Page 149 - Entrophy Analysis in Thermal Engineering Systems

Entropy and fuel cells 143

Likewise, the total enthalpy of the products at the exit of the SOFC (state 3) is

+ _n H 2 O h H 2 O (9.30)
H P ¼ _n H 2 1 U f h H 2 + _n O 2 O 2 + _n N 2 N 2
3
h
h
Applying an energy balance to the SOFC operating under adiabatic and
steady-state conditions gives
(9.31)
H R ¼ _ W FC,dc + H P
The DC (direct current) electric power production of the SOFC is related to
the operating voltage and current of the cell.

(9.32)
_ W FC,dc ¼ V I N c
where N c is the number of the cells.
The molar flowrates of the hydrogen and oxygen fed to the anode are
obtained using Eqs. (9.33) and (9.34), respectively.

1
¼ IN c (9.33)
_ n H 2
2F U f

1
¼ IN c (9.34)
_ n O 2
4F U a
Given the O 2 /N 2 ratio of the air, one may determine the molar flowrate of
the nitrogen as

0:79
¼ (9.35)
_ n N 2 _ n O 2
0:21
For each mole of hydrogen participated in the cell reaction, one mole
water is formed. So, the molar flowrate of the water at the SOFC exit is
obtained by

ð (9.36)
3
_ n H 2 O Þ ¼ U f _n H 2
On the other hand, half mole of oxygen participates in the cell reaction per
each mole of hydrogen. So, the oxygen flowrate at the SOFC exit is

1
Þ ¼
ð _ n O 2 3 IN c 1 (9.37)
4F U a
Because nitrogen does not participate in the reaction, its flowrate at the inlet
and outlet of the fuel cell is the same.