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Tuesday, January 18, 2011

Hydropower

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 to produce compressed air from falling water, which could then be used to power other machinery at a distance from the water.
In hydrology, hydropower is manifested in the force of the water on the riverbed and banks of a river. It is particularly powerful when the river is in flood. The force of the water results in the removal of sediment and other materials from the riverbed and banks of the river, causing erosion and other alterations.

History

Early uses of waterpower date back to Mesopotamia and ancient Egypt, where irrigation has been used since the 6th millennium BC and water clocks had been used since the early 2nd millennium BC. Other early examples of water power include the Qanat system in ancient Persia and the Turpan water system in ancient China.
Waterwheels and mills
Hydropower has been used for hundreds of years. In India, water wheels and watermills were built; in Imperial Rome, water powered mills produced flour from grain, and were also used for sawing timber and stone; in China, watermills were widely used since the Han Dynasty. The power of a wave of water released from a tank was used for extraction of metal ores in a method known as hushing. The method was first used at the Dolaucothi gold mine in Wales from 75 AD onwards, but had been developed in Spain at such mines as Las Medulas. Hushing was also widely used in Britain in the Medieval and later periods to extract lead and tin ores. It later evolved into hydraulic mining when used during the California gold rush.
In China and the rest of the Far East, hydraulically operated "pot wheel" pumps raised water into irrigation canals. At the beginning of the Industrial revolution in Britain, water was the main source of power for new inventions such as Richard Arkwright's water frame.[1] Although the use of water power gave way to steam power in many of the larger mills and factories, it was still used during the 18th and 19th centuries for many smaller operations, such as driving the bellows in small blast furnaces (e.g. the Dyfi Furnace) and gristmills, such as those built at Saint Anthony Falls, which uses the 50-foot (15 m) drop in the Mississippi River.
In the 1830s, at the peak of the canal-building era, hydropower was used to transport barge traffic up and down steep hills using inclined plane railroads.
Hydraulic power pipes
Hydraulic power networks also existed, using pipes carrying pressurized liquid to transmit mechanical power from a power source, such as a pump, to end users. These were extensive in Victorian cities in the United Kingdom. A hydraulic power network was also in use in Geneva, Switzerland. The world famous Jet d'Eau was originally the only over pressure valve of this network.
Compressed air hydro
Where there is a plentiful head of water it can be made to generate compressed air directly without moving parts. A falling column of water is mixed with air bubbles generated through turbulence at the inlet. This is allowed to fall down a shaft into a subterranean chamber where the air separates from the water. The weight of falling water compresses the air in the top of the chamber. A submerged outlet from the chamber allows water to flow to the surface at a lower height than the intake. An outlet in the roof of the chamber supplies the compressed air to the surface. A facility on this principal was built on the Montreal River at Ragged Shutes near Cobalt, Ontario in 1910 and supplied 5,000 horsepower to nearby mines. 
Modern usage

There are several forms of water power currently in use or development. Some are purely mechanical but many primarily generate electricity. Broad categories include:
Hydroelectricity

A conventional dammed-hydro facility (hydroelectric dam) is the most common type of hydroelectric power generation.
Main article: Hydroelectricity
Conventional hydroelectric, referring to hydroelectric dams.
Run-of-the-river hydroelectricity, which captures the kinetic energy in rivers or streams, without the use of dams.
Pumped-storage hydroelectricity, to pump up water, and use its head to generate in times of demand.
Tidal power, which captures energy from the tides in horizontal direction.
Tidal stream power, usage of stream generators, somewhat similar to that of a wind turbine.
Tidal barrage power, usage of a tidal dam.
Dynamic tidal power, utilizing large areas to generate head.
Marine energy

A Pelamis wave device under test at the European Marine Energy Centre (EMEC), Orkney, Scotland.
Main article: Marine energy
Marine current power, which captures the kinetic energy from marine currents.
Osmotic power, which channels river water into a container separated from sea water by a semi-permeable membrane.
Ocean thermal energy, which exploits the temperature difference between deep and shallow waters.
Tidal power, which captures energy from the tides in horizontal direction. Also a popular form of hydroelectric power generation.
Tidal stream power, usage of stream generators, somewhat similar to that of a wind turbine.
Tidal barrage power, usage of a tidal dam.
Dynamic tidal power, utilizing large areas to generate head.
Wave power, the use ocean surface waves to generate power.
Calculating the amount of available power

A hydropower resource can be measured according to the amount of available power, or energy per unit time. In large reservoirs, the available power is generally only a function of the hydraulic head and rate of fluid flow. In a reservoir, the head is the height of water in the reservoir relative to its height after discharge. Each unit of water can do an amount of work equal to its weight times the head.
The amount of energy, E, released when an object of mass m drops a height h in a gravitational field of strength g is given by

The energy available to hydroelectric dams is the energy that can be liberated by lowering water in a controlled way. In these situations, the power is related to the mass flow rate.

Substituting P for E⁄t and expressing m⁄t in terms of the volume of liquid moved per unit time (the rate of fluid flow, φ) and the density of water, we arrive at the usual form of this expression:

or
A simple formula for approximating electric power production at a hydroelectric plant is:
P = hrgk
where P is Power in kilowatts, h is height in meters, r is flow rate in cubic meters per second, g is acceleration due to gravity of 9.8 m/s2, and k is a coefficient of efficiency ranging from 0 to 1. Efficiency is often higher with larger and more modern turbines. 
Some hydropower systems such as water wheels can draw power from the flow of a body of water without necessarily changing its height. In this case, the available power is the kinetic energy of the flowing water.

where v is the speed of the water, or with

where A is the area through which the water passes, also

Over-shot water wheels can efficiently capture both types of energy.

See also


(source:wikipedia)

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