Page 99 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

76 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

are the main operational and design parameters. The system efficiency qsyst
is plotted against excess air h in Figure 3.12 with qAH as a parameter. The
basic chemical thermodynamics shows that A'G - and therefore wtFcreL, (Eq. (5))
and wtsysrev (Eq. (78)) - is independent of the excess air h. This can be used
to prove the model because qsyst is independent of h for qAH = 1 as expected.
qsyst decreases with increasing h for all AH < 1. The influence of qAH
increases with increasing h. The behaviour of the system for qAH = 0.85 is
shown Figure 3.12.

-x- rlAHd.90
HE3 (environmeni
out of sewice
I all HE out of service auxilliaty burner on I
air heater
1 2 3 4 5 6 7 8 efficiency
excess air A [-]
6,,,, = 0.6, HE= 0.7
water surplus nw = 2 integrated reforming
TSOFC 900 "C, Tref = 750 "C, Tevap = 200 "C
=
Figure 3.12 The injuence of the excess air I and the eficiency qm of the heat transfer in the air heater on
the system eficiency qspst the SOFCheat engine hybridcycle.
of
First qsyst decreases slightly with an increasing h, because the work of the
heat engine HE3 decreases by compensating increasing heat losses. The
other heat engines are operating at full load. The decrease of qsyst becomes
sharper for an excess air h M 3 because the heat engine HE3 goes out of service by
a lack of available heat. The total waste heat of the SOFC must be used to supply
the heat engines HE1 and HE2 which operate between the SOFC and the reformer
and the evaporator respectively and to compensate the increasing heat loss of the
air heater. This causes the sharper decrease of the system efficiency qsyst with an
increasing h in the region 3 < h < 6 by a decreasing supply of work by HE1 and
HE2. In the region h > 6 there is no heat engine in operation. All further heat
losses (increasing with increasing excess air h) must be compensated by the
auxiliary burner. qsyst drops to values lower than 50% as shown in Figure 3.12.
These results show that it is important to assure a good heat recovery in the air
heater system and to avoid a very high excess air h.
is
The system efficiency qsyst influenced by the exergetic efficiency of the heat
engines (HE1, HE2, HE3) I;HEI, rHE2, I;HE~ as well. qsyst is plotted against SHE for
each of the three heat engines in Figure 3.1 3. The maximum difference of about
9% in qsyst occurs if CHE3 (heat sink environment) is varied between 0 and 1. The
difference is only about 2% if I;HEI (heat sink reformer) is varied. The variation of
cHE2 (heat sink evaporator) leads to differences in qsyst about 8%. The order
of
of magnitude of these differences corresponds to the difference of the respective