The combined-cycle power station uses gas turbines to increase the efficiency
of the power-generation process. Like many other machines that we
assume to be products of the twentieth century, the gas turbine isn't that
new. In fact, Leonardo da Vinci (1452-1519) sketched a machine for
extracting mechanical energy from a gas stream. However, no practical
implementation of such a machine was considered until the nineteenth
century, when George Brayton proposed a cycle that used a combustion
chamber exhausting to the atmosphere. In 1872 Germany's F. Stolze
patented a machine that anticipated many features of a modern gasturbine
engine, although its performance was limited by the constraints of
the materials available at the time.
Many other developments across Europe culminated in the development
of an efficient gas turbine by Frank Whittle at the British Royal
Aircraft Establishment (RAE) in the early 1930s. Subsequent developments
at RAE led to viable axial-flow compressors, which could attain
higher efficiencies than the centrifugal counterpart developed by Whittle.
All these gas turbines employed the Brayton cycle, whose pressure/
volume characteristic is shown in Figure 1.5. Starting at point A in this
cycle air is compressed isentropically (A-B) before being fed into a combustion
chamber, where fuel is added and burned (B-C). The energy of the
expanding air is then converted to mechanical work in a turbine (C-D).
From C to D heat is rejected, and in a simple gas-turbine cycle this heat is
lost to the atmosphere.
The rotation of the gas turbine can be used to drive a generator (via
suitable reduction gearing) but, when used in a simple cycle with no heat
recovery, the thermal efficiency of the gas turbine is poor, because of the
heat lost to the atmosphere. The gases exhausted from the turbine are not
only plentiful and hot (400-550°C), but they also contain substantial
amounts of oxygen (in combustion terms, the excess air level for the gas
turbine is 200-300%). These factors point to the possibility of using the
hot, oxygen-rich air in a steam-generating plant, whose steam output
drives a turbine.
generator (HRSG) is the basis of the 'combined-cycle gas-turbine'
(CCGT) plant which has been a major development of the past few
decades. With the heat used to generate steam in this way, the whole plant
becomes a binary unit employing the features of both the Rankine and the
Brayton cycles to achieve efficiencies that are simply not possible with
either cycle on its own. In fact, the addition of the HRSG yields a thermal
efficiency that may be 50% higher than that of the gas turbine operating
in simple-cycle mode.
Once again, there is nothing really new about this concept• From the
moment when the gas turbine became a practical reality it was very
obvious that the hot compressed air it exhausted contained huge amounts
of heat. Therefore, the combined cycle was considered in some depth
almost as soon as the gas turbine was released from the constraints of
military applications. However, because of their use of gases at extremely
high temperatures, early machines suffered from limited blade life and
they were therefore used only in applications where no other source of
power was readily available. With improvements in materials technology
this difficulty has been overcome and, nowadays, combined-cycle plants employing gas turbines form the mainstream of modern power-station development.
But whether it is in a combined-cycle plant or a simple-cycle power
station, our interest in this chapter is in steam and its use, and this vapour
will now be examined in more detail. We shall see that what seems a fairly
simple and commonplace thing is, in fact, quite complex.
In spite of its complexities it is important to tackle this subject in some
depth, because the power-plant control and instrumentation engineer will
need to deal with the physical parameters of steam through the various
stages of designing or using a practical system.
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