Also called a free-jet turbine or Pelton wheel, a type of impulse turbine, named after L. A. Pelton who invented it in 1880. Water passes through nozzles and strikes spoon-shaped buckets or cups arranged on the periphery of a runner, or wheel, which causes the runner to rotate, producing mechanical energy. The runner is fixed on a shaft, and the rotational motion of the turbine is transmitted by the shaft to a generator.
Pelton turbines are suited to high head, low flow applications. Typically, to work this type of turbine, water is piped down a hillside so that at the lower end of the pipe it emerges from a narrow nozzle as a jet with very high velocity. The Pelton turbine can be controlled by adjusting the flow of water to the buckets. In order to stop the wheel a valve is used to shut off the water completely. Small adjustments, necessitated by alterations in the load on the generator, are more safely made by a device which deflects part of the water jet away from the buckets.
Pelton wheels are are used in storage power stations with downward gradients up to 2,000 meters and can contain up to 6 nozzles. Compare with the Francis turbine and the Kaplan turbine.
Pelton turbines are medium to high head free jet impulse turbines. The jet(s) strike the splitter edge of the double bucket and is turned through an angle of nearly 180 degrees before falling under gravity into the discharge channel or tailrace.
The Pelton Wheel is a high head, free jet impulse turbine.
This hydro turbine, which is of the free-jet type, had been invented around the year 1880 by the American Lester Pelton. Bucket-like blades are attached to its runner, subdivided into two half-shells respectively by a sharp edge.
The water flow may be influenced through one or several needle jets that may be controlled finely. The water leaves the nozzles, hitting the subdivided runner blades tangentially
The water jet is deflected in the hollows of the blades by almost 180 degrees, transmitting its energy to the turbine. So the water exclusively acts on the turbine runner through the deflecting pressure that is produced.
Presently Pelton turbines are available within the following ranges:
Heads of between 100 and 400 meters
Flows between 0.02 and 1.0 m³/s
Nominal power outputs of 10 to 1500 kW
The Pelton wheel is among 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 the principle of Newton's second law to extract energy from a jet of fluid. Although the one-piece cast impulse water turbine was invented by Samuel Knight in Sutter Creek, in the California Mother Lode gold mining region, Pelton modified this invention to create his more efficient design. Knight Foundry is the last water-powered foundry known to exist in the United States and is still operated using Knight impulse turbines, used to extract power from high heads and low discharge water flows.
The 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. As water flows into the bucket, the direction of the water velocity changes to follow the contour of the bucket. When the water-jet contacts the bucket, the water exerts pressure on the bucket and the water is decelerated as it does a "u-turn" and flows out the other side of the bucket at low velocity. In the process, the water's momentum is transferred to the turbine. This "impulse" does work on the turbine. For maximum power and efficiency, the turbine system is designed such that the water-jet velocity is twice the velocity of the bucket. A very small percentage of the water's original kinetic energy will still remain in the water; however, this allows the bucket to be emptied at the same rate it is filled, (see conservation of mass), thus allowing the water flow to continue uninterrupted. Often two buckets are mounted side-by-side, thus splitting the water jet in half (see photo). This balances the side-load forces on the wheel, and helps to ensure smooth, efficient momentum transfer of the fluid jet to the turbine wheel.
Because water and most liquids are nearly incompressible, almost all of the available energy is extracted in the first stage of the hydraulic turbine. Therefore, Pelton wheels have only one turbine stage, unlike gas turbines that operate with compressible fluid.
Pelton wheels are the preferred turbine for hydro-power, when the available water source has relatively high hydraulic head at low flow rates. Pelton wheels are made in all sizes. There exist multi-ton Pelton wheels mounted on vertical oil pad bearings in hydroelectric plants. The largest units can be up to 200 megawatts. The smallest Pelton wheels are only a few inches across, and can be used to tap power from mountain streams having flows of a few gallons per minute. Some of these systems utilize household plumbing fixtures for water delivery. These small units are recommended for use with thirty metres or more of head, in order to generate significant power levels. Depending on water flow and design, Pelton wheels operate best with heads from 15 metres to 1,800 metres, although there is no theoretical limit.
The Pelton wheel is most efficient in high head applications (see the "Design Rules" section). Thus, more power can be extracted from a water source with high-pressure and low-flow than from a source with low-pressure and high-flow, even though the two flows theoretically contain the same power. Also a comparable amount of pipe material is required for each of the two sources, one requiring a long thin pipe, and the other a short wide pipe.