Hydroelectricity

Hydropower, commonly used to refer to hydroelectric power, is by far the largest source of renewable energy (96% in the US). Hydroelectric power comes from flowing water, winter and spring runoff from mountain streams and clear lakes. Water, when it is falling by the force of gravity, can be used to turn turbines and generators that produce electricity. When flowing water turns blades in a turbine, the kinetic (moving) energy is changed to mechanical (machine) energy. The turbine turns the generator rotor which then converts this mechanical energy into another energy form, electricity.

Although hydropower is one of the oldest ways to produce energy, many favourable characteristics continue today to make hydroelectric projects an attractive source of power:
 - they do not pollute the air;
 - the water is not consumed;
 - projects have long lives compared to other forms of electricity generation; and
 - they can respond quickly to any surge in power demand.

Because of these favourable characteristics, many countries have developed their hydropower capacity near to the maximum. For example, in Norway 99.6% of electricity comes from hydropower. There are 25 countries in the world that rely on hydropower for more than 90% of their electricity needs.

Although estimates vary, there is still a lot of underdeveloped capacity that could be used in the future: non-developed capacity is estimated at around 35% of total available capacity in Europe, 60% in Australia, 80% in South America, 93% in Africa, 80% in Asia and 40% in North America.

So why is hydropower not even more widely used? Hydropower has been critized for its impact on the local environment, as it often requires flooding an entire region. It can also damage the fish habitat through altering of streams and lake levels. This technology can also become problematic in case of droughts, as happened recently in Brazil, a country that relies heavily on hydropower. Finally, the high initial investment, added to potential environmental costs, often deters companies and governments from pursuing new projects.