Hydropower is generated from the water flowing in the river or oceans. There are two water cycles involved in the generation of electricity. One water cycle is the natural cycle and the other is inside the hydropower station.
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Before seeing various types of hydropower in details let us see the water cycle that occurs during the generation of hydropower. There are two water cycles, one in nature and the other in the hydraulic power plant. Let us see both of them:
1) Water cycle in nature: In our natural environment there are number of oceans and rivers and then there is sun above them all. Due to solar energy the surface of the rivers and oceans get heated due to which the water on the surface evaporates. The evaporated water vapors move towards the upper layers of the atmosphere where they form the clouds. Upon precipitation of clouds rain occurs. Due to heavy rains large quantities of water flows through various parts of the earth in the forms of streams, channels and rivers. All the water from rivers is collected back to the oceans, where evaporation of water occurs again.
All these are seasonal processes with evaporation occurring mainly during the hot seasons, and rain falling during rainy seasons. In many geographical areas evaporation of water and rain can occur even during short periods like within days.
2) Water cycle in the hydraulic power plant: The water in rivers posses two types of energies: kinetic energy and potential energy. The energy of water due to its motion is called as kinetic energy. The energy of water due to its high level is called as potential energy. Depending on the type of hydraulic power plants kinetic and/or potential energy of water is used to generate electricity.
The most commonly used method of production of electricity from hydropower is by dams, which are constructed across the large rivers. The large quantities of water from river are diverted by pipelines also called as penstock, towards the main plant where large turbines are located. The water from penstock is allowed to fall on the large turbine blades that start rotating. The shaft of the turbine rotates in the electric generators where electricity is generated. This electricity is then passed to the transformers from where it is connected to the main national grid.
The water leaving the turbine flows back to the river at the lower levels. In almost all the plants where water is used to generate electricity, the motion of water is used to rotate the turbine which generates the electricity in the generators and the water flows back to the river or ocean.
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Water Cycle in Nature and Hydropower Plant (Source: Energy Kid's Page)