IEEE Electrification Magazine - June 2020 - 34

operating range, which is not beyond where the power
curve drops off.
The shape of the polarization curve depends on the
operating temperature. Figure 7 depicts the FC overall efficiency based on rated power. It is seen that the maximum
efficiency region is between 9-20 kW, which is from one
third to two thirds of the rated power.

Power System Architecture
A typical electric powertrain with FC is illustrated in Figure 8. The FC converts the chemical energy of its fuel to dc

1
Cell Voltage (V)
Power Density (W/cm2)

Power Density
0.8

Cell Voltage

0.6
0.4
0.2
0

500
1,000
Current Density (mA /cm2)

1,500

Figure 6. A typical FC polarization curve and related power density curve
for PEM.

Maximum
Optimum Operating Range

FC System Efficiency

0.9
0.8

Maximum
Efficiency

0.7

Maximum
Power

0.6
0.5
0.4
0.3
0.2
0.1
0

10
15
20
FC Power (kW)

Figure 7. The FC efficiency based on rated power.

FC
Fuel
FC
Air

Input Filter
IFC
+
VFC
-

Converter

Load Filter
LFC RFC

CFC

dc
dc

Figure 8. An electrical diagram of a typical FC powertrain.

dc Bus

electricity. The output voltage of the FC depends on the dc
current drawn. For several shortcomings of the FC, power
converters are often necessary to provide a stiff applicable
dc power source.
One of the main challenges of the FC is the produced
power. The typical output voltage of a single cell is about
1.2 V. As a result, it becomes necessary to stack many cells
in cascaded series and parallel forms to increase the
power capacity. Due to the FC's very low voltage, the
power electronic interfacing is also needed to boost the
voltage. To get rid of phase synchronization and large lowfrequency transformers, designers prefer integrating the
FC to the dc distribution with the dc-dc converter. This is
because the FC produces dc output power and, for optimum performance, it is preferred that a dc-dc converter
be used. Therefore, it is connected to the dc bus by a unidirectional dc-dc converter since there is no need for charging the FC. The dc-dc converter modifies the output
voltage to meet the load requirements in terms of power
density and load transients. Depending on the required
load, the converter should either decrease (buck mode) or
increase the FC voltage (boost mode). In the case of being
within the range of FC voltage, a buck-boost mode converter can be used. The polarization curve can also be
used to determine which type of converter should be chosen for better system performance.
As depicted in Figure 8, a filter capacitor C fc is installed
to mitigate the voltage changes at the input of the dc-dc
converter. It is also used to compensate for the influence
of the load changes on the FC. The FC is highly sensitive to
the high-frequency load changes, because the FC is not
able to regulate the pressure level as fast as is required to
react to the current variation.
However, the dc-dc converter may inject a ripple current into the FC. This capacitor may help mitigate the current ripples. Another shortcoming is that the FC is not
able to accept current in a reverse direction since it does
not have overload capability. A diode inserted in series
with the FC can be used to block the reverse current. On
the other hand, the efficiency decreases due to the additional losses caused by the diode. An inductive-capacitive
filter is also normally installed at the point of the load
converter to protect the power system from the load transients and oscillations, thereby improving the stability and
reliability of the system. Unlike the low-temperature FCs,
such as PEMFCs, when it comes to high-temperature FCs,

I E E E E l e c t r i f i cati o n M agaz ine / J UN E 2020

Cdc

CFC

dc
ac

Electric Propulsion

IEEE Electrification Magazine - June 2020

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