, P., 

Massardo, A.F., “Transient Analysis of Solid Oxide Fuel 

cell Hybrids, Part C: Whole-Cycle Model,” ASME 

GT2004–53845, ASME Turbo Expo 2004, Vienna, 

Austria, 2004. 

8. Roberts, R.A., Brouwer, J., Liese, E.A., Gemmen, R.S., 

“Dynamic Simulation of Carbonate Fuel Cell-Gas Turbine 

Hybrid Systems,” ASME GT2004–53653, ASME Turbo 

Expo 2004, Vienna, Austria, 2004. 

9. Kemm, M., Hildebrandt, A., Assadi, M., “Operation and 

Performance Limitations for Solid Oxide Fuel Cells and 

Gas Turbines in a Hybrid System,” ASME GT2004–

53898, ASME Turbo Expo 2004, Vienna, Austria, 2004. 

10. Claus, R.W., McBride, B.J., Foster, R. “Application of a 

Chemical Equilibrium Analysis to In-Situ Lunar Resource 

Utilization,” 1st AIAA Space Exploration Conference: 

Continuing the Voyage of Discovery, Orlando, FL, USA, 

2005. 

11. Hartvigsen, J., Khandkar, A., Elangovan, S., 

“Development of an SOFC Stack Performance Map for 

Natural Gas Operation,” Proceedings of the 

Electrochemical Society, PV 99–19, Honolulu, HI, USA, 

1999. 

12. Kays, W.M. and London, A.L., Compact Heat 

Exchangers, 3rd ed., Kreiger Publishing Co., Florida, 

USA 1998. 
NASA/TM—2005-213805 8

TABLE 1.—SEA LEVEL FULL POWER, CRUISE FULL POWER, AND CRUISE PART POWER 

SYSTEM RESULTS. THE PHYSICAL PARAMETERS, SUCH AS “ACTIVE AREA PER 

CELL”ARE CONSTANT FOR EACH CONDITION SINCE THE 

SYSTEM IS SIZED AT SEA LEVEL FULL POWER. 

System Sea Level, 440kW Cruise, 440kW Cruise, 250kW

SOFC net electrical power 429 kW 279 kW 168 kW

Generator net electrical power 11 kW 161 kW 82 kW

Total net electrical power 440 kW 440 kW 250 kW

Fuel flow rate (Jet-A) 0.0241 kg/s 0.0141 kg/s 0.00798 kg/s

Water flow rate 0.0219 kg/s 0.0128 kg/s 0.00722 kg/s

Air flow rate 1.81 kg/s 1.01 kg/s 0.816 kg/s

System thermal efficiency (LHV) 42.4% 72.6% 72.8%

Air preheater heat duty 963 kW 579 kW 501 kW

Steam generator heat duty 57.0 kW 33.3 kW 18.8 kW

SOFC

Active area per cell 324 cm2 Same as sea level Same as sea level

Average Nernst voltage 0.985 V 0.971 V 0.965 V

Cell voltage 0.750 V 0.834 V 0.887 V

Number of cells per stack 149 Same as sea level Same as sea level

Number of stacks 20 Same as sea level Same as sea level

Ideal stack voltage 112 V 124 V 132 V

Current density 589 mA/cm2 344 mA/cm2 195 mA/cm2

Inlet temperature 700 C 700 C 700 C

Exit temperature 850 C 837 C 788 C

Inlet pressure (cathode) 2.86 bar 1.35 bar 0.979 bar

Exit pressure (cathode) 2.82 bar 1.33 bar 0.964 bar

Specific power 0.276 kW/kg 0.276 kW/kg 0.157 kW/kg

Power density 1.11 kW/L 1.11 kW/L 0.632 kW/L

Anode utilization 85.0% 85.0% 85.0%

Cathode utilization 9.55% 10.10% 6.83%

Reformer

H2O/C ratio 0.70 0.70 0.70

O/C ratio 0.95 0.95 0.95

Exit temperature 700 C 700 C 700 C

Inlet pressure 2.86 bar 1.35 bar 0.979 bar

Exit pressure 2.81 bar 1.32 bar 0.959 bar

Reactor exotherm 31.4 kW 4.16 kW -0.09 kW

Compressor

Pressure ratio (total-total) 2.88 1.83 1.33

Inlet temperature 20 C 20 C 20 C

Exit temperature 144 C 93.5 C 63.8 C

Adiabatic efficiency (total-total) 83.1% 74.7% 56.5%

Catalytic combustor

Inlet temperature 850 C 837 C 788 C

Exit temperature 1170 C 1156 C 1077 C

Estimated eq. ratio 1.00 1.00 1.00

Inlet pressure 2.76 bar 1.30 bar 0.945 bar

Exit pressure 2.76 bar 1.30 bar 0.945 bar

Turbine

Expansion ratio (total-total) 2.37 6.00 3.66

Inlet temperature 412 C 359 C 246 C

Exit temperature 294 C 145 C 110 C

Adiabatic efficiency (total-total) 83.7% 87.7% 86.9%

Nozzle

Exit area 163 cm2 354 cm2 180 cm2

Exit static pressure 1 bar 0.178 bar 0.178 bar

Pressure ratio (total-static) 1.10 1.16 1.38
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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NASA TM—2005-213805

FUELCELL2005–74099

E–15163

WBS–22–066–20–03

14

Off-Design Performance Analysis of a Solid-Oxide Fuel Cell/Gas Turbine Hybrid

for Auxiliary Aerospace Power

Joshua E. Freeh, Christopher J. Steffen, Jr., and Louis M. Larosiliere

Solid oxide fuel cell; Simulation; Turbomachinery; Aircraft power supplies

Unclassified -Unlimited

Subject Category: 07

Prepared for the Third International Conference on Fuel Cell Science, Engineering, and Technology cosponsored by the

ASME, ASME Nanotechnology Institute, Mechanical Engineering Magazine, RIT, NFCRC, Kettering University, SME,

and Fuel Cell Markets, Ypsilanti, Michigan, May 23–25, 2005. Joshua E. Freeh and Christopher J. Steffen, Jr., NASA

Glenn Research Center; and Louis M. Larosiliere, General Electric Gas Turbines, 300 Garlington Road, Greenville,

South Carolina 29602. Responsible person, Joshua E. Freeh, organization code PBP, 216–433–5014.

A solid-oxide fuel cell/gas turbine hybrid system for auxiliary aerospace power is analyzed using 0-D and 1-D system-level models.

The system is designed to produce 440 kW of net electrical power, sized for a typical long-range 300-passenger civil airplane, at both

sea level and cruise flight level (12,500 m). In addition, a part power level of 250 kW is analyzed at the cruise condition, a requirement

of the operating power profile. The challenge of creating a balanced system for the three distinct conditions is presented, along with the

compromises necessary for each case. A parametric analysis is described for the cruise part power operating point, in which the system

efficiency is maximized by varying the air flow rate. The system is compared to an earlier version that was designed solely for cruise

operation. The results show that it is necessary to size the turbomachinery, fuel cell, and heat exchangers at sea level full power rather

than cruise full power. The resulting estimated mass of the system is 1912 kg, which is significantly higher than the original cruise

design point mass, 1396 kg. The net thermal efficiencies with respect to the fuel LHV are calculated to be 42.4 percent at sea level full

power, 72.6 percent at cruise full power, and 72.8 percent at cruise part power. The cruise conditions take advantage of pre-compressed

air from the on-board Environmental Control System, which accounts for a portion of the unusually high thermal efficiency at those

conditions. These results show that it is necessary to include several operating points in the overall assessment of an aircraft power

system due to the variations throughout the operating profile.