Page 152 - Entrophy Analysis in Thermal Engineering Systems

146 Entropy Analysis in Thermal Engineering Systems

Like the conventional power plants discussed in Chapter 8, it can be read-
ily deduced from Fig. 9.9 that the maximum thermal efficiency design is
identical to that of the minimum SEG for the hybrid cycle. The optimum
pressure ratio in Fig. 9.9 is 21.7 at which the cycle efficiency is 0.612
(i.e., 61.2%). The distribution of the SEG within the cycle at the maxi-
mum efficiency operation is shown in Fig. 9.10.The exhaustgases,with
46% share, are the largest source of the efficiency losses, followed by the
SOFC with 30% contribution to the inefficiencies.
The temperature of the exhaust gases leaving the turbine is relatively
high, 735.3K, which explains why the exhaust gases are responsible for
a significant portion of the efficiency losses. A modified design may then
include a recuperator to partially recover the thermal energy of the hot
exhaust gases by preheating the air upstream of the SOFC. The modified
cycle would indeed be an integrated regenerative gas turbine and SOFC
cycle. Assuming an effectiveness of 0.85 for the recuperator, the maximum
thermal efficiency raises to 0.658. This is 4.6 percentage points higher than
the maximum efficiency of the original hybrid cycle of Fig. 9.8.The key
features of the original and the modified hybrid cycles are compared in
Table 9.3.
The optimum pressure ratio of the regenerative hybrid cycle, i.e., 5.6, is
almost a quarter of that without a recuperator. The exhaust gases in the mod-
ified cycle are over 120K cooler than those in the hybrid cycle of Fig. 9.8.

Fig. 9.10 Distribution of the SEG within the cycle at maximum efficiency.