Hydroelectric plants harness energy by passing lowing water through a turbine. Thewater turbine rotation is delivered to a generator, which generates electricity. Thequantity of electricity that can be produced at a hydroelectric plant relies upon twovariables. These variables are the vertical distance that the water falls, called the"head", and the low rate, calculated as volume over time. The amount of electricitythat is produced is thus proportional to the head product and the low rate.
So, hydroelectric power stations can normally be separated into two kinds. Themost widespread are "high head" plants and usually employ a dam to stock up water atan increased height. They also store water at times of rain and discharge it during drytimes. This results in reliable and consistent electricity generation, capable of meetingdemand since low can be rapidly altered. At times of excess electrical system capacity,usually available at night, these plants can also pump water from one reservoir toanother at a greater height. When there is peak electrical demand, the higher reservoirreleases water through the turbines to the lower reservoir.
"Low head" hydroelectric plants usually exploit heads of just a few meters or less.These types of power station use a weir or low dam to channel water, or no dam at alland merely use the river low. Unfortunately their electricity production capacity fluctuateswith seasonal water flow in a river.
Until only recently people believed almost universally that hydroelectric power wasan environmentally safe and clean means of generating electricity. Hydroelectric stationsdo not release any of the usual atmospheric pollutants emitted by power plants fuelledby fossil fuels so they do not add to global warming or acid rain. Nevertheless, recentstudies of the larger reservoirs formed behind dams have implied that decomposing,Iooded vegetation could give off greenhouse gases equal to those from other electricitysources.
The clearest result of hydroelectric dams is the Iooding of huge areas of land. Thereservoirs built can be exceptionally big and they have often Iooded the lands ofindigenous peoples and destroyed their way of life. Numerous rare ecosystems are alsoendangered by hydroelectric power plant development.
Damming rivers may also change the quantity and quality of water in the riversbelow the dams, as well as stopping fish migrating upstream to spawn. In addition, silt,usually taken downstream to the lower parts of a river, is caught by a dam and so theriver downstream loses the silt that should fertilize the rivers Iood plains during highwater periods.
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