Hydroelectric energy, also known as hydroelectricity, is a form of energy that generates energy by harnessing the power of moving water. People in Greece utilized flowing water to turn the steering wheel of their mill to grind wheat into flour over two thousand years ago.

Hydropower is by far the most commonly used clean energy source. China is the world’s largest hydroelectric power producer, followed by America, Brazil, Canada, India, and Russia. Hydropower accounts for over 71% of all renewable electricity produced on the planet.

China’s Three Gorges Dam is the world’s largest hydroelectric dam in terms of energy generation. It holds back the Yangtze River. The dam is 2,335 meters long (7,660 feet) and 185 meters tall (607 feet), with enough generators to generate 22,500 megawatts of electricity.

Hydropower facilities generate electricity by capturing the energy of flowing water. The kinetic energy of flowing water is converted into mechanical energy by a turbine. This mechanical energy is hence converted into electrical energy by a generator. Hydropower plants range from small ‘micro-hydros’ to ‘massive dams’ in size.

Table of Contents


1. Reservoir

Water Reservoir
Water Reservoir | Image Credit – WhitcombeRD

One of the essential components of a hydropower station is the reservoir. It holds the water and sends it down to the water turbine, generating energy. Natural lakes in hilly places can serve as reservoirs, or they can be created intentionally by building a dam over a body of water. The hydroelectric plant reservoirs can also be used for flood control, irrigation, industrial, and aquaculture.

The height at which this water reservoir is the hydraulic head. Hydraulic head is usually measured in meters above sea level and is one of the most important parameters in determining how much energy a dam can generate. Since water kept at a greater elevation has a certain amount of potential energy translated into turbine rotational motion as it falls and spins the blades.  The Hydro Power Plant (HPP) may generate more electricity if it has more potential energy.

2. Forebay

A forebay is a storage facility for water. Before the water is pumped down to the turbine, it is sored in the forebay. It reserves extra water during the rainy season and supplies it during dry seasons. 

Forebay regulates the amount of water needed according to the load area’s requirements. When the hydropower plants are positioned too far from the reservoir, the forebay is built; otherwise, when the plant is located close to the reservoir, the reservoir acts as the forebay.

3. Dam

Dam | Image Credit – Sustainable Operations Energy Blog

The most expensive component of a hydroelectric power plant is the dam. It is a barrier built across water bodies to restrict naturally flowing water and elevate the water level in reservoirs. Concrete, rocks, soil, or stonemasonry are commonly used to make dams. 

The sort of material to be utilized in its construction is determined by the area’s geography, transport availability, and the likelihood of natural hazards such as floods or floods occurring in that location. A stonemasonry dam, for example, is appropriate in areas with small canyons, but an earth dam is preferable in areas with large valleys.

Reservoirs built by dams prevent floods and provide water for agriculture, household purposes, commercial use, fisheries, and navigation. It is also viable to distribute the water stored in dam equitably between different sites. Dams are used to retain water, while floodgates and levees restrict or prohibit water flow into certain land areas.

4. Spillways

Spillway | Image Credit – Engineering New Record

When a dam’s reservoir is full, a spillway allows excess water to flow over or around it. Spillways are critical safety elements for a variety of dams. The erosive force of the water at the foot of the spillway is dispersed away from the foundation by leading over the dam or a portion of it, or via a conduit around the dam. 

If a dam is overtopped, it can have profound effects. A dam that was not intended to allow unrestricted river flow on its downstream side is likely to fail in case of an embankment. 

Overtopping can severely damage dams and their foundations if the spillway is inadequate or missing, potentially leading to disastrous failure. Dams built before current flood data became available had problems related to spillway capacity. 

Floodwater splattering over a concrete gravity dam is also dangerous because it erodes the base at the downstream toe. After overtopping, arch dams are more resistant to failure. 

There are four general elements of spillways worth mentioning.

  • The uncontrollable outflow of surplus water across the dam ought to be automatic rather than regulated by humans. 
  • The spillway inflow should be large enough to allow the floods to flow through without raising the water level in the reservoirs.
  • The flood flow frequency shall not exceed the flow experienced before the dam’s construction. 
  • Floodwater discharged over a dam’s height can cause damage to the dam’s structure and the riverbed unless it is controlled and dispersed in harmless turbulence.

5. Tailrace

Tail Race in a hydropower plant
Tail Race in a hydropower plant | Image Credit – Raquel Coelho Loures

The tailrace is a conduit that carries the water left at the hydroelectric power plant after the hydro turbine has generated energy.  The tailrace is located behind the dams and lower than the reservoir. 

The water through the tailrace runs at the natural speed of the water. Since the hydro turbine uses the potential energy of water due to the reservoir’s elevation, the water in the tailrace flows naturally and joins another water stream. 

6. Penstocks

Penstock Pipes
Penstock Pipes | Image Credit – Fire2Fusion

The penstocks at a hydroelectric plant are the channels or large pipes that carry water from the reservoir to the water turbines at the power station. Reinforced cement concrete (RCC) or steels are commonly used to construct penstocks. 

The dam’s water-head determines the type of material used to build penstocks. The water-head is the vertical distance traversed by the water from the reservoir elevation to the turbine; it is commonly measured in meters or feet. 

The water in the penstock has kinetic energy due to flow and potential energy due to height. Steel penstocks are suitable for any water head or working pressure, whereas RCC penstocks are suitable for low water heads of less than 30 meters. 

The sudden opening and closing of the gates at the terminals of the penstock might generate a water hammer effect because a considerable amount of water runs through the penstocks. The penstocks are carefully built to withstand the water hammer effect; for example, short-length penstocks have thick walls, and long-length penstocks have surge tanks.

7. Water Intakes

Trash Racks
Trash Racks | Image Credit – Cink Hydro Energy

The water intake consists of the structures that receive water from the forebay or reservoir and transfer it to the turbines through the penstocks. Water intakes are made up of several gates, filters, screens, sluices, booms, and trash racks that regulate the amount of water that reaches the turbines. 

They also divert debris containing waste products, trunks, or branches to the bypass chute. The screens and trash racks are put at the penstock’s entry to prevent waste from entering the penstock, as debris can harm essential hydraulic parts, including nozzle, turbine blades, and turbine runners. Trash racks are typically formed of rod-shaped structural steel. Intake structures are generally classified as follows:

  • high-pressure intakes (for large storage) 
  • low-pressure intakes (for smaller storage) 

During the colder seasons, there’s a potential that ice will build on the water’s surface. The waste racks are heated to prevent ice from reaching the penstock, and the ice ultimately melts when it comes into contact with them.

8. Sluice

Sluice gate in hydropower plant
Sluice gate in hydropower plant | Image Credit – Ollo

Water intake structures also include sluices. The sluice controls the flow of water through the penstocks. It is a gate located at the ends of the penstocks and can be raised or lowered depending on the water demand at the turbine. When the sluice is open, water flows freely through the penstocks; however, when the sluice is partially closed. 

In dry seasons, they are left open to allow water to run through the penstocks, but they are slightly closed to prevent flooding in wet seasons. The sluice installation in the penstocks minimizes overall dam failures. It ensures that penstocks can be quickly cleaned, examined, and repaired in the event of any problems such as holes or cracks.

9. Surge Tank

Surge Tank in a Hydropower Plant
Surge Tank in a Hydropower Plant | Image Credit – Syed Ahmad Amin Shah

A surge tank is a device that stores water. In a hydropower conveyance system, a surge tank acts as a pressure neutralizer to prevent excessive pressure rise and pressure decrease in a hydropower water conveyance system. 

The placement of the surge tank is crucial in achieving better results. It is placed in a way that allows for easy access.

  • Surge tanks are strategically placed near the power plant to shorten the length of the penstocks.
  • There are no restrictions on the height of the surge tank.
  • It is placed where a flat-sloped conduit meets a steep-sloped penstock.

Sudden water surges caused by fluctuations in water flow can cause pressure variances, which might harm the hydropower plant’s components. Surge tanks are compact cylindrical storage tanks used to handle pressure variations. 

Surge tanks are used to regulate turbines and are open from the top to reduce or negate pressure changes in the reservoir. They protect the channel against excessive internal pressure and can also be used to store water in the event of a pressure decrease. 

Before the water turbine, surge tanks are generally situated in the center of the penstock. The length of the pipeline determines the sort of surge tanks that will be utilized in the plant.

10. Powerhouse

Turbine Hall of a hydropower house in a hydropower plant
Turbine Hall of a hydropower house in a hydropower plant | Image Credit – Colin Cuthbert

The powerhouse is a facility that protects the hydraulic and electrical equipment. In most cases, the entire equipment is maintained by the power house’s foundation or substructure. 

Some machinery, such as draft tubes and scroll casing, are fixed within the foundation laid in reaction turbines. As a result, the foundation is built in large dimensions. 

Generators are installed on the ground floor beneath vertical turbines in the superstructure. The turbines are also placed in the powerhouse.  A control room is available on the first or mezzanine floors.

A hydroelectric station’s powerhouse is where water’s potential and kinetic energy running through the water-conducting system are converted into mechanical energy by rotating turbines. This mechanical energy is subsequently transferred to electrical power by generators. 

The auxiliaries of the powerhouse include a Turbine or Generator hall, Maintenance bay, Bus-bars, Pipes, Control room, Electrical room, Workshop, Store, Manager’s Office

11. Turbines

Turbine in Hydroelectric station
Turbine in Hydroelectric station | Image Credit – Direct Industry

These are the components of the prime mover, which converts water energy into mechanical energy and is used to operate a hydroelectric generator. Because the turbine runner and the generator rotor usually are positioned on the same shaft, the entire unit is called a turbo-generator. 

These are the components of the prime mover, which converts water energy into mechanical energy and is used to operate a hydroelectric generator. Hydroelectric facilities harness the energy of water falling over a level difference of a few meters to 1500 meters or even 2000 meters. 

Different turbines with different functioning designs are used To manage such a wide variety of pressure heads. There are two types of modern hydraulic turbines. They are mentioned below:

  • Impulse Turbine

The driving energy for an impulse turbine comes from the kinetic energy of the water 

  • Reaction Turbine

The driving energy for a reaction turbine comes partly from the kinetic energy of the water and partly from the pressure energy. 

Impulse Turbine and Reaction Turbine differ from each other in the following ways: 

SNImpulse TurbineReaction Turbine
1Steam enters through the nozzle of an impulse turbine and strikes the moving blades.The steam in a reaction turbine passes through the guiding mechanism before passing through moving blades.
2With kinetic energy, steam strikes the buckets.With both kinetic and pressure energy, steam flows over the spinning blades.
3The pressure of steam as it passes through moving blades remains constant.The steam pressure is reduced as it passes through moving blades.
4The impulse turbine blades are symmetrical.The reaction turbine blades are not symmetrical.
5The steam flow is radial to the turbine wheel’s direction.Steam flows both radially and axially around the turbine wheel.
6It needs minimal maintenance.It requires significant maintenance.
7It’s best for low-discharge situations.It can handle mild and heavy discharges.
8Pelton Wheel as an example of Impulse turbine.Reaction turbines include Francis turbines Kaplan turbines.

The basic types of Impulse and reaction turbine are given below:

Propeller (Kaplan)Reaction10m-60m
PeltonImpulseAbove 300m

Table showing different types of turbines 

The hydropower designer needs to select the sort of turbine that will be used for a particular project. The designer’s task is to choose an optimal turbine type and series  of power generating units, the runner diameter, rotational speed, and runner axis elevation based on head requirements. 

12. Draft Tube

Components of hydropower plant showing Draft Tube
Components of hydropower plant showing Draft Tube | Image Credit – The Engineer’s Post 

The draft tube connects the water turbine’s outlet to the tailrace. It transfers the kinetic energy of the water at the turbine’s outflow into static pressure and is usually found near the outlet or exit of the turbines. Cast steel and cemented concrete are the main components of a draft tube. 

The draft tube’s primary function is to transfer kinetic energy from water into pressure energy. The pipe is used to gradually increase the cross-sectional area of the water to reduce the velocity and pressure of the water before it enters the tailrace. 

The draft tube equalizes the water pressure with that of the atmosphere. The tube must be sturdy enough to withstand the amount of pressure and speed of the water.

13. Generator

Hydropower Generator
Hydropower Generator | Image Credit – Erlend Løklingholm Engevik

The hydroelectric power plant’s electric generator turns the water turbine’s mechanical energy into electrical energy. The generator operates according to Faraday’s law, which states that the voltage produced in an electric circuit is proportional to the rate of change of magnetic in the circuit. 

The generator consists of two major components. They are the stator and rotor. The turbine shaft’s mechanical force is applied to the generator’s revolving structure, i.e., the rotor. 

At the same time, the stator is the generator’s stationary section, where the voltage is induced when the rotor is excited or magnetized. 

The field poles (electromagnets) are mounted on the inside of the rotor’s edges, and when the rotor rotates, the field poles rotate around the stator’s conductors. The induced voltage and current flow at the output terminal are the outcomes of this.

14. Transformer

Transformer in Hydropower Plant
Transformer in Hydropower Plant | Image Credit – SEA

Because the electricity generated by hydroelectric power plants is not of a voltage suitable for use in homes or other common applications, transformers are utilized at hydroelectric power plants. 

While maintaining constant electric power, the transformers transform the alternating current (A.C) into the appropriate voltage. This power source is linked to the national grid and then supplied for industrial or home consumption.

15. Electricity Transmission Lines

Transmission lines
Transmission lines | Image Credit – Twilight Productions 

After the power has been converted to the needed voltage, the next and last stage is to distribute it to the required places. 

Dams are frequently built-in remote regions, therefore the power generated at the hydroelectric plant is delivered by long cables or transmission lines to the home or industrial sectors.

Working Principle of Hydropower Plant

Illustrations of a Hydroelectric Power Plant
Illustrations of a Hydroelectric Power Plant | Image Credit – Nandine Kanesalingam

To comprehend the operating principle of a hydroelectric power plant, it is necessary to understand the concepts of types of energy in a hydropower plant.

  • Potential Energy

Potential energy is the energy that a body possesses due to its position in relation to other objects. The water in a reservoir has potential energy due to its altitude.

  • Kinetic Energy

Kinetic energy is the energy acquired by the body due to its motion. The higher the body’s speed, the greater is the kinetic energy. KInetic energy is because of the flow of water.

  • Mechanical Energy

The turbine rotates when the high-pressure water hits the turbine blades. The mechanical energy is due to the rotation of the turbine. 

  • Electrical Energy

A generator is connected to the rotating turbine. When rotational motion is in the turbine, the generator also spins, generating electricity. 

The hydroelectric power plant’s working mechanism is that it uses turbines to convert the water’s potential energy (because of the elevations of water from the channels) and kinetic energy (owing to fast-flowing water) into mechanical energy. 

Water from the forebay or reservoir behind the dam flows through the penstock, resulting in high pressure on the turbine blades, causing the turbine runner to rotate. The runner is connected to the center shaft. 

The shaft is  linked to the generator, which ultimately generates power, converting the turbine’s mechanical energy into electricity via electric generators. After voltage regulation by transformers, the generated electrical energy is provided for home or industrial usage via transmission lines. The amount of electricity generated by hydroelectric facilities is dependent on the rate of water flow and the altitude drop.

How much Electricity can a Hydropower plant generate?

The rate at which energy is produced is referred to as power. Watts (W) and kiloWatts (KW) are the units of measurement for power (kW).  It is measured in kilowatt-hours (kWh) or megawatt-hours (MWh).

2 factors determine the maximum hydroelectric power output:

  • Potential Head
  • Water Flow

P = (m x g x Hnet )x η


P =power, measured in Watts (W).

m =mass flow rate in kilogram per second (kg/s)

g =The gravitational constant (meter per square second), = 9.81m/s2

Hnet =Net head. 

This is the gross head and is determined physically at the site. For reference, we can consider head losses to be 10%, so Hnet=Hgross x 0.9

η =The product of the component efficiencies, which are normally the generator, turbine, and drive system

Advantages of Hydropower

Working of Hydropowerplant
Working of Hydropowerplant | Image Credit – Panakkal Chandramohan

The advantages of Hydropower are given below:

  • Source of renewable energy.
  • Extremely efficient.
  • The cost of operation is lower.
  • The expense of maintenance is lower.
  • Pollution is almost non-existent.
  • The powerhouse just takes up a small amount of area.
  • Water is reused over and over.
  • Energy is renewable.
  • Reduces the number of greenhouse gases
  • In a position to fulfill peak demand.
  • The reservoir assists in flood management.
  • Facilitates Irrigation
  • Robust life span of sound 60 years
  • Efficiency does not compromise with the age of the plant

Disadvantages of Hydropower

  • Cost of plant installation or the initial investment is relatively high.
  • The payback period is long because of higher initial expenditure.
  • It can’t be made anywhere because it needs a lot of water.
  • A dam’s construction is difficult and time-consuming.
  • More time is required for building.
  • Failure risks are present.
  • Natural disaster dangers.
  • Emissions of methane
  • Droughts are a possibility.
  • The incidence of local droughts is one of the major drawbacks of constructing hydroelectric power plants.
  • Damage to the environment
  • Wetland destruction
  • In the river, there is an issue with marine life.
  • By any chance, relocation is not an option.
  • Locals may be forced to migrate if there is a downstream flood problem while the dam is being built.
  • The amount of water available determines the output.

Closing statement

Hydropower is a clean as well as a renewable energy source. Because hydropower plants possess the inherent ability to operate in real-time, they are more responsive than most other energy sources in terms of meeting peak power demands and improving power system reliability.

Though we can count upon the cons of hydropower plants, hydropower remains a vital source of energy for the world. Water is a fuel that is both efficient and highly reliable. Thus, we should focus indefinitely on the use, construction, and advancement of hydropower stations.

(Last Updated on April 30, 2022 by Sadrish Dabadi)

Suraksha Pal is an Industrial Engineer currently pursuing my master’s degree in Renewable Energy Engineering at the Institute of Engineering, Pulchowk Campus. She has a keen interest in Renewable Energy and is passionate about sustainable development. She loves to express her views on these subjects through articles and blogs.