To state it in a simple way, turbines are machines that convert some form of energy into mechanical energy. Based on the working medium used we have steam turbines, hydraulic turbines and gas turbines. Of these hydraulic turbines are those turbines which convert hydraulic energy (energy possessed by water) to mechanical energy. Mechanical energy is produced in the form of a rotating shaft. Hydraulic turbines are one of the older but still used mechanical devices for power extraction. Read on to know more about how hydraulic turbines play a role in hydroelectric power generation.
The development of hydraulic turbines spreads across centuries. Primitive forms of these were used since medieval times and even during the time of the Roman Empire. Simple water wheels were the first to be developed. Water wheels served as the basis of the modern hydro turbines. In water wheels the energy possessed by flowing water was used to turn a large wheel fitted with several buckets. Ever since the development of water wheels the development of hydro machines was very slow. The Romans invented two helix mills, which were used to grind grains in mills. Various new turbines were later on developed, but all of them had practical difficulties and reduced efficiencies.
Finally, it was in the year 1849 that James B Francis came up with a much more efficient and improved version of hydraulic turbine. The turbine was named after him, and thus the Francis turbine came into operation. Further research works on hydraulic turbines concentrated upon extracting the energy possessed by a water jet. This eventually led to the development of the impulse turbine by Lester Pelton in 1879 which later on came to be known as Pelton Wheel. Various other turbines were developed during the following years. Francis turbine and Pelton Wheel are still widely in use.
POWER GENERATION USING HYDRAULIC TURBINES
All hydraulic turbines require water from a reservoir or lake at a considerable height for their working. This
is obtained by either construction dams/reservoirs or by taking water from a lake on a mountain. Dams are the most common way for generating hydropower. The basic layout of a hydroelectric dam is shown here. Water is maintained at a great height in the reservoir. For beginners we can simply call this height as the head of water available for a turbine. Higher the head of water available, higher is the energy available from water. Water from the reservoir is carried through large diameter pipes called penstock. This water under pressure is fed to the turbine. The energy possessed by water is then used to turn the turbine shaft. The turbine shaft is coupled to an electric generator. As the shaft rotates, electricity is generated by the generator which is then fed to the power grid. The water leaves the turbine and is expelled onto a tail race.
BASIC PARTS OF A HYDRAULIC TURBINE
Although various forms of turbines are there, all turbines essentially consist of the following parts:
- Vanes: Vanes are fitted on to the sides of a runner (the rotating part). Water acts upon these vanes and impart its energy to the vanes. This in turn causes a rotating effect on the runner.
- Shaft: this is the rotating part connected to the runner which is coupled to the electric generator.
- Casing: Most turbines are enclosed in a casing which prevents spillage of water. In certain type of turbines such as reaction and axial turbines (which will be explained later) these casing is an essential part which converts the pressure energy of water into kinetic energy.
- Tailrace: after passing through the vanes the water exits the turbine into the tailrace.
DIFFERENT TYPES OF ENERGY IN A FLOWING FLUID:
A fluid in motion generally has three kinds of energy associated with it. These are:
- Potential energy: Energy possessed by the fluid by virtue of its position (same as the energy possessed by a stone falling under gravity).
- Kinetic energy: Energy possessed by the fluid by virtue of its motion (just like a moving bullet has some energy associated with it).
- Pressure energy: Energy possessed by the fluid on account of its pressure.
Bernoulli’s equation governs the energy changes of a flowing fluid. This equation has very large applications and is familiar to all high school students. In simple a simple way Bernoulli’s equation states that the total energy of a flowing fluid is a constant. That is the sum of kinetic, potential and pressure energies of a flowing fluid is a constant.
Kinetic energy + Potential energy + Pressure energy = constant
Bernoulli’s theorem helps us in understanding energy changes during flow inside a turbine.
TYPES OF HYDRAULIC TURBINES AND THEIR WORKING:
1. IMPULSE TURBINE (PELTON WHEEL):
In impulse turbines, the kinetic energy of high velocity water jet is used to rotate the runner which in turn develops mechanical power. The basic construction consists of a large wheel around the periphery of which a number of buckets are attached. These buckets (which act as vanes in this case) are so shaped that the water jet gets deflected sideways when it hits them. Water from the penstock is passed through a nozzle which increases the velocity (and hence the kinetic energy) of water. This produces a high energy water jet. This jet is directed tangentially at the wheel.
Usually in hydroelectric power stations, a number of jets are used around a runner.
2. RADIAL FLOW REACTION TURBINES:
In reaction turbines such as Francis turbine, water under pressure is made to pass through the runner and create the turning effect. A reaction turbine consists of a scroll casing which is filled with water. The casing is scroll shaped (that is its cross-section goes on reducing). Water at inlet had both kinetic as well as pressure energy. As the water moves through the scroll casing, a part of the pressure energy continuously changes to kinetic energy. A special arrangement of guide vanes is fitted around the runner of the turbine. Water is guided on to the curved blades of the runner by means of these guide vanes. The swirling water moves the blades and creates a rotating effect on the runner. A shaft attached to the runner is coupled to the generator. For large turbines the shaft is made vertical.
3. AXIAL FLOW REACTION TURBINES: In axial-flow reaction turbine water flow is parallel to the axis
of the turbine. The runner is shaped just like a propeller. Guide vanes direct water axially on to the runner from the scroll casing. As water flows through the runner its pressure energy goes on changing to kinetic energy(according to Bernoulli’s theorem). The best example of this turbine is Kaplan turbine, named after the Austrian engineer V Kaplan. It’s most suited for large water flow under small heads. The blades on the runner (which is called a hub in this case) are adjustable so as to control the amount of water flowing through it.
Power generation from hydraulic turbines is by far one of the cleanest methods of power generation since it does not pollute water in any form. But the construction of dams may lead to disruption of the nearby environment and obstruct natural river flows.