Water power

Hydropower, hydraulic power or water power is power that is derived from the force or energy of moving water, which may be harnessed for useful purposes.

Prior to the widespread availability of commercial electric power, hydropower was used for irrigation, and operation of various machines, such as watermills, textile machines, sawmills, dock cranes, and domestic lifts.

Another method used a trompe, which produces compressed air from falling water, which could then be used to power other machinery at a distance from the water.

Hydroelectric power now supplies about 715,000 megawatts or 19% of world electricity. Large dams are still being designed. The world's largest is the Three Gorges Dam on the third longest river in the world, the Yangtze River. Apart from a few countries with an abundance of hydro power, this energy source is normally applied to peak load demand, because it is readily stopped and started. It also provides a high-capacity, low-cost means of energy storage, known as "pumped storage".

Hydropower produces essentially no carbon dioxide or other harmful emissions, in contrast to burning fossil fuels, and is not a significant contributor to global warming through CO2.

Hydroelectric power can be far less expensive than electricity generated from fossil fuels or nuclear energy. Areas with abundant hydroelectric power attract industry. Environmental concerns about the effects of reservoirs may prohibit development of economic hydropower sources.

The chief advantage of hydroelectric dams is their ability to handle seasonal (as well as daily) high peak loads. When the electricity demands drop, the dam simply stores more water (which provides more flow when it releases). Some electricity generators use water dams to store excess energy (often during the night), by using the electricity to pump water up into a basin. Electricity can be generated when demand increases. In practice the utilization of stored water in river dams is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands. Not all hydroelectric power requires a dam; a run-of-river project only uses part of the stream flow and is a characteristic of small hydropower projects. A developing technology example is the Gorlov helical turbine.