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 flows back towards the sea. Large scale hydro makes use of large reservoirs, usually created by damming rivers. Water is allowed to flow out of the reservoir in a controlled manner, turning turbines that drive electrical generators as it does so.
The storage of water in the reservoir allows generation to be timed to meet the demands of the power system. Energy storage capacity 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 possible to place turbines in a river flow directly with no dams or penstocks, extracting only a very small amount of the predominantly kinetic energy as the river flows by. Such turbines operate at effectively zero head and the installations are known as run of river schemes.