The hydroelectric dam, or "impoundment facility," is the most widely used water-powered technology. Typically, water from a river is impounded behind a barricade, or dam, creating a reservoir. Water is released from the reservoir as needed to flow through a turbine, causing it to spin, thus activating a generator, which produces electricity.
In 2009 such dams generated 35% of all the power from renewable sources in the U.S. This represents about 3% of the total energy consumed in the country that year. Dam reservoirs also provided secondary value in the form of recreation, water supplies, irrigation support, and flood control for their region. However, hydroelectric dams also have some disadvantages:
- Dam reservoirs cause large loss of land.
- Dams are usually located in remote areas, requiring heavy investment in transmission lines to carry power to population centers.
- There is significant damage to ecological systems, both upstream and downstream of the power plant.
- Drought can adversely affect the river flow and cause power shortages.
- In tropical regions, substantial amounts of methane, a powerful greenhouse gas, can be released, as submerged flora decay under anaerobic conditions.
- Development of a reservoir is often disruptive to the lives of people who must be relocated.
- There is the potential for failure of the dam, leading, possibly, to great loss of life.
The "diversion facility," the second means of harnessing major water power, uses a conduit to divert, parts of a river to the turbines which set the generator in motion. Not surprisingly, the power plants at Niagara Falls, New York, are diversion facilities.
Inventive new forms of hydropower are being pioneered to take advantage of hydropower's reliability and renewability. New turbine designs are coming on line to maximize the energy-generating capabilities of existing facilities. Various marine sources including ocean tides and waves, marine currents, osmotic and ocean thermal systems are perhaps the most visionary of water-power possibilities, but developers are deploying experimental prototypes in hopes of finding economically viable methods of producing clean, cheap and steady energy.