Environmental Engineering Reference
In-Depth Information
The design of generation plant does affect its time response and the extent that it can con-
tribute to the regulation demands of the power system. Load following capabilities refl ect in
the main the thermal mass of the central plant elements. Thus coal stations with large boilers
can take many hours to reach full output from cold. In contrast, gas turbines can reach rated
power in minutes and combined cycle plants lie somewhere between coal fi red and direct gas
fi red plant. Innovations in plant design aimed at improving conversion effi ciency can have
the effect of reducing fl exibility. This important issue is discussed in detail in Reference
[1] .
Although the term nuclear power formally covers both fi ssion and fusion, fusion power is
still decades away and may never be technologically viable or cost effective, and it certainly
cannot be regarded as conventional. Nevertheless, considerable international research
has been and continues to be directed towards this technology, in particular through the
recently agreed ITER consortium. In contrast fi ssion based reactors have been generating a
signifi cant proportion of electricity worldwide. It remains a controversial technology due to
the dangers of radiation and the challenge of radioactive waste disposal. A number of differ-
ent technologies have been developed including boiling water reactors and gas cooled
reactors.
Such plant sustain a fi ssion chain reaction within a controlled environment. Fission takes
place in the reactor core which is normally contained within a pressure vessel safety shield.
Moderators, usually graphite or water, slow down the neutrons and help regulate the chain
reaction. Control rods are inserted into the core to regulate the nuclear reaction; these are
made of a material such as boron which absorbs neutrons. The core also comprises fuel rods
of fi ssile material. A coolant, normally water or gas, passes through the reactor. It then passes
to the boiler where steam is raised. Electrical generation is provided by steam turbines and
in this regard nuclear power stations are similar to large coal fi red stations; it is simply the
source of heat for raising the steam that is different.
In general nuclear reactors are regarded as infl exible and they normally operate on base
load. The fact that capital and installation costs far outweigh fuel and operational costs is
further motivation for operating such plant as continuously as possible. Load factors are
around 80% but would almost certainly fall if they were used to any extent to load follow,
as advocates of nuclear power in France have claimed is possible. The activity of load fol-
lowing is sometimes confused with the steady reduction of nuclear power station output over
some weeks, which is possible, indeed advisable, if the station is being run down prior to
maintenance.
2.3 Hydroelectric Power
As mentioned in Chapter 1, hydroelectric generation is an indirect form of solar energy.
Incident solar radiation evaporates water from the sea, and to a lesser extent from land areas,
and the warmed water vapour rises; as it ascends it expands and cools, eventually condensing
in the form of clouds. Some of the resulting rain falls on high ground. This water has thus
gained potential energy as a result of solar input. Hydro power is the result of extracting some
of this energy as the water fl ows back towards the sea. Large scale hydro makes use of large
reservoirs, usually created by damming rivers. Water is allowed to fl ow out of the reservoir
in a controlled manner, turning turbines that drive electrical generators as it does so.
