(Transcript of the lesson commentary.)
From waterwheel to turbine
The use of water wheels as a source of mechanical energy extends into the distant past. Water wheels were widely used for irrigation purposes and to power machinery, but with the arrival of the industrial revolution, the limits of these imposing structures began to be apparent. During the nineteenth century, the need for more powerful sources of power, simple experimentation, and the application of new scientific discoveries gradually led to the development of modern turbines.
By definition a water turbine is a rotary mechanism that transforms the potential and kinetic energy of water into mechanical work. In all types of water turbines, flowing water is aimed at the blades of a runner, creating the force that makes it turn. In this way, energy is transferred from the water to the turbine.
The term “turbine” was first used by French engineer Claude Burdin in his dissertation at the beginning of the nineteenth century. The word is based on the Latin “turbis” — something that turns, a vortex. This vortex-like component of moving water allowed substantially smaller turbines to provide the same amount of power as water wheels. Their higher rotor speed also allowed turbines to process more water and use much greater head. In other words, they could generate much more power than old water wheels. Depending on the energy transfer method and pressure in the runner, water turbines are classified into two basic groups: reaction turbines and impulse turbines.
In reaction turbines, part of the pressure energy of water changes to kinetic energy as it passes through the turbine (pressure declines and velocity increases), which is transferred to the rotor. Due to the changing pressure these turbines must be enclosed and their outlet should be connected to a draft tube so that the entire height difference (head) between the upper and lower reservoir can be utilized. These turbines can be used for a head of up to 400 metres.
At the supply side of impulse turbines, nozzles convert the pressure energy of water into kinetic energy. The fast jet of water is then aimed at the blades of the runner, transferring energy and causing it to spin. The turbine need not be enclosed because the pressure of the water as it passes through the runner does not change. Impulse turbines are used for a head greater than 300 metres.
The history of water turbines
Likely the oldest references to hydraulic machines closely resembling turbines are from the era of the Roman Empire. They had a horizontal runner with slanted blades, located at the bottom of a circular shaft with tangential water flow. The first reactive water turbine was developed in the eighteenth century by Johann Andreas Segner. His cylindrical design with bent radial nozzles became an important step in the evolution of all modern turbines. In the 1820s French mathematician, mechanic, and geometrician Jean-Victor Poncelet invented a water turbine with internal flow. In 1826 French engineer Benoit Fourneyron made a significant advance in turbine design with his turbine with outward flow. He designed a vertical reaction turbine with centrifugal flow and a runner spinning freely in the air. In 1844 Uriah Atherton Boyden improved the Fourneyron turbine by designing a water turbine model with outward flow.
The first modern reaction water turbine with inward (radial) flow was developed in 1849 by James Bicheno Francis. His vertical spiral turbine used adjustable wicket gates to direct water inward to a runner with fixed blades. The Francis turbine achieved up to 90% efficiency and became a commonly used pressurized hydraulic motor. In 1876, a quarter-century after Francis designed his turbine, John Buchanan McCormick designed the first mixed-flow reaction turbine.
Around 1913 Austrian engineer and inventor Viktor Kaplan designed a high-speed propeller-type turbine, evolving and improving the Francis turbine. To achieve better efficiency, it uses adjustable blades on both the wicket gate and the runner. The Kaplan turbine led to a revolution in the development of hydraulic sites with low head and variable flow. Standard hydraulic machines being perfected until the end of the nineteenth century were basically reaction hydraulic machines. However, the invention and development of constant-pressure impulse turbines was also taking place.
The first impulse system was developed in 1866 by Samuel Knight, a miller in California. His high-speed cast iron impulse water wheel with buckets was the predecessor of the Pelton turbine. In 1879, when experimenting with Knight’s wheel, Lester Pelton developed the Pelton impulse turbine. In it he placed nozzles spraying a jet of water tangentially on the bucket-shaped blades of the runner. Fifteen years later, William Doble improved the Pelton turbine. His double elliptical buckets that included a cut defined the form of the Pelton turbine still in use today, achieving efficiency of up to 92%.
Kaplan turbine
The Kaplan turbine is a reaction turbine where both the wicket gate blades and the runner blades can be adjusted. Its head ranges from 5 to 80 metres and flow rates of up to several hundred cubic metres per second. Kaplan turbines are best used in locations with a small head and a relatively high and variable flow. Thanks to its dual regulation it is more complex and expensive than the Francis turbine, but has a stable efficiency that exceeds 90% in larger machines.
Water from a reservoir is conducted to the turbine via high-pressure pipes, and at the turbine is equally distributed along its circumference. A system of adjustable wicket gates accelerates the water and aims it at the blades of a runner reminiscent of a ship's screw propeller. A mechanism inside the hollow shaft of the runner automatically adjusts the pitch of the runner blades, optimizing the angle of attack for various values of head and flow to maximize efficiency. Once it has transferred its energy to the turbine rotor, the water flows through pipes to a bottom reservoir. These pipes, called draft tubes, help increase turbine efficiency by creating suction where the water exits the rotor.
A Kaplan turbine belongs in the category of high-speed hydraulic motors because the circumferential velocity of the turbine’s runner is roughly twice that of the water flowing past it. At the cost of reduced efficiency under marginal flow conditions, smaller and lighter Kaplan turbines can be equipped with fixed-angle wicket gates or runner blades.
Francis turbine
The Francis turbine is a reaction turbine, which means that as water passes through the turbine, its pressure decreases. As pressure declines the pressure energy is transformed into kinetic energy — part is transformed in the distributor and part as it passes through the runner. The turbine has a combined radial-axial design and is intended for a head from 20 to 700 metres. The wicket gates at the turbine inlet have variable angle, allowing them to be optimized for a given flow rate. The specially shaped runner has fixed blades.
The water from a pressure feeder flows into a concrete or metal spiral turbine casing, where it is equally distributed along its circumference to all inter-blade channels, adjusted by turning the wicket gates to correspond to the current flow. As it flows through these channels, the water gains optimum velocity and direction before hitting the fixed blades of the runner. After passing through the runner and giving up the maximum amount of its energy, the water is conducted away through a draft tube running parallel to the turbine axis into the bottom reservoir.
The Francis turbine is one of the most commonly used turbines in the area of hydro power, with approximately 90% efficiency. Its reversible design is often also used in pumped storage hydroelectricity applications, as it operates equally as a turbine and in reverse as a pump.
Pelton turbine
A Pelton turbine is a constant-pressure impulse turbine with partial tangential jets, i.e. the water flow into it in the direction of the tangent to the circumference of the runner. Constant pressure means that the water that strikes the blades of the runner has the same pressure as the water leaving it. Pelton turbines are used for very high head values of up to 1800 metres and small flow rates, typically for mountainous areas. Larger installations with these turbines can achieve up to 95% efficiency.
The water is conducted to the turbine from the upper reservoir by a pressure feeder to nozzles situated around the turbine. There, the pressure energy of the water is transformed into kinetic energy and the water sprays out of the nozzle in the form of a jet aimed at the buckets located on the circumference of the runner. The buckets have a double shape, and a blade situated between the buckets splits the jet into two. The shape of the buckets makes the water do a “u-turn”. Because water is nearly incompressible, as it reverses direction it give up almost all of its energy to the buckets and exits the turbine with minimal residual velocity.
The output of a Pelton turbine is regulated by slowly moving a conical regulation spear in the nozzle. This reduces or increases water flow to the turbine. If the turbine needs to be stopped immediately, the water jet is simply diverted from the runner buckets using various deflectors.
