contribution while increasing system efﬁciency. To achieve opti-
mal efﬁciency, the higher efﬁciency device, the FC, should con-
tribute the greatest to the system output. This does not imply that
using a smaller compressor will make a hybrid system perform
better; often this would cause the hybrid to fail completely. What
this trend implies is that designing a system that requires less air
ﬂow, and therefore a smaller compressor, implies an increase in
FC efﬁciency. From a capital cost perspective, the turbomachinery
will be cheaper, per kW, than the FC stack and related compo-
nents, and therefore, designs maximizing the power from the tur-
bine will likely reduce the initial system cost.
The primary considerations when designing a hybrid FC/GT
system are stack-power density, operating temperature, stack tem-
perature rise, system pressure, fuel utilization, and the relative
size of the turbomachinery. These design selections and the FC
structure and material set, determine the operating voltage and
therefore the operating efﬁciency of the FC. These decisions, in
turn, determine the air-ﬂow requirements and heat available to
drive the turbomachinery, with the difference in preheating and
air ﬂow provided by the cathode recirculation. High voltage, typi-
cally achieved by operating at low power density, resulted in the
highest achievable system efﬁciency but the largest necessary FC
stack size. Higher system pressure improves voltage and efﬁ-
ciency but requires sturdier components and applies mainly to
large systems utilizing axial ﬂow turbomachinery. Typical axial
turbines are designed for high-pressure ratios, but utilization of
the low pressure spool only could produce system pressures ame-
nable to SOFC integration. System pressures between 4 and 8
atmospheres would bring the design within the operating regime
of existing hardware. Higher fuel utilization actually has a nega-
tive impact on fuel cell voltage for the conﬁgurations considered
here but improves system performance by employing more fuel in
the electrochemical reactions. It is important to note that the efﬁ-
ciency penalty associated with reduced fuel utilization is less in a
hybrid system than in a stand-alone FC system. It is extremely
likely that the optimal operating condition for a speciﬁc FC stack
will be at lower fuel utilization when hybridized with a gas tur-
bine. The side beneﬁts of lower fuel utilization include reduced
degradation effects, less chance of fuel starvation, more even spa-
tial current and temperature distributions in the stack, and greater
dynamic operating ﬂexibility. Higher operating temperatures
reduce ionic resistance, increase the turbine inlet temperature
closer to nominal conditions, and raise overall hybrid system efﬁ-
ciency. The drawbacks of high temperature operation include
accelerated voltage degradation and the requirement of potentially
exotic interconnect and sealant materials. An optimal system may
be able to achieve ultrahigh fuel-to-electricity conversion efﬁ-
ciency but fall short of economic viability. The fuel cell stack rep-
resents the single largest capital cost, so that minimizing the stack
size requirement reduces cost signiﬁcantly. Achieving size reduc-
tions primarily occurs by raising power density. In the current
study, increasing power density from 400 to 500 mW/cm
the stack size by 25%, with only a 2.6% efﬁciency penalty. Elimi-
nating the air preheating heat exchanger reduces the cost signiﬁ-
cantly but may diminish the ability to sufﬁciently control stack
operating temperature. Replacing air preheating with additional
compression heating raises efﬁciency if the fuel cell can safely
handle the pressure without cracking. Increased pressure improves
power density, allowing for additional trade-offs between efﬁciency
and reducing stack size even further. Optimizing the system with
cost-minded design choices can produce a highly efﬁcient (65%
LHV or better) system with a substantially higher speciﬁc power
output than a stand-alone FC system.
The authors thank the U.S. Department of Defense Fuel Cell
Program and Mr. Frank Holcomb of the Construction Engineering
Research Laboratory of the Engineer Research and Development
Center for partial support of the current work under contract num-
¼speciﬁc heat (constant pressure, constant volume)
Flow ¼turbomachinery ﬂow rate
N¼normalized turbomachinery parameter
PR ¼pressure ratio
Q¼sensible enthalpy of ions
RPM ¼shaft speed
¼universal gas constant
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041005-10 / Vol. 10, AUGUST 2013 Transactions of the ASME