Environmental Engineering Reference
In-Depth Information
The storage of water in the reservoir allows generation to be timed to meet the demands
of the power system. Energy storage capacity
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is limited so the aim is usually to generate at
times of high load and so maximize the income generated. Since water availability is limited
and seasonal, complex algorithms making use of rainfall prediction are used to optimize hydro
operation. Some countries, for example Norway and Switzerland, have bountiful hydro
resources and as a result there are times when electricity is so cheap it is almost given away.
This has encouraged the development of industries that require abundant cheap electricity.
Energy intensive processes such as aluminium smelting and silicon production are often
located in countries with plentiful hydro resources for exactly this reason.
Dams are attractive because they can provide a large
head
(equal to the fall in height of
the water) but whether they can be built depends on the local topography. Rivers are the
natural way in which water loses potential energy and it is possible to extract a proportion
of this by means of so called low head schemes which are usually small scale. It is also pos-
sible to place turbines in a river fl ow directly with no dams or penstocks,
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extracting only a
very small amount of the predominantly kinetic energy as the river fl ows by. Such turbines
operate at effectively zero head and the installations are known as run of river schemes.
2.3.1 Large Hydro
Large scale hydro is a well developed and widely used form of generation. Depending on
estimates, between 20 and 25% of the world's large scale hydro potential has already been
developed, although the resource reasonably located geographically in relation to electricity
demand has according to many commentators already been largely exploited. Hydroelectric
stations currently contribute about 20% of world electricity generation. Large scale hydro
power is operationally the most desirable of all renewable energy sources with respect to
availability and fl exibility of supply.
As explained above, water can be stored in reservoirs and used when required, either con-
tinuously if the reservoir is large or when most required by the demand for electricity. The
advantage of this storage arrangement is compounded by the natural capability of hydro plant
to respond within minutes to demand increase or decrease. Such plant is therefore invaluable
as a means of fl exible generation to follow both predicted and unexpected changes in
consumer demand. This feature is so valuable that in countries such as the UK where the
topography is unsuitable for the installation of large conventional hydro,
pumped storage
schemes driven by the bidirectional transfer of water between two reservoirs have been
developed.
The downside of large hydro schemes is that they involve substantial upfront capital invest-
ments with profi ts accrued over long periods in the future. Their development can also be
environmentally undesirable because of the fl ooding of large areas and the displacement of
populations, such as has occurred with the construction of the Three Gorges scheme in China.
However, this established renewable energy technology is unlikely to provide substantially
increased contributions in the future since many of the attractive sites have already been
developed; the areas with remaining potential lie mainly in the former Soviet Union and the
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Capacity for a store is the energy that can be stored (i.e. MW h), rather than the rating.
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A penstock is a sluice or gate used to control the water together with the pipe to take the water to the turbine.
