Hydropower is the generation of electricity from the capture of the kinetic energy of rivers water. Derived from the water mills of old, hydropower is, by far, the largest renewable source of electricity in the world.

There is a whole legend around hydropower, and the building of big dams always had a kind of epic dimension. That used to be the case for the big projects carried out in the USA during the New Deal, and it remains true of the giant endeavours of Itaipu, at the border between Brazil and Paraguay, and the Three Gorges in China. In France, the hydropower equipment was the epitome of the post-war reconstruction effort.

 

In 2005, the world electricity generation amounted to 18 235 billion kilowatt-hours (TWh), out of which 40% came from coal, 20% from natural gas, 16% from hydropower, 15% from nuclear power, 6.5% from oil and 2.5% (biomass, Windpower, solar power, geothermal, etc...).

Having produced 3 000 TWh, hydropower is, by far, the largest renewable source of electricity in the world. China, Canada, Brazil and the USA are the largest producers, while two countries have an outstanding share of hydropower in their electricity generation: Norway, with 99% and Brazil, with 84%. As a matter of fact, hydropower is important for the whole Latin America. In 2000, hydropower ranked second in world electricity production but now it is third, behind gas.

Hydropower is a concentrated form of solar energy, since the water flowing down rivers or stored behind dams comes from the evaporation of the oceans heated by the Sun. The water vapour must, of course, condensate into clouds and rainfall must be collected in valleys and dales to produce this considerable concentration. The rest of the story does not involve the Sun but the Earth, whose gravity gives this stored water a potential energy, transformed into kinetic energy by flowing down back to the oceans. Turbines coupled to alternators convert this kinetic energy into electricity. Hydropower is therefore truly renewable. Dams, on the other hand, do not last for ever but their useful lifetime is of the order of one century.

There are two different ways to generate hydropower. Mountain reservoirs allow water to drop from a significant height through forced pipes. It is possible to produce power at will when it is most needed, for instance during demand peaks when electricity becomes very expensive. It is even possible to reverse the process and pump water back to the reservoirs when power is abundant and cheap (this is called: pumped storage). “Run of the mill” dams are built across large rivers with low water drops. They must produce power continuously, otherwise the water is lost.

While Europe and the United States have already harnessed most promising hydropower sites, there remains a huge potential in Asia, Russia and especially Africa, but the most promising sites are remote and far away from the electricity consumers.

Hydropower has many advantages:

- It is renewable.
- It causes little greenhouse emissions (only those linked to the actual building of dams and power plants).
- In the best sites, it generates very cheap power.
- It is very flexible: a hydro power plant needs only minutes to generate at full capacity. That makes it the ideal complement to intermittent electricity sources like windpower.
- It can supply either peak or baseload power.
- Pumped storage is today the only available way to store large amounts of electricity.

Furthermore, dams play a key role in water management for agriculture and flood control (as a matter of fact, only one fourth of the dams in the world produce electricity), and some reservoirs have become important tourism attractions (witness Lake Powell).

Despite those qualities, environmentalist organizations more and more oppose large hydropower projects, with the following arguments:

- They modify the original landscape (but sometimes for the better).
- They flood large areas which were often agricultural lands.
- They cause massive displacements of the people who used to live on these lands (The famed Three Gorges dam caused the forced migration of at least 1.5 million Chinese farmers: on the same scale, it would be unconceivable to move 75 000 French people to build a dam).

In some subtropical sites, the anaerobic fermentation of the drowned biomass might lead to the emission of more greenhouse gases than the electrical production would prevent.

Environmentalists, therefore, accept only “small hydropower” which may have some local benefits but is quite negligible in the world energy mix.