Power Plant Engineering

2.17.4 EFFICIENCY OF CELLS
The performance evaluation of fuel cells is represented in terms of current density at electrode
surface (range 100 to 400 mA/cm
2
) at specified temperature and reactant partial pressures and voltage.
Let,
V
o
= No load voltage of cell, Volts, DC
V
c
= Cell voltage on load
I
c
= Cell current on load, Ampere
P
c
, = Cell power, Watts
V
p
= Polarization voltage = Voltage drop in the cell?
= No load voltage V
o
– O
n
load voltage V,
A = Surface area of on face of an electrode, m
I
d
= Current density of cell, = I
c
/A.... A/m....
η
= Efficiency
During no current (no load or open circuit), the cell voltage is maximum and is called no load
voltage (V
o
).
The performance is illustrated by actual V
c
vs. I
d
curve. Increase in operating temperature and
partial pressure, improves the fuel cell performance (increase in V
c
and P
c
). There is a trade-off be-
tween the higher performance and higher cost (for high temperature, pressure design).
VOLTAGE V
C
-CURRENT DENSITY I
d
 CHARACTERISTIC (POLARIZATION CURVE)
The performance of a fuel cell is evaluated by the cell voltage V
c
vs. electrode current density Id
curve (Fig. 2.22). Cell voltage V
c
drops with increase in current density due to polarization within the
cell. Hence, the curve is also called the polarization curve of the fuel cell.

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NON-CONVENTIONAL ENERGY RESOURCES AND UTILISATION
77
Polarization is internal chemical, electrical, thermal effect within the fuel cell resulting in inef-
ficiencies. Polarization the cause of internal energy loss and is measured by in terms of polarization
voltage V
p
.
V
p
= v
o
– v
c
V
P
= Polarization voltage of the cell = Voltage Drop
= No load voltage V
o
– On load voltage V.
 
0
0.1
0
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2
A
B
C
∆
V
p
V
c
V
o
C
e
ll V
ol
tage V
c
Current Density I
d