Hydroelectricity is electricity that has been generated through capturing the kinetic energy from flowing water in order to produce electricity[sc:1]. It is considered to be a renewable energy source because under normal environmental conditions, water is always being recharged and recycled back into the water cycle, thus renewing this resource that can generate electricity[sc:2].
Some hydroelectric plants are very large, such as the Hoover Dam in the United States, but others are small and can be built to suit the needs of each community and the conditions of each site[sc:1].
Most hydropower facilities work by altering the natural flow of a river or other water body through the use of dams or other diversions[sc:3]. These diversions raise the level of the flowing water in order to capture the energy of falling water, and the water is stored behind the dam or diversion in a reservoir[sc:1]. As the water flows through the diversion structure, it turns a turbine that then produces electricity by a generator that is sent through the power grid[sc:1].
With pumped storage hydroelectric systems, water can be stored in the reservoir to produce electricity when demand for electricity is low, such as at night. Water is then pumped back into the reservoir after use to be used again when more power is needed. This storage of energy works like a battery and allows for predictable energy production[sc:3].
Because of the ability of hydroelectric plants to reuse the same water more than once, they are more efficient at supplying energy for peak power demands than fossil fuels and nuclear power plants can[sc:3]. This increased efficiency of hydroelectricity also provides a relatively inexpensive power source in many locations[sc:4].
The potential of a hydroelectric plant to make electricity depends on how far the water falls, since water falling a greater distance contains greater energy. The distance that the water falls depends on the size of the dam[sc:1].
The amount of electricity produced also depends on the amount of water that falls through the dam, which itself depends upon the amount of water that is flowing in the river. Thus, bigger rivers with more water have the potential to generate more electricity than smaller rivers do[sc:1].
Other types of hydropower facilities are “dam-less” and work by channeling only a portion of the river’s water flow through the facility before the water rejoins the river[sc:4].
Engineers are currently working on developing dams that are even more efficient and have reduced impacts on natural aquatic ecosystems, such as the creation of new fish-friendly turbines and fish ladders to help fish swim around dams[sc:4].
In some countries, including the U.S, governments are working to increase the efficiency of turbines and generators by encouraging neighboring hydropower facilities to work together, rather than each individual facility working alone[sc:4].