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Hydroelectricity is the term used to define the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy, accounting for 16% of global electricity production. There are three main methods of using hydropower to produce electricity:
Conventional – dammed water drives a water turbine and generator, creating electrical energy. This is the most common form.
Pumped-storage – produces electricity to supply high peak demands by moving water between reservoirs at different elevations.
Run-of-the-river – water coming from upstream is used to generate electricity. These stations have small or no reservoir capacity, so must be used for generation at that moment.
Once a hydroelectric plant has been constructed, the project produces no direct waste, and has a considerably lower output of CO2 than fossil fuel powered energy plants. Studies show that hydropower produces some of the lowest amounts of greenhouse gases of any energy source at 41gCO2-e/kWh. In addition, hydropower is a flexible source of energy as plants can adapt to changing demands. However, damming a river may cause serious harm to the local ecosystems as flooding can involve displacement of people and wildlife. Thus, the initial impacts can be serious.
Large-scale hydroelectric power stations, such as the Three Gorges Dam in China, are considered to be the largest power producing facilities in the world, with some hydroelectric facilities capable of generating more than double the installed capacities of the current largest nuclear power stations. In addition, as of 2015 hydropower is one of the cheapest forms of renewable power at $0.08/kWh, and the most efficient form of electricity generation of any power source, at over 90% efficiency.