Water Turbines in Hydroelectric Power Plants

(Transcript of the video commentary.)

Humanity has been using water as a source of mechanical energy for a long time. In the past, water wheels were widely used for irrigation purposes and to power machinery, but with the arrival of the industrial revolution in the nineteenth century, their limitations resulted in the gradual switch to more modern and efficient turbines. The first to use the term “turbine” was French engineer Claude Burdin at the beginning of the nineteenth century. The word is based on the Latin “turbis” — something that turns, a vortex. Turbines are based on the use of this vortex-like component of the movement of water, allowing them to be small, faster, and much more powerful than the old water wheels.

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 rotating wheel called a runner, creating the force that moves the entire turbine rotor. Depending on the energy transfer method and pressure in the runner, water turbines are classified into two basic groups: reaction (high-pressure) turbines and impulse (constant-pressure) turbines.

In reaction turbines, part of the pressure energy of water changes to kinetic energy as it passes through the turbine, which is transferred to the rotor. Due to the changing pressure these turbines must be enclosed and their outlet is usually connected to a draft tube so that the entire height difference (head) can be utilized. Reaction turbines can be used for a head of up to 400 metres.

In impulse turbines, the water’s pressure energy is changed to kinetic energy using nozzles. The accelerated stream of water is then aimed at the blades of the runner, transferring its 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.

Like all technology, water turbines also underwent certain historical development. Let’s go over at least the most important milestones. Likely the oldest references to hydraulic machines closely resembling turbines are from the era of the Roman Empire. The first reactive water turbine was developed in the eighteenth century by Johann Andreas Segner. His simple concept became an evolutionary step on the road to more modern turbines. In the 1820s French mathematician and mechanic Jean-Victor Poncelet developed a water turbine with inward flow. in 1826 French engineer Benoit Fourneyron made a significant advance in turbine design by assembling a vertical reaction turbine with centrifugal flow and a runner spinning freely in the air. In 1844 Uriah Atherton Boyden designed a water turbine model that improved the Fourneyron turbine The first modern reaction water turbine was developed in 1849 by James Bicheno Francis. His vertical spiral turbine was the first to use adjustable guide vanes to direct water inward to a runner with fixed blades. In 1876, a quarter-century after Francis designed his turbine, John Buchanan McCormick designed the first mixed flow reaction turbine. The last key milestone was in 1913, when Austrian engineer and inventor Viktor Kaplan designed a high-speed propeller-type turbine using adjustable blades on both the wicket gate and the runner.

All hydro machines being perfected until the end of the nineteenth century were basically reaction turbines. However, the development of impulse turbines was also taking place. The first impulse system resembling a water wheel with buckets was developed in 1866 by Samuel Knight, a miller in California. In 1879, when experimenting with Knight’s wheel, Lester Pelton developed the Pelton impulse turbine with 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.

One of the most frequently used water turbines in the world is the Kaplan turbine. It 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. 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. Kaplan turbines are best used in locations with a small head and a relatively high and variable flow. 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.

Another important hydro power turbine is the Francis turbine. This, too, is a reaction turbine, meaning that as the pressure energy of water is transformed into kinetic energy as it passes through the turbine — part is transformed in the distributor and part as it passes through the runner. The turbine has a combined radial-axial design with adjustable wicket gates. It is intended for a head from 20 to 700 metres and achieves good efficiency of around 90%.

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, not only in standard hydro power plants, but thanks it its reversible design, often also in pumped storage hydroelectricity applications. This is because it works equally well as a turbine and a powerful pump.

The last water turbine being presented is the Pelton turbine. This is a constant-pressure impulse turbine with partial tangential jets striking 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 1,800 metres and small flow rates, typically for mountainous areas. They can achieve up to 95% efficiency.

The water is conducted to the turbine from the upper reservoir by a pressure feeder. In nozzles situated around the turbine, the pressure energy of the water is transformed into kinetic energy. The nozzles aim the water jet at blades shaped like double spoon-shaped buckets mounted on the outer rim of the runner. A blade situated between the buckets splits the jet into two and the shape of the buckets makes the water do a “u-turn” and exit the bucket. 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 moving a conical regulation spear in the nozzle, which functions as a valve that reduces or increases the flow of water on the turbine. If the turbine needs to be stopped immediately, the water jet is simply diverted from the runner buckets using various deflectors.

Water turbines are a necessary and fundamental part of every hydroelectric power plant, allowing energy to be generated using a clean, relatively available, and most likely the most important renewable resource — water. Water, which as a primary source gives up its potential and kinetic energy in a water turbine, but which through the natural cycle of evaporation and condensation is constantly being renewed.