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 flexibility of supply.
As explained above, water can be stored in reservoirs and used when required, either continuously 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 flexible 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 investments with profits accrued over long periods in the future. Their development can also be environmentally undesirable because of the flooding 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
5 Capacity for a store is the energy that can be stored (i.e. MWh), rather than the rating.
6A penstock is a sluice or gate used to control the water together with the pipe to take the water to the turbine.
developing countries  . The optimal way of integrating this technology in power systems has been covered in many books and scientific papers published over the last century and will not be dealt with further here. Reference  presents an up to date approach to hydro scheduling.
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