Page 29 - Advanced Gas Turbine Cycles

6 Advanced gas turbine cycles

where Qo is equal to Mf[CVl0 = [-AH0] = HR0 - Hpo, the change in enthalpy from
reactants to products, at the temperature of the environment.
The overall efficiency of the entire gas turbine plant, including the cyclic gas turbine
power plant (within Y) and the heating device (within Z), is given by

W QB
170 = F = (E)( -> = 77%. ( 1.4)

The subscript 0 now distinguishes the overall efficiency from the thermal efficiency.

1.2.2. Eficiency of an open circuit gas turbine plant

For an open circuit (non-cyclic) gas turbine plant (Fig. 1.3) a different criterion of
performance is sometimes used-the rational eficiency (m). This is defined as the ratio of
the actual work output to the maximum (reversible) work output that can be achieved
between the reactants, each at pressure (po) and temperature (To) of the environment, and
products each at the same po, To. Thus

W
7)R’- (1.5a)
WREV

(1.5b)

where [-AGO] = GRO - Gpo is the change in Gibbs function (from reactants to products).
(The Gibbs function is G = H - TS, where H is the enthalpy and S the entropy.)
[- AGO] is not readily determinable, but for many reactions [- AH01 is numerically
almost the same as [- AGO]. Thus the rational efficiency of the plant is frequently
approximated to

where [-AH01 = HRo - Hpo. Haywood [3] prefers to call this the (arbitrary) overall
eficiency, implying a parallel with 170 of Eq. (1.4).
Many preliminary analyses of gas turbines are based on the assumption of a closed
‘air standard’ cyclic plant, and for such analyses the use of 77 as a thermal efficiency is
entirely correct (as discussed in the early part of Chapter 3 of this book). But most
practical gas turbines are of the open type and the rational efficiency should strictly be
used, or at least its approximate form, the arbitrary overall efficiency 770. We have
followed this practice in the latter part of Chapter 3 and subsequent chapters; even
though some engineers consider this differentiation to be a somewhat pedantic point
and many authors refer to 70 as a thermal efficiency (or sometimes the ‘lower heating
value thermal efficiency’).