Harnessing energy from moving water is an ancient technology. Hundreds of years ago, water-powered mills were used to produce flour from grain, operate pumps, saws and other tools. Since the late 19th century, the kinetic energy in flowing water has been used to generate electricity. A hydroelectric system involves putting up a dam to store vast quantities of water in a reservoir. The power of the "falling" water, which is released on a regular basis, is used to turn the blades of a turbine. The turbine in turn drives a generator, which produces electricity.

Hydroelectric power plants can generate electricity depending on the demand, offering a clear advantage over other alternative energy sources.

In a pumped-storage facility, a special type of hydropower plant which contains two reservoirs, the plant can pump water back to the upper reservoir using a reversible turbine. This is done in off-peak hours. By pumping water back to the upper reservoir, the plant has more water to generate electricity during periods of peak consumption.

Water-rich mountainous countries such as Norway can meet almost their entire electricity needs from hydro power. In Alpine nations such as Switzerland and Austria, it accounts for almost two thirds of electricity generation. By contrast, hydro power makes up just 3.5 percent of the energy mix in Germany, according to the environment ministry. Hydroelectric power supplies a sixth of the world's electricity demands.

Gigantic dams such as the Three Gorges in China don't emit any harmful greenhouse gas emissions. But generating electricity from dammed rivers remains controversial. Several mega dam projects, such as one in the Amazon rainforest in Brazil, involve flooding large areas. That often destroys natural habitats and displaces people. Find out more in the GLOBAL IDEAS' dossier on hydro power.