What is a hydroelectric power plant?

A hydro electric power plant derives the energy from the force of fast moving water to harness energy. The demand for energy needs has impacted the environment resulting in global warming. Extensive use of non-renewable energy sources like petrol and diesel is the primary reason for environmental pollution. A major shift from non-renewable energy sources to renewable energy sources is the only solution to encounter the rising issue of environmental damage.

Renewable energy sources are those sources that never get exhausted upon usage, unlike non-renewable energy sources. Solar energy, tidal energy, geothermal energy, wind energy, and hydropower energy are some examples of renewable energy sources. This article primarily emphasizes the energy usage from hydropower. The term hydropower means the generation of electricity from water flow. Hydroelectric plants or hydropower plants are specially designed plants that make use of the water flow to generate electricity.

Architecture of hydroelectric power plant
CC BY-SA 4.0 | Image Credits: https://commons.wikimedia.org | Aung Kyaw Soe mm

Major parts of hydropower plant

A hydroelectric plant or a hydropower plant functions with various equipment. This section summarizes the different main components that make up a hydropower plant.


A reservoir is a large water-storing area usually created using a dam in most of the hydropower plants. A reservoir can be created by forming a barrier across a natural water body like lakes or a river. The reservoir generally stores water on the upstream side of the barrier. There are generally two kinds of reservoirs impounding reservoirs and balancing reservoirs. Impounding reservoirs are large reservoirs whose water flow is controlled by operating gates mechanically. Opening and closing of gates release the stored water in a controlled manner according to needs. Balancing reservoirs are those reservoirs that are constructed downstream of the flow. These reservoirs are mainly used when there are flow fluctuations of water.

The water from the dam or the reservoir is collected in a temporary storage tank or a substructure known as a forebay that has a connected penstock. The penstock serves the purpose of caring water to the powerhouse of the plant.

A Reservoir
CC BY-SA 3.0 | Image Credits: https://commons.wikimedia.org | Nilfanion


Penstocks are the long pipelines or channels which carry water from the forebay of the reservoir to the powerhouse of the hydropower plant. The powerhouse typically consists of hydraulic turbines. The penstock primarily carries water under pressure and makes it available at the inlet of a hydraulic turbine. Penstocks are purposefully built circular which minimizes the circumferential stress concentrations and evenly distributes the pressure. The circular cross-section of the penstock also reduces the possibility of scaling which is more in square cross-sections.

Penstocks are normally made of mild steel due to their high corrosion resistance and low magnitudes of head loss due to friction. One of the ends of the penstock is connected to the gates of the reservoir upstream and forms the intake structure, while the other end of the penstock runs down to the hydraulic turbines and has a control valve at its end. The control valve allows stopping the water flow during repair and maintenance. The channel of the penstock can be exposed to the environment or berried underground depending upon the geographical and environmental conditions.

A Penstock
CC BY-SA 4.0 | Image Credits: https://commons.wikimedia.org | Leijurv

Surge tank

Surge tanks are the storage containers installed in the pathway of the penstock. These surge tanks serve as pressure neutralizers in the hydropower stations. The surge tank contains inlet valves that function by bypassing excessive water from the channel and storing it for future use. If there is a drop in water pressure in the channel, the outlet valve of the surge tank can be operated to make water exit out of the tank and join the flow in the channel. Hence the pressure in the channel can be increased or decreased with the help of surge tanks.

A surge tank
CC BY-SA 2.5 | Image credits: https://commons.wikimedia.org | Qurren

Hydraulic turbines

Turbines are energy conversion devices that convert one form of energy into another. There are various categories of turbines like gas turbines, wind turbines, propeller turbines, steam turbines, and hydraulic turbines.
A hydraulic turbine is a turbine that converts the hydraulic energy of water into mechanical energy in terms of turbine shaft rotation. Hydraulic turbines primarily consist of blades arranged around the periphery of the shaft. Depending on power requirements, a hydraulic turbine can have multiple sets of blades. The rotating shaft of a turbine and its constituent blades form the runner of the hydraulic turbine.
Water possessing either kinetic energy or pressure acts as an inlet to the turbine, the energy of the water creates a force component which is perpendicular to the central axis of the shaft, this force component pushes the blades which in turn causes the shaft to possess a net angular velocity.

Hydraulic turbines
CC-BY-PD 1.0 | Image Credits: https://commons.wikimedia.org| U.S. Army Corps of Engineers

Draft tube

A draft tube is a long tube having a diverging cross-sectional area. The primary purpose of a draft tube in a hydropower plant is to carry the outlet water from the turbine towards the tailrace. A tailrace is a water collection unit that stores water that moves out of the turbine. The diverging nature of the draft tube converts the kinetic energy of water from the turbine into pressure energy. The draft tube simply makes use of Bernoulli's theorem for the energy conversion.

Draft tube
CC BY-SA 3.0 | Image Credits: https://commons.wikimedia.org | Adhish24


The generator is the major energy conversion unit of the entire hydropower plant. A generator is connected to the shaft of a hydraulic turbine. Generators are the devices that convert the mechanical energy of the shaft rotation of the turbine into electrical energy, preferably known as hydroelectricity. A generator has huge magnets with a series of coils along with an armature. The rotation of the armature of the generator changes the magnetic flux and induces a current in the coils. The current is transferred to a step-up transformer and transmitted through electrical lines. The current output from the generator is usually rated in bigger units like kilowatt and megawatt.

CC BY-SA 3.0 | Image Credits: https://commons.wikimedia.org | Loracco

Types of hydraulic turbines

Impulse turbine

In the impulse turbine, the energy that is available at the turbine blades is kinetic. The blades of such turbines are shaped in the form of buckets which deflect the water into various ways to create the force component which turns the turbine blades. Due o the shape of the blades the water imparts an impulsive force to the buckets. The Pelton turbine and the Crossflow turbine are the two famous turbines in this category. These kinds of turbines are majorly preferred to form high head applications.

Impulse turbine
CC BY-SA 4.0 | Image Credits: https://commons.wikimedia.org | Ashumech

Reaction turbine

This type of turbine generally employs Newton's third law of motion for its operation. This kind of turbine makes use of pressure energy as the inlet to the turbine. The expense of the pressure energy is used to create torque in the turbine shaft. The pressure energy exerts a pressure force at the turbine blades, the exact opposite reaction induced in the blades sets the shaft in motion. This kind of turbine is well suited for low head applications. The Kaplan turbine and Francis turbines are popular categories of this type.

Hydraulic efficiency

The hydraulic efficiency of a hydraulic turbine signifies how well the turbine can deliver power with the provided hydraulic energy at its inlet.
The ratio between the power developed at the turbine shaft and the hydraulic energy available at the turbine inlet denotes the hydraulic efficiency of the turbine.

ς=output power Hydraulic power

Where, ς denotes hydraulic efficiency of the hydraulic turbine.

Context and Applications

This topic is primarily taught in various undergraduate and post-graduate degree courses like-

  • Bachelor of Technology (Mechanical engineering)
  • Master of Technology (Mechanical engineering)
  • Master of Science in Rotodynamic devices
  • Bachelor of Technology (Civil engineering)
  • Master of Science in Hydraulic energy

Practice Problems

Q1) Which of the following is a reaction turbine?

  1. Kaplan turbine
  2. Pelton turbine
  3. Francis turbine
  4. Both a and c

Answer: Option d

Explanation: Both Kaplan turbine and Francis turbine are the examples of reaction turbines.

Q2) Which of the following is an impulse turbine?

  1. Cross-flow turbine
  2. Propeller turbine
  3. Francis turbine
  4. None of these

Answer: Option a

Explanation: Cross-flow turbine is an example of impulse turbine.

Q3) Which of the following device is used for pressure regulation in hydropower plants?

  1. Draft tube
  2. Spillway
  3. Surge tank
  4. Penstock

Answer: Option c

Explanation: Surge tanks are large storage tanks across the channels which are used for pressure regulations.

Q4) Which of the following combinations are known as the powerhouse of a hydropower plant?

  1. Spillway and penstock
  2. Surge tank and reservoir
  3. Hydraulic turbine and generator
  4. Trash racks and draft tube

Answer: Option c

Explanation: The hydraulic turbine and the generator of a hydropower plant forms the primary equipment of a hydropower plant.

Q5) By which of the following name is the substructure near to a reservoir called?

  1. Trash racks
  2. Forebay
  3. Surge tank
  4. None of these

Answer: Option b

Explanation: Forebays are the secondary reservoirs or substructures build near to the primary reservoirs.

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