Water Turbines and Water Wheels

Water wheels and water turbines extract energy from moving water, converting the flow or fall of a stream into rotary motion that can grind grain, drive machinery, or turn a generator to produce electricity. The water wheel is the older technology, used for centuries to power mills, while the modern turbine is a more compact and efficient enclosed device developed during the industrial age.

Traditional wheels are classified by where the water meets them. An undershot wheel is turned by water flowing past its lower paddles, relying on the current's speed. An overshot wheel receives water at the top, so the weight of water filling its buckets does most of the work, making it more efficient where a sufficient drop, or head, is available. A breast wheel takes water at about axle height, a compromise between the two. A tub wheel sits horizontally and is spun by water directed against its blades.

Modern hydroelectric turbines are designed for specific combinations of head and flow. The Pelton wheel is an impulse turbine in which high-pressure jets of water strike cup-shaped buckets around its rim, suited to a tall head and modest flow. Reaction turbines, in which water flows through curved blades within a casing, suit lower heads and larger volumes. The power available from any of these depends on both the height the water falls and the rate at which it flows. Small do-it-yourself versions can drive a generator for off-grid power where a reliable stream is present.

Frequently asked questions

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

An undershot wheel is driven by water flowing past its bottom and depends on current speed, while an overshot wheel is filled at the top and driven mainly by the weight of falling water, which is generally more efficient.

When is a Pelton wheel used?

A Pelton wheel suits sites with a high head and relatively low flow. Jets of water strike its cup-shaped buckets, making it efficient where water falls from a considerable height.

What determines how much power a turbine can produce?

Output depends on the head, meaning the vertical distance the water falls, and the flow rate, the volume of water passing per second. Greater head and flow mean more available power.