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Alternative Fuels for Transportation
11.4.1.5 PEFC System Efficiency
System efficiency is defined as
P
η
net
system =
,
P fuel
where P net = FCPS net power (W); and P fuel = fuel power, which is equal to
fuel flow rate × fuel heating value (W). The automotive standard is to use
the lower heating value, which is 242 kJmol –1 for hydrogen. Stack efficiency
is defined as
P gross
P
η k =
net
stac
,
P H c
2 onsumed
P gross
where P gross = fuel cell stack gross power (W); and P H c
2 onsumed = hydrogen fuel
power consumed electrochemically.
V
η stack =
cell
,
1 2
. 3V
where V cell is in volts. The system efficiency can be written as:
η system = η H u
2 til η stack η parasitic ,
where η H u
2 til = P n et /P gross .
For a typical direct-hydrogen system, η H u
2 til is near unity and η parasitic is normally dominated by the compressor auxiliary power and may vary between
80 and 95%. For operating cell-voltages between 1 V at open circuit and 0.6–
0.7 V at peak power, η stack varies from nearly 80% at open circuit to 50–60%
at peak power.
11.4.1.6 Stack Sizing/Packaging
Stack sizing is perhaps the most important system design component. As
the stack happens to be an expensive part in FCV, it needs to be small in
size. This results in reduced peak efficiency, which adversely impacts the
balance of plant. The actual design choice will in part depend on the system
efficiency target, but is likely to be driven by factors such as cost, packaging,
and thermal management.
After stack size, the most fundamental system trade-offs are stack operating pressure, temperature, humidification, stoichiometry, and their interactions. Higher temperature operation is desirable for effective heat rejection.
Alternative Fuels for Transportation
11.4.1.5 PEFC System Efficiency
System efficiency is defined as
P
η
net
system =
,
P fuel
where P net = FCPS net power (W); and P fuel = fuel power, which is equal to
fuel flow rate × fuel heating value (W). The automotive standard is to use
the lower heating value, which is 242 kJmol –1 for hydrogen. Stack efficiency
is defined as
P gross
P
η k =
net
stac
,
P H c
2 onsumed
P gross
where P gross = fuel cell stack gross power (W); and P H c
2 onsumed = hydrogen fuel
power consumed electrochemically.
V
η stack =
cell
,
1 2
. 3V
where V cell is in volts. The system efficiency can be written as:
η system = η H u
2 til η stack η parasitic ,
where η H u
2 til = P n et /P gross .
For a typical direct-hydrogen system, η H u
2 til is near unity and η parasitic is normally dominated by the compressor auxiliary power and may vary between
80 and 95%. For operating cell-voltages between 1 V at open circuit and 0.6–
0.7 V at peak power, η stack varies from nearly 80% at open circuit to 50–60%
at peak power.
11.4.1.6 Stack Sizing/Packaging
Stack sizing is perhaps the most important system design component. As
the stack happens to be an expensive part in FCV, it needs to be small in
size. This results in reduced peak efficiency, which adversely impacts the
balance of plant. The actual design choice will in part depend on the system
efficiency target, but is likely to be driven by factors such as cost, packaging,
and thermal management.
After stack size, the most fundamental system trade-offs are stack operating pressure, temperature, humidification, stoichiometry, and their interactions. Higher temperature operation is desirable for effective heat rejection.