Page 138 - Advanced Gas Turbine Cycles

110 Advanced gas turbine cycles

QHR = QL

I QLR‘ QA

Fig. 7.1. Ideal combined cycle plant.

The thermal efficiency of the combined plant is greater than that of the upper cycle
lw).
alone, by an amount ~ ( -

7.3. A combined plant with heat loss between two cyclic plants in series

Consider next two cyclic plants operating in series, but with unused heat QuN (or heat
‘loss’) between the two plants, so that QHR = QL + em. as shown in Fig. 7.2.
The overall thermal efficiency of the combined plant is by definition
Wn + WL
TCP =
QB ’
and the efficiencies of the higher and lower plant, respectively, are
Wn WL
w=-9 %=-e
QB QL
However, the heat supplied to the lower cycle is now

QL = QHR - QUN = QB(~ - TH) - QUN~
so that
+
M~B %[QB(~ - w) - Qml
TCP = = + m - %TL - VUN~, (7.6)
QB
where vW = em&. Thus there is a loss in efficiency of vuN%, in comparison with the
‘ideal’ cycle with no heat loss between plants H and L.