Saturday, 26 April 2008
Hydropower BasicsFlowing water creates energy that can be captured and turned into electricity.
This is called hydropower.
Hydropower is currently the largest source of renewable power, generating nearly 10% of the electricity used in the United States.
The most common type of hydropower plant uses a dam on a river to store water in a reservoir.
Water released from the reservoir flows through a turbine, spinning it, which, in turn, activates a generator to produce electricity.
But hydropower doesn't necessarily require a large dam.
Some hydropower plants just use a small canal to channel the river water through a turbine.
Another type of hydropower plant—called a pumped storage plant—can even store power.
The power is sent from a power grid into the electric generators.
The generators then spin the turbines backward, which causes the turbines to pump water from a river or lower reservoir to an upper reservoir, where the power is stored.
To use the power, the water is released from the upper reservoir back down into the river or lower reservoir.
This spins the turbines forward, activating the generators to produce electricity.
Types of HydropowerImpoundmentAn impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir.
The water may be released either to meet changing electricity needs or to maintain a constant reservoir level.
DiversionA diversion, sometimes called run-of-river, facility channels a portion of a river through a canal or penstock.
It may not require the use of a dam.
Pumped StorageWhen the demand for electricity is low, a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir.
During periods of high electrical demand, the water is released back to the lower reservoir to generate electricity.
Sizes of Hydropower PlantsFacilities range in size from large power plants that supply many consumers with electricity to small and micro plants that individuals operate for their own energy needs or to sell power to utilities.
Large Hydropower
Although definitions vary, DOE defines large hydropower as facilities that have a capacity of more than 30 megawatts.
Small Hydropower
Although definitions vary, DOE defines small hydropower as facilities that have a capacity of 0.1 to 30 megawatts.
Micro HydropowerA micro hydropower plant has a capacity of up to 100 kilowatts (0.1 megawatts).
Turbine TechnologiesThere are many types of turbines used for hydropower, and they are chosen based on their particular application and the height of standing water—referred to as "head"—available to drive them.
The turning part of the turbine is called the runner.
The most common turbines are as follows:
Pelton TurbineA Pelton turbine has one or more jets of water impinging on the buckets of a runner that looks like a water wheel.
The Pelton turbines are used for high-head sites (50 feet to 6,000 feet) and can be as large as 200 megawatts.
Francis TurbineA Francis turbine has a runner with fixed vanes, usually nine or more.
water enters the turbine in a radial direction with respect to the shaft, and is discharged in an axial direction.
Francis turbines will operate from 10 feet to 2,000 feet of head and can be as large as 800 megawatts.
Propeller TurbineA propeller has a runner with three to six fixed blades, like a boat propeller.
The water passes through the runner and drives the blades.
Propeller turbines can operate from 10 feet to 300 feet of head and can be as large as 100 megawatts.
Kaplan TurbineA Kaplan turbine is a type of propeller turbine in which the pitch of the blades can be changed to improve performance.
Kaplan turbines can be as large as 400 megawatts.
How Hydropower WorksMost hydropower projects use a dam and a reservoir to retain water from a river.
When the stored water is released, it passes through and rotates turbines, which spin generators to produce electricity.
Water stored in a reservoir can be accessed quickly for use during times when the demand for electricity is high.
Dammed hydropower projects can also be built as power storage facilities.
During periods of peak electricity demand, these facilities operate much like a traditional hydropower plant — water released from the upper reservoir passes through turbines, which spins generators to produce electricity.
However, during periods of low electricity use, electricity from the grid is used to spin the turbines backward, which causes the turbines to pump water from a river or lower reservoir to an upper reservoir, where the water can be stored until the demand for electricity is high again.
A third type of hydropower project, called "run of the river", does not require large impoundment dams (although it may require a small, less obtrusive dam).
Instead, a portion of a river's water is diverted into a canal or pipe to spin turbines.
How Hydropower is UsedHydropower is currently
the largest and least expensive source of renewable electricity produced in the United States. Large and small-scale hydropower projects are most commonly used by clean power generators to produce electricity.
Our Buying Clean Electricity section provides information on buying electricity generated from hydro and other renewable resources in your state.
Many large-scale dam projects have been criticized for altering wildlife habitats, impeding fish migration, and affecting water quality and flow patterns.
As a result of increased environmental regulation, the National Hydropower Association forecasts a decline in large-scale hydropower use through 2020.
Research and development efforts have succeeded in reducing many of these environmental impacts through the use of fish ladders (to aid fish migration), fish screens, new turbine designs, and reservoir aeration.
Although funding has been limited, current research focuses on the development of a "next generation turbine, which is expected to further increase fish survival rates and improve environmental conditions.
A very small hydropower (called microhydro) project can also be installed to meet the electricity needs of a single home or small business, and is especially useful for those in remote areas.
Reference Web site :
http://people.howstuffworks.com/http://www.eere.energy.gov/RE/hydropower.html
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