Water Wheels and Hydropower

A water wheel is a machine that converts the energy of moving or falling water into rotary mechanical motion. Water possesses energy by virtue of its position and its flow: water held at a height has potential energy, and as it falls or runs downhill that energy becomes kinetic energy of motion. A water wheel intercepts this moving water so that it pushes against blades or buckets around the rim, turning the wheel and producing useful work, such as grinding grain or driving machinery.

Several designs developed according to the available flow and drop. In an overshot wheel, water is delivered to the top so that its weight in the buckets turns the wheel, making efficient use of a high fall. In an undershot wheel, the lower blades dip into a stream and are pushed by its current, suited to a fast flow with little drop, while breast wheels and middle-shot wheels take water at an intermediate height. For centuries water wheels were a principal source of mechanical power for mills and early industry, before steam and electric power.

The same principle underlies modern hydroelectric power, where flowing water spins a turbine, a more efficient successor to the water wheel, which drives a generator to produce electricity. Dams raise the water level to increase the available fall, and tidal schemes harness the rise and fall of the sea. Because the water is replenished by rainfall and the natural water cycle, hydropower is regarded as a renewable source of energy.

Frequently asked questions

How does a water wheel work?

Moving or falling water pushes against blades or buckets around the wheel's rim, turning it. This converts the water's energy into rotary motion that can drive machinery.

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

An overshot wheel takes water at the top so its weight turns the wheel, suiting a high fall; an undershot wheel is pushed by a current striking its lower blades, suiting a fast flow with little drop.

Is hydropower renewable?

Yes. The water is continually replenished by rainfall and the natural water cycle, so the energy source is not used up.