Page 24 - Advanced Gas Turbine Cycles

Chapter 1

A BRIEF REVIEW OF POWER GENERATION
THERMODYNAMICS

1.1. Introduction

A conventional power plant receiving fuel energy (F), proaucing work (W) and
rejecting heat (QA) to a sink at low temperature is shown in Fig. 1.1 as a block diagram.
The objective is to achieve the least fuel input for a given work output as this will be
economically beneficial in the operation of the power plant, thereby minimising the fuel
costs. However, the capital cost of achieving high efficiency has to be assessed and
balanced against the resulting saving in fuel costs.
The discussion here is restricted to plants in which the flow is steady, since virtually all
the plants (and their components) with which the book is concerned have a steady flow.
It is important first to distinguish between a closed cyclic power plant (or heat engine)
and an open circuit power plant. In the former, fluid passes continuously round a closed
circuit, through a thermodynamic cycle in which heat (QB) is received from a source at a
high temperature, heat (QA) is rejected to a sink at low temperature and work output (W) is
delivered, usually to drive an electric generator.
Fig. 1.2 shows a gas turbine power plant operating on a closed circuit. The dotted chain
control surface (Y) surrounds a cyclic gas turbine power plant (or cyclic heat engine)
through which air or gas circulates, and the combustion chamber is located within the
second open control surface (a. Heat QB is transferred from Z to Y, and heat QA is rejected
from Y. The two control volumes form a complete power plant.
Usually, a gas turbine plant operates on ‘open circuit’, with internal combustion (Fig.
1.3). Air and fuel pass across the single control surface into the compressor and
combustion chamber, respectively, and the combustion products leave the control
surface after expansion through the turbine. The open circuit plant cannot be said to
operate on a thermodynamic cycle; however, its performance is often assessed by
treating it as equivalent to a closed cyclic power plant, but care must be taken in such an
approach.
The Joule-Brayton (JB) constant pressure closed cycle is the basis of the cyclic gas
turbine power plant, with steady flow of air (or gas) through a compressor, heater,
turbine, cooler within a closed circuit (Fig. 1.4). The turbine drives the compressor and
a generator delivering the electrical power, heat is supplied at a constant pressure and is
also rejected at constant pressure. The temperature-entropy diagram for this cycle is also

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