Hydro Energy and Hydropower

Hydropower is energy obtained from moving or falling water. It is one of the oldest forms of mechanical power and is now a major source of renewable electricity. The principle rests on the conversion of energy: water held at a height possesses potential energy, and as it descends this is converted into kinetic energy, the energy of motion. When the moving water strikes a turbine or wheel, that kinetic energy is transferred as rotational motion, performing useful work.

In a hydroelectric power plant, water is usually held behind a dam to create a difference in height. Released water flows through a water turbine, spinning it and driving a connected generator that produces electricity. The amount of power available depends on both the height the water falls and the rate at which it flows. Because the water cycle continually replenishes rivers and reservoirs, hydroelectricity is classed as a renewable resource, though dams can alter local environments.

Before electricity, the same energy was harnessed mechanically by water wheels to grind grain and drive machinery. Designs differed in where the water met the wheel. Overshot mills fed water onto the top of the wheel, relying largely on the weight of the water; undershot mills placed the wheel in a flowing stream so the current pushed its lower blades; and breast-shot and middle-shot wheels took water at an intermediate height. A related approach, tidal energy, captures the rise and fall of the sea, which is also driven by the movement of water on a large scale.

Frequently asked questions

How does a hydroelectric power plant make electricity?

Water held behind a dam flows down through a turbine, spinning it. The turbine drives a generator, which converts the rotational motion into electricity.

What is the difference between an overshot and an undershot water wheel?

An overshot wheel receives water at the top and is turned mainly by the water's weight, while an undershot wheel sits in a stream and is pushed around by the current at its base.

Why is hydropower considered renewable?

The water cycle continually refills rivers and reservoirs through rainfall and snowmelt, so the source is naturally replenished rather than used up.