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Saturday, 25 June 2011

Introduction to hydropower

Introduction to hydropower
To know the power potential of water in a river it is necessary to know the flow in the river and the available head.
The flow of the river is the amount of water (in m3 or litres) which passes in a certain amount of time a cross section of the river. Flows are normally given in cubic meters per second (m3/s) or in litres per second (l/s).
Head is the vertical difference in level (in meters) the water falls down.

Components of a typical high head hydro installation.
(click for enlargement)
The theoretical power (P) available from a given head of water is in exact proportion to the head H and the flow Q.
P=Q × H × c       c = constant
The constant c is the product of the density of water and the acceleration due to gravity (g).
If P is measured in Watts, Q in m3/s and H in meters, the gross power of the flow of water is:
P=1000 × 9.8 × Q × H
This available power will be converted by the hydro turbine in mechanical power. As a turbine has an efficiency lower than 1, the generated power will be a fraction of the available gross power.
 
 


Different sizes hydropower installations
Hydropower installations can be classified as follows:
 
name description
Large all installations with an installed capacity of more than 1000 kW (according to some definitions more than 10,000 kW)
Small general term for installations smaller than 1000 kW (or < 10,000 kW). Also used for installations in the range between 500 and 1000 kW.
Mini capacity between 100 and 500 kW
Micro hydropower installations with a power output less than 100 kW (or less then 1000 kW)

Hydro Power

Hydro Power
Breastshot waterwheel animated gif
When it rains in hills and mountains, the water becomes streams and rivers that run down to the ocean. The moving or falling water can be used to do work. Energy, you'll remember is the ability to do work. So moving water, which has kinetic energy, can be used to make electricity.
For hundreds of years, moving water was used to turn wooden wheels that were attached to grinding wheels to grind (or mill) flour or corn. These were called grist mills or water mills.
In the year 1086, the Domesday Book was written. The multi-volume books are very large. Hand-written on the pages of the books are lists of all properties, homes, stores and other things in England. The Domesday Book listed 5,624 waterwheel-driven mills in England south of the Trent River. That was about one mill for each 400 people.
Water can either go over the top of the wheel like in the photograph on the left, or the wheel can be placed in the moving river. The flow of the river then turns the wheel at the bottom like in the moving graphic on the right.
Today, moving water can also be used to make electricity.
Hydro means water. Hydro-electric means making electricity from water power.
Hydroelectric power uses the kinetic energy of moving water to make electricity. Dams can be built to stop the flow of a river. Water behind a dam often forms a reservoir Like the picture of Shasta Dam in Northern California pictured on the right. Dams are also built across larger rivers but no reservoir is made. The river is simply sent through a hydroelectric power plant or powerhouse. You can see this in the picture of The Dalles Dam on the Columbia River along the border of Oregon and Washington State.

Hydro Power6276143 Hydro is one of the largest producers of electricity in the United States. Water power supplies about 10 percent of the entire electricity that we use. In states with high mountains and lots of rivers, even more electricity if made by hydro power. In California, for example, about 15 percent of all the electricity comes from hydroelectric.
The state of Washington leads the nation in hydroelectricity. The Grand Coulee, Chief Joseph and John Day dams are three of six major dams on the Columbia River. About 87 percent of the electricity made in Washington state is produced by hydroelectric facilities. Some of that electricity is exported from the state and used in other states.

Calculating the amount of available 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[5] is given by
\, E = mgh
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.
\frac{E}{t} = \frac{m}{t}gh
Substituting P for Et and expressing mt 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:
P = \rho\, \phi\, g \, h
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. [6]
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.
P = \frac{1}{2}\,\rho\,\phi\, v^2
where v is the speed of the water, or with
 \phi = A\, v
where A is the area through which the water passes, also
P = \frac{1}{2}\,\rho\, A\, v^3
Over-shot water wheels can efficiently capture both types of energy

Modern usage

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.
  • 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.
  • 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.

History

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)[2] 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.[3]

Hydropower

Hydropower

From Wikipedia, the free encyclopedia
Saint Anthony Falls, United States.
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 development of 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.