Page 137 - Advanced Gas Turbine Cycles

Chapter 7

THE COMBINED CYCLE GAS TURBINE (CCGT)

7.1. Introduction

The modification to single cycles described earlier may not achieve a high enough
overall efficiency. The plant designer therefore explores the possibility of using a
combined plant, which is essentially one plant thermodynamically on top of the other, the
lower plant receiving some or all of the heat rejected from the upper plant. If a higher mean
temperature of heat supply and/or a lower temperature of heat rejection can be achieved in
this way then a higher overall plant efficiency can also be achieved, as long as substantial
imversibilities are not introduced.
In this chapter, a short review of the thermodynamics of CCGTs is given. However, the
author recommends readers to refer to two books which deal with combined plants in
greater detail [1,2].

7.2. An ideal combination of cyclic plants

Consider a combined power plant made up of two cyclic plants (H, L) in series
(Fig. 7.1). In this ideal plant, heat that is rejected from the higher (topping) plant, of
thermal efficiency w, is used to supply the lower (bottoming) plant, of thermal efficiency
w, with no intermediate heat loss and supplementary heating.
The work output from the lower cycle is

but
QHL = QB<~ - TI+),
where QB is the heat supplied to the upper plant, which delivers work

wH = %&?Be (7.3)
Thus, the total work output is
W = WH + WL = VHQB + ~(1- TI+)QB = QB(TI+ + TL - TI+%). (7.4)
The thermal efficiency of the combined plant is therefore
W
-
TCP = - = %I+% (7.5)
QB
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