Turbines

Pelton turbine

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,[1] 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.

Function

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.

Applications

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.

Kaplan turbine

The Kaplan turbine is a propeller-type water turbine that has adjustable blades. It was developed in 1913 by the Austrian professor Viktor Kaplan.

The Kaplan turbine was an evolution of the Francis turbine. Its invention allowed efficient power production in low-head applications that was not possible with Francis turbines.

Kaplan turbines are now widely used throughout the world in high-flow, low-head power production.

Development

Viktor Kaplan living in Brno, Moravia, now Czech Republic, obtained his first patent for an adjustable blade propeller turbine in 1912. But the development of a commercially successful machine would take another decade. Kaplan struggled with cavitation problems, and in 1922 abandoned his research for health reasons.

In 1919 Kaplan installed a demonstration unit at Poděbrady, Czechoslovakia. In 1922 Voith introduced an 1100 HP (about 800 kW) Kaplan turbine for use mainly on rivers. In 1924 an 8 MW unit went on line at Lilla Edet, Sweden. This marked the commercial success and wide spread acceptance of Kaplan turbines.



Theory of operation

The Kaplan turbine is an inward flow reaction turbine, which means that the working fluid changes pressure as it moves through the turbine and gives up its energy. The design combines radial and axial features. 

The inlet is a scroll-shaped tube that wraps around the turbine's wicket gate. Water is directed tangentially through the wicket gate and spirals on to a propeller shaped runner, causing it to spin. 

The outlet is a specially shaped draft tube that helps decelerate the water and recover kinetic energy. 

The turbine does not need to be at the lowest point of water flow, as long as the draft tube remains full of water. A higher turbine location, however, increases the suction that is imparted on the turbine blades by the draft tube. The resulting pressure drop may lead to cavitation. 

Variable geometry of the wicket gate and turbine blades allow efficient operation for a range of flow conditions. Kaplan turbine efficiencies are typically over 90%, but may be lower in very low head applications. 

Current areas of research include CFD driven efficiency improvements and new designs that raise survival rates of fish passing through. 

Because the propeller blades are rotated by high-pressure hydraulic oil, a critical element of Kaplan design is to maintain a positive seal to prevent emission of oil into the waterway. Discharge of oil into rivers is not permitted. 

Applications 

Kaplan turbines are widely used throughout the world for electrical power production. They cover the lowest head hydro sites and are especially suited for high flow conditions. 

Inexpensive micro turbines are manufactured for individual power production with as little as two feet of head. 

Large Kaplan turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%. They are very expensive to design, manufacture and install, but operate for decades. 

Variations 

The Kaplan turbine is the most widely used of the propeller-type turbines, but several other variations exist: 

Propeller turbines have non-adjustable propeller vanes. They are used in where the range of head is not large. Commercial products exist for producing several hundred watts from only a few feet of head. Larger propeller turbines produce more than 100 MW. 

Bulb or Tubular turbines are designed into the water delivery tube. A large bulb is centered in the water pipe which holds the generator, wicket gate and runner. Tubular turbines are a fully axial design, whereas Kaplan turbines have a radial wicket gate. 

Pit turbines are bulb turbines with a gear box. This allows for a smaller generator and bulb. 

Straflo turbines are axial turbines with the generator outside of the water channel, connected to the periphery of the runner. 

S- turbines eliminate the need for a bulb housing by placing the generator outside of the water channel. This is accomplished with a jog in the water channel and a shaft connecting the runner and generator. 

VLH turbine an open flow, very low head "kaplan" turbine slanted at an angle to the water flow. It has a large diameter, is low speed using a permanent magnet alternator with electronic power regulation and is very fish friendly (<5%>

A type of turbine, developed around 1915 by the Austrian enginer Viktor Kaplan (1876-1934), that has two or more blades, the pitch of which is adjustable; it resembles a marine propeller. The turbine may have gates to control the angle of the fluid flow into the blades. 

Kaplan turbines are well suited to situations in which there is a low head and a large amount of discharge. The adjustable runner blades enable high efficiency even in the range of partial load, and there is little drop in efficiency due to head variation or load.

As a result of recent developments, the range of Kaplan turbine applications has been greatly increased. They are being applied, for example, in exploiting many hydro sources previously discarded for economic or environmental reasons, and have also been used as wind turbines. The adjustable runner blades add to the complexity of construction of a Kaplan turbine. The runner blade operating mechanism consists of a pressure oil head, a runner servomotor, and the blade operating rod inside the shaft. 



Francis turbine

The Francis turbine is a type of water turbine that was developed by James B. Francis. It is an inward flow reaction turbine that combines radial and axial flow concepts.

Francis turbines are the most common water turbine in use today. They operate in a head range of ten meters to several hundred meters and are primarily used for electrical power production.

Development

Water wheels have been used historically to power mills of all types, but they are inefficient. 19th century efficiency improvements of water turbines allowed them to compete with steam engines (wherever water was available).

In 1826 Benoit Fourneyron developed a high efficiency (80%) outward flow water turbine. Water was directed tangentially through the turbine runner causing it to spin. Jean-Victor Poncelet designed an inward-flow turbine in about 1820 that used the same principles. S. B. Howd obtained a U.S. patent in 1838 for a similar design.

In 1848 James B. Francis improved on these designs to create a turbine with 90% efficiency. He applied scientific principles and testing methods to produce the most efficient turbine design ever. More importantly, his mathematical and graphical calculation methods improved the state of the art of turbine design and engineering. His analytical methods allowed confident design of high efficiency turbines to exactly match a site's flow conditions.

Theory of operation

The Francis turbine is a reaction turbine, which means that the working fluid changes pressure as it moves through the turbine, giving up its energy. A casement is needed to contain the water flow. The turbine is located between the high pressure water source and the low pressure water exit, usually at the base of a dam.

The inlet is spiral shaped. Guide vanes direct the water tangentially to the runner. This radial flow acts on the runner vanes, causing the runner to spin. The guide vanes (or wicket gate) may be adjustable to allow efficient turbine operation for a range of water flow conditions.

As the water moves through the runner its spinning radius decreases, further acting on the runner. Imagine swinging a ball on a string around in a circle. If the string is pulled short, the ball spins faster. This property, in addition to the water's pressure, helps inward flow turbines harness water energy.



Application

Large Francis turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%. 

Francis type units cover a wide head range, from 20 meters to 700 meters and their output varies from a few kilowatt to 1,000 megawatt. 

In addition to electrical production, they may also be used for pumped storage; where a reservoir is filled by the turbine (acting as a pump) during low power demand, and then reversed and used to generate power during peak demand. 

Francis turbines may be designed for a wide range of heads and flows. This, along with their high efficiency, has made them the most widely used turbine in the world. 


A type of hydropower reaction turbine that contains a runner that has water passages through it formed by curved vanes or blades. The runner blades, typically 9 to 19 in number, cannot be adjusted. As the water passes through the runner and over the curved surfaces, it causes rotation of the runner. The rotational motion is transmitted by a shaft to a generator. 

The Francis turbine has a wide range of applications and can be used for fall heights of 2–800 meters. The largest Francis turbines have an output of 750 MW. Compare with Kaplan turbine and Pelton turbine. 

The Francis turbine is an inward flow turbine that is the most efficient and widely used water turbine in the world today. It works best in higher head (pressure) applications, and hydroelectric power plants at big dams use these guys to make lots of watt-hours. 

The term "inward flow" means that the turbine itself (the thing with the blades that the water "presses on" to make the thing turn) has the water directed from the outside of the turbine wheel in onto the blades, and through them to a "center area" for the water to exit. The unit is darn efficient. Pictures are a mouse click away. Use the link to surf on over to our friends at Wikipedia (from which some of the data here was gathered) for more information and the cool pics that really show a viewer how the unit works.