The most efficient form of water wheel is a pelton wheel or water turbine. Nozzles direct forceful streams of water against a series of pairs of buckets. The buckets are mounted around the edge of a wheel. Each bucket forces a flow of water to completely reverse its direction. Usually, the buckets are mounted in pairs, to keep the forces on the wheel balanced.
Since water is not a compressible fluid, almost all of the available energy is extracted in the first stage of the turbine. Therefore, pelton wheels have only one wheel, unlike turbines that operate with compressible fluids.
Pelton wheels are made in all sizes. There are multiton pelton wheels mounted on vertical oil pad bearings in the generator houses of hydroelectric plants. The smallest pelton wheels, only a few inches across, are used with household plumbing to tap power from mountain streams with a few gallons per minute of flow, but thirty meters or more of head.
In general, as the height of fall increases, less volume of water can generate the same power. Energy is force times distance, and the higher pressure can apply the same force with less mass flow.
Every installation therefore has its own most-efficient pressure, speed and volume. Usually small installations use standardized buckets, and adapt the turbine to one of a family of generators and wheel sizes by using custom plumbing. Small turbines can be adjusted somewhat by varying the number of nozzles and buckets per wheel, and choosing different diameters of wheel. Large installations custom design the turbine's torque and volume to turn a standard generator.
Many small installations also can double as a developed source of pressurized household wash water, or animal feed water.
In areas where household water is sufficiently cheap, a pelton wheel can generate power from household water.
The unusual item in most pelton wheel installations is the governor. It usually controls the flow of water based on the speed of the wheel.
Supposedly Pelton discovered his wheel by noticing how water splashed out of the nostrils of a cow as it drank from a stream.
