Power Plant Engineering

Fig. 2.22. Polarisation curve.
As the load on the cell (I
s
× A = I) increases, the internal electro-chemical reactions trying to
oppose the cause also increase. The internal losses increase and the terminal voltage V
c
drops. These
internal losses and inefficiencies increasing with current are called Polarization. The drop in voltage V
p
is called Polarization Voltage VP.
Power per Cell P
c
,
Power = Voltage × Current
P
c
= V
c
× 4
The power of a cell increase with the increase in current density, and reaches a saturation point
at due to polarization effects.
 
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0
1000
2000
3000
4000
5000
6000
7000
P
o
we
r De
n
s
it
y 
(k
W/
m
)
2
Current Density (A/m )
2
I
d
Power
Voltage
0
1
2
3
Ce
ll V
o
lt
a
g
e
(
V
o
lt
s
) 
V
c
Fig. 2.23. Power-Current Density Curve.

---

78
POWER PLANT ENGINEERING
Input power – Polarization losses = Output power
and,
Output power/Input power = Efficiency(
η
)
Efficiency of Cells. In terms of the final energy output these options are only 40% to 60%
efficient — the exception being electrolysis which can be up to 80% efficient. The argument of fuel cell
advocates is that this cycle still represents an improvement for cars. The overall efficiency is something
like 40% if running fuel rather than 20% if running an internal combustion engine.
The simple method of calculating the efficiency (
η
) of a fuel cell is
η
= 
Cell voltage on load
No load voltage of cell
= 
c
o
V
V
η
= 
No load voltage – Polarization voltage
No load voltage
The efficiency of a fuel cell varies with the current density at electrode surface due to the
Polarization Effect. Fig. 2.24 gives a typical characteristic. The power loss is converted to waste heat
and released to atmosphere.
Po
w
e
r
Ef
fi
c
ie
n
c
y
P
c
B
A
Efficiency
Power
Current Density
 Fig. 2.24. Power and Efficiency Curve.
After reaching saturation level, the and power per cell starts decreasing. The losses increase and
are converted to waste heat.