IEEE Electrification Magazine - June 2020 - 38

energy in hydrogen. It is considered that the efficiency of
the FC is between 45 and 60%, and the efficiency of the
electric power system is about 95-98%. As can be seen, the
efficiency for releasing hydrogen from compressed hydrogen 350 bar and 700 bar is 88 and 85%, respectively. For
releasing hydrogen stored as liquid and LOHC, the efficiency is 60 and 70%, respectively.
It has become a technical challenge to utilize the waste
heat produced. As depicted in Figure 2, LOHC, metal
hydrates, and ammonia need thermal energy to release
hydrogen. In high-temperature FCs, such as MCFCs and
SOFCs, the operating temperature is high enough temperature drive the reforming reaction.
A common solution for reusing waste heat is combined
heat and power (CHP), which is normally used in systems
producing heat to increase the overall efficiency by reusing
the waste heat. CHP typically has been used in thermal
power plants to generate electricity, and useful thermal
energy in a hybrid power system consists of a combination
of two or more power-generation technologies. This strategy can make efficient use of the power system's characteristics, which leads to the higher efficiency than could be
obtained from a single power source. For example, a combination of a high-temperature FC with a gas turbine (GT)
would increase the combined efficiency of the system.
Apart from using the waste heat for reforming the fuel,
the compressed air, which is drawn using an air compressor, can be heated by the exhaust heat of the GT before the
chemical reaction occurs on the cathode in an SOFC or
MCFC. The GT also drives the generator to generate electricity, which can be used for several purposes onboard.
The problem is that CHP can work well when it comes
to high-quality heat. Generally, the heat that is produced
by low-temperature FCs is not considerable. For instance,
the operating temperature of a PEM is between 50 and 85
°C, and the waste heat recovery in a PEM is not considered
in several works. However, there are several methods that
can be employed to use this small amount of heat that, if
met, could improve the efficiency.
A solution could be the organic Rankine cycle (ORC)
system, which has superior performance in the area of
reusing low-temperature waste heat. As in Figure 15, the

Fuel

FC
Stack

Grid
Interface

G
Generator

Turbine

Waste Heat

Air
Condensor
Pump
Figure 15. A schematic of the FC/GT hybrid power system.

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

ORC is based on the vaporization of a high-pressure
liquid. The waste heat, which is generated during the
electrochemical reaction inside the FC, is simultaneously
absorbed by the working fluid and becomes vapor. Then,
the vapor is passed through the turbine to produce electricity at the condensing pressure. After that, it is passed
through the condenser so that heat can be removed by the
water cooling system, where it is finally recondensed.
This strategy is able to increase efficiency by about 5% in
steady-state operation compared to that of the PEM FC
stack without a waste heat recovery system.
A microturbine generator needs a low turbine inlet
temperature, which can also be supplied with the exhaust
of a high-temperature FC and is well matched to the
requirements required for integration with high-temperature FCs in hybrid systems. When it comes to high-temperature FCs, there is at least a 22% gain in efficiency
when using the ORC for heat recovery.
Although the FC is an energy source with high specific
energy that provides reliable power at a steady state, it has
many dynamics modes that operate in different time
scales. The time scale for internal electrochemical and
thermodynamic characteristics is slow. As a consequence
of its slow dynamics, FC is not capable of responding to
electrical load transients as quickly as desired. Fast dynamic load demands lead to a notable voltage drop, which
causes FC stack shutdown. According to Table 1, PEMFCs
and SOFCs have higher power densities than other types of
FCs. However, in both systems, it is extremely challenging
to respond to quick load changes while maintaining a safe
and optimal operation. For instance, an SOFC may be able
to respond quickly to fast load transients, but, due to
stresses, which may be caused by thermal expansion on
the cell materials, the FC stack's efficiency and lifetime will
decrease. Moreover, when current is drawn from the FC,
the air supply system should replace the reacted oxygen;
otherwise, the cathode will suffer from oxygen starvation,
which damages the stack, limits the power response of the
FC, and is evidently hazardous for the FC stack.
One of the methods for improving the oxygen transient
response is to control the airflow with the air compressor.
The drop in voltage, which is a consequence of oxygen
starvation, can be avoided by controlling the voltage of the
compressor, Vcm . However, the compressor is not capable
of responding as quickly as desired to the load demand
since oxygen is depleted instantaneously. The reason is
that, during the sudden startup, the compressor should
operate at a very high mass flow rate, and it reaches a
point that will not allow more airflow through, and the
airflow cannot be increased anymore.
Furthermore, even the mentioned hybrid FC/GT system
is not able to make fast load transients without having a
negative impact on the FC stack and, consequently, its lifetime and overall efficiency. A recent analysis from the literature revealed that an FC/GT system has a limited
dynamic capability due to the dynamic characteristics of

IEEE Electrification Magazine - June 2020

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