Hydropower: untapped resource and environmental hazard

Hydropower is frequently neglected when sustainable energy sources are discussed, but it has a number of advantages which other "renewables" cannot yet match: it is a fully mature technology with high levels of reliability and mechanical efficiency; and there are substantial unexploited reserves. Last year the World Energy Conference estimated that hydropower supplies approximately six exajoules (10 super(18) joules) of electricity per year, or roughly 23 percent of the world supply; by the year 2020 that figure is expected to be almost quintupled, to an estimated 28.30 exajoules.