Environmental Engineering Reference
In-Depth Information
Example 16.2
An SOFC operating with natural gas produces 2 W electric power ( P e ) with an
electrical efficiency (
η e ) of 52% based on the lower heating value (LHV) of the
natural gas.
1. Calculate the heat released by the FC per unit time.
2. If 70% of the available heat can be recovered by heating water from 20 to
70 C, calculate the amount of hot water (g) produced when the FC is oper-
ating for 1 h. The specific heat capacity of water, c p,w , is 4.186 J g −1
K −1 .
Solution
1. The total chemical power of the natural gas ( U : )is
U : = P e
η e
2
=
52 =3
:
85 W
0
:
The amount of heat ( Q ) released by the FC is
Q = U : 1
ð
η e
Þ
=3
:
85 × 1
ð
0
:
52
Þ
=1
:
848 W
2. 70% heat recovered is 0.7 × 1.848 = 1.30 W.
The mass flow rate of water
φ m is
Q
c p , w Δ
1
:
30
2×10 −3 g
s −1
φ m =
T =
=6
:
4
:
186 × 70
ð
20
Þ
The amount of hot water produced during 1 h of operation is
2×10 −3 × 3600 = 22
:
:
6
4g
16.2.2 Anode
Cermet (ceramic
metallic) anodes are often used for SOFCs to ensure thermal and
chemical compatibility in high-temperature SOFC environments. While anodes are
necessary for the electrochemical fuel oxidation to take place, they also cause losses
in power production. The anodic losses are due to internal electrical resistance, contact
resistance, mass transfer limitations, and the limited rate of the electrochemical reac-
tions (Zhu and Deevi, 2003). The internal resistance is the resistance to the transport of
electrons and oxide ions within the anode. The contact resistance is caused by poor
adherence between the anode and the solid electrolyte and is generally not affected by
fuel variations. The mass transport limitation is related to the diffusion of gas-phase
species through the porous electrode. The porosity or microstructure of the electrode is
an important parameter affecting gas diffusion. Mass transfer losses may become sig-
nificant at higher current flows and at greater fuel utilization. Anodic activation losses
-
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