Environmental Engineering Reference
In-Depth Information
not permitted to operate at too low a load. Nuclear plant in particular offers very limited
capability to follow changes in load.
The remainder of this chapter explores in detail the characteristics of the different sources
of generation with specifi c regard to their contribution to meeting the demands of a large
power system. All the renewables are dealt with except geothermal power, which is very
geographically limited, and where it does exist can be treated very much like fossil fuelled
thermal power. It is after all simply a way of raising steam using large naturally occurring
thermal gradients.
2.2 Conventional Sources: Coal, Gas and Nuclear
The diverse characteristics of generators operating from different fossil fuel sources defi ne
their function in an integrated power system. Nuclear powered stations, dealt with in more
detail at the end of the section, are generally infl exible and designed to run at constant power.
This mode of operation is known as
base load
since it contributes to that fraction of the load
that can be regarded as always present.
Conventional coal plant effi ciency, as reviewed in Chapter 1, is in the range 30-40% and
depends signifi cantly on how it is operated.
2
The higher effi ciencies would correspond to the
best plant in base load operation; increased
cycling
3
and low load operation of the plant sig-
nifi cantly lowers operational effi ciency. Peak system loads generally occur for only short
periods of time and it turns out that such loads are best met by plant of low capital cost;
4
the
high operational costs associated with typically low effi ciency generation of this peaking
plant is acceptable since total operation time tends to be limited.
Thermal conversion has been substantially improved over the last two decades through the
development of combined cycle gas turbines (CCGTs). In this arrangement the gas, usually
natural gas, is burnt at temperatures of around 1000 °C to drive a turbine and the exhaust
gases are subsequently used to raise steam for a traditional steam turbine. Both turbines drive
separate generators feeding power to the grid. By increasing the temperature at which the
fuel is burnt effi ciencies of around 50% can currently be achieved, with plant effi ciencies
towards 60% being projected for the next generation. Such plant are currently highly control-
lable, but because of their high effi ciency they are constrained by fuel supply contracts that
dictate running on high load. Most gas powered stations are therefore run at constant power
and thus also contribute to the base load, although it is worth noting that recent rises in gas
prices within Europe have recently changed the situation to one where it is, at least for the
time being, economically preferable to run coal fi red stations at base load in place of CCGT
plant.
2
Most conventional modern coal plants burn pulverized coal; future plant using gasifi ed coal could reach a con-
siderably higher overall effi ciency.
3
Plant cycling is the process by which a plant is run up from cold to meet load and subsequently run down when
not required. During the transitional periods plant is generally operating at far from optimal conditions and energy
is required to prepare the plant for generation. All this contributes to poor overall effi ciency.
4
Open cycle gas turbines provide cheap peaking capacity but are relatively expensive to operate. The impact of
plant costs on their role in the system is explored in more detail in Chapter 7.
