Page 30 - Advanced Gas Turbine Cycles

Chapter 1. A brief review of power generation thermodym'cs 7

1.2.3. Heat rate

As an alternative to the thermal or cycle efficiency of Eq. (1. l), the cyclic heat rate (the
ratio of heat supply rate to power output) is sometimes used:

Heat rate = - = -.
QB
QB
w w
This is the inverse of the closed cycle thermal efficiency, when QB and W are expressed in
the same units.
But a 'heat rate' based on the energy supplied in the fuel is often used. It is then defined
as
_-
Heat rate = Mf[CVIO - F
W W'
which is the inverse of the (arbitrary) overall efficiency of the open circuit plant, as defined
in Eq. (1.6).

1.2.4. Energy utilisation factor

For a gas turbine operating as a combined heat and power plant, the 'energy utilisation
factor' (EUF) is a better criterion of performance than the thermal efficiency. It is defined
as the ratio of work output (W) plus useful heat output (eU) the fuel energy supplied (F),
to
W+Qu
EUF= -
F '
and this is developed further in Chapter 9.

13. Ideal (Carnot) power plant performance

The second law of thermodynamics may be used to show that a cyclic heat power plant
(or cyclic heat engine) achieves maximum efficiency by operating on a reversible cycle
called the Carnot cycle for a given (maximum) temperature of supply (T-) and given
(minimum) temperature of heat rejection (Tmin). Such a Carnot power plant receives all its
heat (QB) at the maximum temperature @.e. TB = Tmm) and rejects all its heat (QA) at the
minimum temperature (i.e. TA = Tmin); the other processes are reversible and adiabatic
and therefore isentropic (see the temperature-entropy diagram of Fig. 1.8). Its thermal
efficiency is

Clearly raising T,, and lowering Thn will lead to higher Carnot efficiency.
The Carnot engine (or cyclic power plant) is a useful hypothetical device in the study of
the thermodynamics of gas turbine cycles, for it provides a measure of the best
performance that can be achieved under the given boundary conditions of temperature.