Environmental Engineering Reference
In-Depth Information
Fuel cell
Stack
Net power
Air compressor
Radiator fan
H
2
and H
2
O pumps
Controller
Fig. 4.10
Main energy losses related to ancillary components of a H
2
FCS
Fig. 4.11 Stack and fuel cell
system efficiency as function
of current density [
46
]
0,8
0,6
0,4
0,2
FCS
stack
0
0
100
200
300
400
500
600
700
800
Current density, mA/cm2
major contribution, in particular an air compressor alone in low pressure plants, or
Compressor Expander Modules in high pressure plants, would spend about 7-10%
of the net produced power by the stack. All the auxiliary components determine an
average consumption of about 25% of the net stack power [
12
,
45
].
The role of auxiliaries can be also clarified by curves of Fig.
4.11
, where stack
and system efficiency are shown as function of current density for a PEMFC stack
of 2.5 kW [
46
]. The FCS curve shows unsatisfactory efficiency values at very low
current density (lower than 100 mA/cm
2
), in particular evidence that electrical
ancillary devices strongly affect the net stack power in the power range corre-
sponding to the non-linear variation region of cell voltage. When net stack power
increases a profile tending almost to a plateau in a wide range of load is obtained
[
12
,
13
,
45
,
47
], corresponding to the linear profile of polarization curve.
Stack needs to operate in this last range in most part of driving conditions in
order to guarantee high conversion efficiency. Efficiency loss due to fuel cells is
then limited to the resistance to proton flow through the polymeric membrane. The
system efficiency profile at low current density can be explained taking into
account that fuel cells are more efficient at part load (see stack efficiency curve),
and the impact of parasitic component consumption on net produced power
