Impoundment systems use a dam to capture and contain water in a reservoir. Gates control the flow of the water into a large pipe (called a penstock) that feeds the pressurized water into the turbine, which spins an electrical generator. The water is then discharged back into the river way.
The depth of the reservoir creates head pressure. The deeper the reservoir, the greater the head pressure and the greater the potential to generate power. The amount of water flowing through the turbine (in combination with the head pressure, which depends on the depth of the water in the dam) can be varied to control the amount of electrical power output of the hydropower generator.
The total potential energy that can be generated is a function of how many gallons of water are in the reservoir times the head pressure. Low depth, wide dams are not as good as high, narrow dams in terms of power output, although the total energy potential may be the same.
Impoundment systems are by far the most common type of hydropower generating system in use. A flowing river with a section amenable to building a dam is the only requirement, which explains why this type of system is found in over 160 countries worldwide. Because, according to estimates, only a third of worldwide hydropower potential has been tapped, the growth in foreign hydropower promises to be very high in the next decades.
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