Environmental Engineering Reference
In-Depth Information
temperatures and air leakage, limiting power consumption and thus optimising the
compressor efficiency [
19
]. On the other hand, the additional power consumption
due to water injection slightly reduces the overall consumption.
4.4 Thermal Management System
The design of the temperature control system has a strong effect on both perfor-
mance and dynamics PEMFC [
20
-
22
]. The electrochemical conversion of hydro-
gen occurring in polymeric electrolyte fuel cells produces heat as by-product, due to
the unavoidable irreversibilities of the process. This heat elevates the temperature
of local reaction sites inside the MEA, and progressively of the entire stack by
conductive transmission through bipolar plats and by convective flow within the
reactant feeding channels. As the cell voltage during stack operation drops well
below the reversible value (about 1.2 V), ranging from 0.9 V to about 0.5 V in the
entire current density range, according to the Ohm's law (see
Sect. 3.3
), the fol-
lowing equation can be used to calculate the thermal power produced inside the
stack:
Q
¼
DV
¼
V
id
V
ð
Þ
I
ð
4
:
7
Þ
where Q
is the produced heat rate, I is the electric current flowing through the
cells, while V
id
and V are the reversible and actual cell voltage values, respectively.
The dynamics of temperature change associated with the produced power is
derived by the following differential equation:
Q
oT
=
ot
¼
ð
4
:
8
Þ
m
c
p
where qT/ qt (time derivative) is the rate of temperature change, m is the whole
mass stack, c
p
represents the stack average specific heat. Equation
4.8
is further
discussed in
Sect. 4.6
, where the dynamic behavior of the overall FCS and the
optimization of integrated management strategies are clarified.
The design issues of thermal management sub-system depend on the total
produced heat and of consequence on the sizing of the stack. The mass flow rate of
coolant (usually deionised water) can be derived by the definition of thermal
capacity described by the following equation:
Q
m
H
2
O
¼
c
p
T
f
T
i
ð
Þ
where Q is the produced heat rate, c
p
is the specific heat of water, while T
f
- T
i
is
the temperature difference of the coolant between stack output (T
f
) and input (T
i
).
A value of T
f
- T
i
not higher than 5 K is advisable [
11
].
