IEEE Electrification Magazine - December 2017 - 74

studied (Garcia-Arregui 2007, Morin et al. 2014) for associating fuel cells with ultracapacitors. Here, the focus is on
the architecture that uses two dc-dc converters: one for
the fuel cell and the other for the ultracapacitor.

Description of the Hybrid System

max (Power) - mean (Power)
max (Power)
70e3 - 40e3
=
= 0.42.
70e3

OVSR =

(1)

An OVSR close to zero means that power consumption does not vary, while an OVSR close to one means
that the sizing power is used only occasionally. In addition, generally speaking, a fuel cell requires fluidic auxiliaries with a slow dynamic response, whereas in an
emergency mission, the required power must be available instantly. As a result, hybridization appears to be
necessary in this case.
The new avionics standard that seems to be appearing in terms of the voltage level is a dc bus voltage of
+270 Vdc/0 Vdc/−270 Vdc (Eid et al. 2010, Ravel 2009). The
ultracapacitor is a good candidate for hybridization due
to its qualities: high specific power, easy management
of the state of charge, long lifetime and life cycles, and
enough storable/returnable energy for our mission profile (Roboam et al. 2011, Langlois 2006, Maxwell Technologies Inc.).
Several solutions are possible for associating a fuel cell
with ultracapacitors, as proposed by Garcia-Arregui. We
take various criteria into account to classify the solutions:
weight, gas consumption, stability of the dc bus, ease of use,
fuel cell load current, filtering capacity of the ultracapacitors, and system reliability. According to Garcia-Arregui, the
solution that offers the best compromise for our specifications is the one that uses two static converters: one for the
fuel cell and the other for the ultracapacitors (Figure 2).
This solution has the best bus stability and filtering
capacity as well as middling weight and gas consumption, but its use and reliability are yet to be proven as the
architecture puts a lot of strain on the power electronics.
We used several criteria to initially size the solution. The issue
+270 V -270 V
was to determine the optimal numdc Bus
ber of cells to be connected in series
to create the fuel cell and their surface area. For example, it is possible
dc-dc Fuel Cell
Auxiliaries
to oversize the cell surface to redc/dc Auxiliary
Converter
Converter
duce gas consumption. Weight is
therefore saved for storing gas, but
the fuel cell is made heavier so a
compromise must be reached. An
Fuel Cell
important criterion in sizing the
dc-dc Storage
power electronics is the minimum
Converter
voltage of the fuel cell. At equivalent
power, the lower the cell voltage,
the higher the current flowing
through the semiconductors.
As for the stored energy, it is
Storage
important to ascertain its capacity
and its minimum reference voltage,
Figure 2. A solution using two dc-dc converters for the hybridization of a fuel cell and
ultracapacitors.
allowing for a discharge of less than
Various loads used in an emergency may be considered
constant. This is because the electronic equipment
(computers) and anti-icing items absorb power that is
constant over time for the duration of the emergency
mission. Other equipment, such as fuel pumps, radio
equipment, and lights, absorb constant power when
used. However, some loads are very variable, in particular the flight control actuators, which require power that
fluctuates substantially depending on the flight conditions and practically throughout the whole mission.
During the ground approach phase, the aircraft's speed is
reduced, which means that a significant deflection of the
flight control surfaces is required to direct the aircraft. This
results in considerable power consumption. In addition,
prior to landing, several maneuvers are performed by the
pilot to adjust the position of the aircraft with respect to
the target. The flight mission we selected (see Figure 1) is
the low-speed ground approach phase of an aircraft flying
in a turbulent environment with maneuvers controlled
by the pilot. We defined this as the main mission for sizing our study (Garcia-Arregui 2007), i.e., highly variable
power consumption that reaches the highest consumption peaks.
This mission profile has a mean power of 40 kW and
consumption peaks reaching 70 kW, i.e., a 30-kW difference between the mean power to be supplied and the
maximum sizing power for the system. An oversizing rate
(OVSR) can be defined as follows:

74

I E E E E l e c t r i f i cati o n M a gaz ine / DECEMBER 2017



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