23 April, 2007

Pelton Wheel

Pelton wheel, also called a Pelton turbine, is one of the most efficient types of water turbines. It was invented by Lester Allan Pelton (1829-1908) in the 1870s, and is an impulse machine, meaning that it uses Newton's second law to extract energy from a jet of fluid.

The pelton wheel turbine is a tangential flow impulse turbine, water flows along the tangent to the path of the runner. Nozzles direct forceful streams of water against a series of spoon-shaped buckets mounted around the edge of a wheel. Each bucket reverses the flow of water, leaving it with diminished energy. The resulting impulse spins the turbine. The buckets are mounted in pairs, to keep the forces on the wheel balanced, as well as to ensure smooth, efficient momentum transfer of the fluid jet to the wheel. The Pelton wheel is most efficient in high head applications.

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.
Peltons are the turbine of choice for high head, low flow sites. However, Pelton wheels are made in all sizes. There are multi-ton Pelton wheels mounted on vertical oil pad bearings in the generator houses of hydroelectric plants. The largest units can be up to 200 megawatts. The smallest Pelton wheels, only a few inches across, are used with household plumbing fixtures to tap power from mountain streams with a few gallons per minute of flow, but these small units must have thirty meters or more of head. Depending on water flow and design, Pelton wheels can operate with heads as small as 15 meters and as high as 1,800 meters.

In general, as the height of fall increases, less volume of water can generate a bit more power. Energy is force times distance, in the instance of fluid flow power is expressed as P = Constant x Pressure x Volume/t. The power P grows linearly with flow rate and grows with f(Pressure^3/2.) Thus it is usually best to seek as much head or pressure as possible in hydro designs then go for flow rate.

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