IEEE Electrification Magazine - June 2020 - 39

A recent analysis
from the literature
revealed that an
FC/GT system has
a limited dynamic
capability due to
the dynamic
characteristics of
the combined system.

the combined system. According to
Figure 12, the turbine is driving both
the compressor and generator. During
load transient, the GT shaft speed
drop might lead to system shutdown.
A load governor for the generator can
be proposed to control the steam
flow to avoid shutdown. A load governor is typically designed to minimize
the load tracking error according to
the system constraints. In a stepdown load, the load governor injects
a signal to bypass the steam, preventing the low speed and avoiding system shut down. In load transient, the
load governor is able to compensate
for the load instead of sending the
signal to increase more oxygen from the air compressor,
which can improve FC efficiency. However, designing a
load governor for the complex hybrid SOFC/GT could be
very computationally demanding for load transients.
This oxygen starvation and overloading during the transient condition can be avoided by restricting its current
dynamics by using ESSs to improve the system performance when electrical loads at a dc bus demand high
power in a short time. Different technologies can be used
as the auxiliary power source to the FC. Then, the FC can
operate near the steady-state condition to both avoid
mechanical stresses and circumvent fast changes of FC
current. Batteries and supercapacitors that have faster
dynamics than FCs are mostly used in combination with
FCs to manage fast transient loads and regulate the dc bus
voltage. Since the response time of an supercapacitor and
battery is faster than that of an FC, both the FC and the
compressor will have time to adjust to the new power level.
In some configurations used to increase the efficiency
of the system, a combination of several ESSs, which is
known as a hybrid ESS (HESS), is used to take advantage of
each ESS. For instance, HESSs that includes an ultracapacitor and a battery can combine the advantages of both.
Although the supercapacitor can supply high specific
power, it cannot store a significant amount of energy.
Unlike supercapacitors, batteries have higher specific
energy; however, the specific power is lower in batteries.
Then, the combined utilization of batteries and supercapacitors may be a proper hybridization system in terms of
high energy and high power density. Then the FC can be
assisted by the presence of ESSs to deliver the stored
energy on demand.

Control and Power Management
Designing proper control systems can make ships greener
and safer by reducing fuel consumption, decreasing
power fluctuation, and increasing the lifecycle. The successful operation of the hydrogen FC as a main source of
power in a shipboard power system highly depends on

advanced integrated control strategies. It is essential to understand the
control problems and identify the
best control variables to design control strategies. Some para--meters can
be controlled by the local FC controller and others through power electronic interfaces.
Knowing the auxiliary components' working principles can help
designers create a well-controlled
hydrogen FC. According to the dynamic phenomena occurring inside the FC
system, two types of balance should
be considered. First, depending on the
operating temperature, which is influenced by heat produced by the chemical reaction, the system design should consider the
temperature management of each cell. Second, the mass
balance, which depends on input flows, is another factor
that should be considered.
Temperature control in FC is usually taken care of by a
dedicated cooling system to remove the heat generated by
the chemical reaction by regulating both the exhaust flow's
temperature and the reactant's partial pressure by controlling valves. An optimal operating temperature must be
specifically chosen for each FC system since, in each FC
design, there is an optimal temperature. A higher operating temperature leads to more vaporized water, which is
the byproduct of the chemical reaction, and less liquid
water. Furthermore, a higher temperature means faster
kinetics and more thermodynamic stress. The optimal
temperature can be designed based on increasing the inlet
flows to cool down the stack and remove the external heat.
To achieve mass balance, a sufficient amount of reactants should be present in both electrodes of the FC (anode
and cathode) to sustain the chemical reaction and operate
at the mass transport limit for the reactant's pressure by
controlling the valves. However, this can make the FC's voltage drop relatively sensitive to small changes in reactant
concentration. Another challenging issue is that when the
cells are in series, the reactant flows are arranged in series
through the cells. Therefore, it is difficult to control the
reactant concentrations in each cell separately since the
reactant should first pass through all of the cells. In the case
of arranging cells in parallel, different flows may be the
result of the differences in construction details. In both
strategies, controlling the mass balance with the reactant
flow may result in low overall efficiency.
The simplest strategy could be to design a control strategy that properly controls the valves by regulating the
hydrogen pressure PH and oxygen partial pressure PO .
The hydrogen pressure, as one of the reactants, can be
regulated by a feedback loop based on the pressure measured by the PT. However, the closed loop for the oxygen
partial pressure control may be challenging because of the
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IEEE Electrification Magazine - June 2020

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