About Hydropower

Did you know:

  • Hydro power is the world’s largest source of renewable electricity generation
  • Hydro power produces 19% of the world’s electricity needs
  • Hydro power produces 16% of India’s electricity needs
  • Hydro power supplies at least 50% of electricity production in 66 countries, and at least 90% in 24 countries.

Hydroelectricity means generating electricity from the moving water of rivers and streams. It is a renewable source of energy as it is constantly being renewed by a river’s water flow.

The water cycle creates the water flow essential for hydropower.

Water Cycle – The Eternal Source of Hydropower

Water constantly moves through a vast natural cycle, evaporating from rivers and oceans due to the sun’s heat, condensing to form clouds in the cooler layers of the atmosphere, precipitating as rain or snow and replenishing the water in the rivers and oceans.  Gravity drives the water, moving it from high ground to low ground. The force of moving water can be extremely powerful. Hydroelectric power systems capture some of the energy embodied in the flow of water and convert it into electrical energy.

Thanks to the water cycle which is an endless, constantly recharging system, hydropower is considered a timeless and renewable energy resource

How Hydropower Works

Hydropower has been used for centuries. More than 2,000 years ago, people first used the power of moving water to turn water wheels to grind wheat into flour. Placed in a river, a water wheel spins as a stream of water hits the blades. The kinetic energy of falling water gets converted into mechanical energy which can then be used to grind grain, saw wood or pump water. Based on a similar process of energy conversion, today’s hydro plants convert the natural flow of water in a river, stream or channel into electricity.

As water flows downwards due to the force of gravity the kinetic energy it carries increases.
A turbine converts the kinetic energy of falling water into mechanical energy.
Then a generator converts the mechanical energy from the turbine into electrical energy.

The structure that houses the turbines and generators is called the powerhouse.

Click here to read more about the functioning of a hydroelectric plant.

Parts of a Hydroelectric Plant

A typical hydroelectric plant includes a dam, reservoir, penstocks, a powerhouse and an electrical power substation. The dam stores water and creates the head; penstock carries water from the reservoir to the turbine inside the powerhouse; the water rotates the turbine which in turn drives a generator that produces electricity. The electricity is then transmitted to a substation where transformers increase voltage to allow transmission to communities.

 Main Components :

a) Dam and Reservoir Most hydropower plants rely on a dam that holds back water flowing down the river, creating a large reservoir or lake. This water is diverted to turbines in power stations. The dam raises the level of the water to create falling water and also controls the flow of water from the reservoir. This enables the water held in the reservoir to become stored energy. The force of the falling water released from the reservoir through the dam spins the blades of a turbine in the power house.

b) Penstock – Penstock is a pressurized pipeline which carries water from the reservoir to the turbine inside the power house. When the gates of the dam are lifted, the force of gravity makes the water flow down the penstock and reach the blades of the turbine. Water builds up high pressure as it flows through the penstock. A tunnel serves the same purpose as a penstock. It is used when an obstruction is present between the dam and power station such as a mountain.

c) Turbine– The force of falling water strikes against the large blades of a turbine and causes it to spin. Turbines can spin as fast as 90 revolutions per minute. The turbine converts the kinetic energy of falling water into mechanical energy. Depending mostly on the available water head, there are various types of turbines used for power generation.  In general, impulse turbines like Pelton are used for high head sites and reaction turbines like Kaplan are used for low head sites.

d) Generator– The generator is connected to the turbine by shafts and so when the turbine blades turn, the generator spins too. The generator converts the mechanical energy from the turbine into electric energy. A series of large electromagnets rotate inside copper coils and produce alternating current (AC) by moving electrons.

e) Transformer – The transformer takes the current produced by the generator and converts it into current of very high voltage for easy transmission through long distances.

f) Power lines – The power lines conduct electricity from the hydropower plant to various power distribution centres.

g) Outflow – After passing through the turbine, used water is carried through an outlet pipe called the tailrace and re-enters the river on the downstream side of the dam.

It may be noted here that ‘run of the river’ hydropower projects do not require a large dam, although some may use a small, less obtrusive dam. Instead some ‘run of the river’ hydropower plants just use a small canal or pipe to channel a portion of the river water to spin the turbines.

How Much Electricity Can a Hydroelectric Plant Make?

Hydro power is obtained from the potential and kinetic energy of water flowing from a height. The energy contained in the water is converted into electricity by using a turbine coupled to a generator.

The amount of electricity a hydropower plant produces primarily depends on:

Head The head refers to the height or the vertical distance through which the water falls.  It is the distance between the water surface and the power-producing turbine.

Flow– The flow refers to the volume of water flow available to the turbine per unit time.

Efficiency It represents how well the turbine and generator convert the energy of the water into electrical energy. For older, poorly maintained hydroplants this might be 60% (0.60) while for newer, well operated plants this might be as high as 90% (0.90).

The higher the head, and the larger the flow, the more energy is available for conversion to electricity. In case of a dam, the higher the dam, the greater the distance over which water falls, the greater the power produced. Bigger rivers have more flowing water and can produce more energy. Efficiency is often higher with larger and more modern turbines and may be lower with very old or small installations due to proportionately higher friction losses.

Hydro Calculator

A simple formula to determine the amount of electric power production at a hydroelectric plant is:

P = hrgk


P The electric power in kilowatts.
h The head or the distance the water falls measured in metres.
r The flow rate measured in cubic metres per second.
g Acceleration due to gravity of 9.8 m/s2.– i.e. each second an object is falling, its speed increases by 9.81 metres per second (until it hits its terminal
k A coefficient of efficiency ranging from 0 to 1.

These same calculations are valid whether you are planning a tiny Pico or Micro Hydro Power system or the next Three Gorges Dam Hydro Project.

Plant Types


Storage Plants

Conventionally, a large hydropower plant uses an impoundment dam to store a large volume of river water in a reservoir to run the power generating facility. The reservoir is located at a higher level and the power generation plant at a lower level. The dam raises the water level of the river to create falling water and also controls the volume of water released from the reservoir to the power station. Water flows from the reservoir through pressurized pipes or tunnels and spins the blades of the turbines in the power plants located in the valleys.

Since storage plants often have consistent flow rates, they provide a stable output of power. They are generally ramped in to counteract peak load spikes when power demand is temporarily high. But these projects come with huge environmental and social costs.

Run-of-River Power Plants

Run of river hydro projects use the natural downward flow of rivers and streams for the generation of electricity. These facilities divert a portion of water flow from the river or stream through a canal or pipe to spin turbines for power generation after which water re-enters the original source. Normally a small barrage is built and if there is a local head, it is exploited. Any dam, if built, is for diversion purpose only; there is no obstruction of water flow. In case of very low heads, it is convenient to put the plant directly in the river.

These plants normally do not need any storage area for reservoirs. Since the energy is directly tapped from the flowing water, these plants need minimal construction, and submerge least area. Installation of such a system is relatively cheap and has minimal environmental impact.

India has a huge potential for generating power through such run of the river schemes. Even though these plants are subject to seasonal fluctuations in water flow, a well-developed grid can absorb the seasonality through a proper load generation balance.

Pumped Storage

Pumped storage plant is a type of dammed hydropower plant which reuses water in contrast to conventional dam plants and run-of-the river plants.

A pumped-storage plant uses two reservoirs, one above the power generating facility and one below.  During periods of peak electricity demand, it operates much like a traditional hydropower plant — water released from the upper reservoir passes through turbines, which spins generators to produce electricity. However water exiting the hydropower plant flows into a lower reservoir rather than re-entering the river and flowing downstream. During off-peak demand hours (normally night), some of the water is pumped back into the upper reservoir to be reused later in the day during periods of increased demand.

Pumped storage schemes have enough storage capacity to off-set seasonal fluctuations in water flow and are an important means of large-scale grid energy storage. They also improve the daily load factor of the generation system.

Types of Hydro Power Plants

Run-of-river power plants

Reservoir power plants

Pumped storage power plants

Sizes of hydropower plant

Hydroplants range in size from “micro-hydros” that power only a few homes to  mammoth dams like the Three Gorges  Dam that provide electricity for millions of people.

The first hydroelectric power plant was built in 1882 in Appleton, Wisconsin to provide 12.5 kilowatts to light two paper mills and a home. Today’s hydropower plants generally range in size from several hundred kilowatts to several hundred megawatts, but a few giant plants have capacities up to 10,000 megawatts lighting millions of homes and establishments.

The 185-metre Three Gorges Dam in China on the upper Yangtze River having an installed capacity of 18,200 MW of power is the largest hydropower dam in the world. The Itaipú Dam between Brazil and Paraguay, which is 190 metres high, generates more than 12,600 MW. Major hydroelectric projects in India above 750 MW are Tehri, Bhakra, Dehar, Koyna, Nagarjunasagar, Srisailam, Sharavarthy, Kalinadi and Idukki.  At the other extreme are the small but sure incremental agents of development – the decentralized, stand-alone Pico and micro Hydel projects providing a few kilowatts to a home or farm.

Though the definitions may vary, hydropower plants can be classified on generation capacity as follows:

  • Mega hydropower plants- More than 500 MW
  • Large hydropower plants- More than 25 MW
  • Small hydropower plants- 2MW- 25 MW
  • Mini hydropower plants- 100 kW- 2 MW
  • Micro hydropower plants- 10 KW-100 KW
  • Pico hydropower plants- Less than 10 KW

In India, Small Hydro Projects are classified based on head as follows:

Ultra low head: Below 3 metres
Low head: Above 3 and up to 40 metres
Medium/high head: Above 40 metres

From falls of water as low as 3 metres with flows as small as 12 Gallons per minute (GPM) check, hydro systems can take your spring, pond, river, or runoff water and turn it into clean, efficient electricity. This can meet the local power needs independently at the lowest cost.

So, size micro mini to jumbo, whether small or big, hydropower wipes out darkness! We just need to harness it!!!

Turbine- The ‘mechanical heart’ of a hydropower plant

Types of Turbines

Hydroelectric power converts the natural flow of water into electricity to light our homes and power
our industries. The energy is produced by the fall of water turning the blades of a turbine. The turning part of the turbine is called the runner. The turbine is the ‘mechanical heart’ of a hydroelectric plant. It converts the energy of the flowing water into mechanical energy. This mechanical energy drives a generator which generates electricity.

Based on their hydraulic action or the means by which this mechanical energy is created, turbines are classified impulse or reaction.

Impulse Turbine

The turbine which runs on impulse force is called an impulse turbine. Water of high head is used to drive an impulse turbine.  Water at high pressure from the penstock is forced on the buckets mounted on the periphery of the turbine runner through a divergent nozzle which converts the high pressure into high speed free jets of water. This high velocity fluid impinges on the bucket wheel and it rotates at very high speed producing mechanical energy.

There is no suction on the down side of the turbine and the water after striking the buckets falls into the tail water. Since, the turbine casing only needs to protect the surroundings against water splashing, it can be very light.

A large amount of pressure is needed to accelerate water to the necessary velocity, therefore impulse turbines achieve greatest efficiency in high head applications and are also called the high head turbine. These hydro turbines do not require high flow-rates and are the most commonly used in micro hydro systems.

Impulse turbines are widely used, and these come in three basic kinds:


The pelton is best used for a thin stream (low flow) of water falling from a reasonable height (high head)  The Pelton turbines are used for high-head sites in the range of 50-1300 metres, normally  for more than 250 m of water head. They can be as large as 200 megawatts.

A turgo is for intermediate water flow and fall (or head) conditions.  The Turgo turbine can operate under a head in the range of 30-300 metres.

Cross-flow is very adjustable, ideal for a variably flowing source of water and can be used for a wide range of flows and heads. It can operate with discharges between 20 litres/sec and 10 cubic meters per second and heads between 1 and 200 meters.

Reaction Turbine

A reaction turbine develops power from the combined action of pressure and moving water. Unlike impulse turbine, in which rotational energy is created by impulse of water along the blade, in a reaction turbine, the runner is placed directly in the water stream flowing over the blades rather than striking each individually. The water pressure applies a force on the face of the runner blades, which decreases as it proceeds through the turbine. The guide vanes change the direction of the flow and cause the water to whirl, improving the turbine’s efficiency. Reaction turbines need higher flow rates than impulse turbines, but can operate with a smaller vertical distance between the diversion and the water turbine.

While impulse turbines spin freely in the air, a reaction turbine is fully immersed in water and is entirely enclosed in housing, so that the full pressure of the water turns the turbine. The turbine casing, with the runner fully immersed in water, must be strong enough to withstand the operating pressure.

Reaction turbines are generally used for sites with ample water supply but a low head.

The two main types of reaction tubines are:

Francis turbines are radial flow reaction turbines, with fixed runner blades and adjustable guide vanes.  The water enters the turbine in a radial direction striking the blades at a tangent to the drive shaft and exits into the tailrace axially. It is generally used for medium to high heads. It can be used for head varying between 2.5 metres to 450 metres and can be as large as 800 megawatts.

Propeller turbines are axial-flow reaction turbines where the shaft through the center of the turbine runs in the same direction as the water flow, much like a boat propeller.  A propeller turbine has a runner with three to six fixed blades. The water passes through the runner and drives the blades. They are generally used for medium to low heads. Propeller turbines can operate from 1.5metres to 30 metres of head and can be as large as 100 megawatts.

Kaplan turbine is a type of propeller turbine that has adjustable runner blades to improve performance.  Similar to francis turbine, flow enters a Kaplan turbine circumferentially and exits axially. But the water flow is directed axially along the drive shaft before it comes in contact with the blades. Kaplan turbines with adjustable blade pitch are well-adapted to wide ranges of flow or head conditions. They are used for heads varying between 1.5 metres to 70 metres.  Kaplan turbines can be as large as 400 megawatts.

Turbine Selection

The type of hydropower turbine selected for a project is based on their particular application and  the height of standing water—referred to as “head”—and the flow, or volume of water, at the site. Turbine selection is based more on the available water head and less on the available flow rate. Other deciding factors include how deep the turbine must be set, efficiency and cost.

In general, impulse turbines are used for high head sites, and reaction turbines are used for low head sites.



Reaction turbine

Propeller hydropower turbine
Credit: GE Energy

Fig. 2.3 shows a schematic of an impulse turbine

Image – www.yourdictionary.com/images/ahd/jpg/A4tubine.jpg

Hydropower Advantages


‘Blue Gold’- A fully functional, viable, clean alternative energy source


Renewable– Hydropower relies on the water cycle which is driven by the sun. Its fuel supply is flowing water which is renewed yearly by snow and rainfall. Thus it is a renewable source of energy. Further, hydropower is based on the non-consumptive use of water. Unlike fossil fuels, water is not reduced or used up in the process.

Non-polluting– Hydropower is clean. It produces very few greenhouse gases and no other air pollutants because it does not burn any fuel.  It leaves behind no waste.

Offsets GHG– Concomitantly, the development of hydroelectric potential avoids Green House Gas (GHG) emissions and other air contaminants from equivalent thermal and other fuel based power plants. Each additional terawatt of hydropower that replaces coal-generated electricity offsets 1 million tonne a year of carbon dioxide equivalent.

Technically Advantageous

Flexible – Hydro plants with storage are unique among energy sources for their operational flexibility. Hydropower plants have the ability to start up, shut down and respond to load variations quickly and economically. For instance, while coal based thermal plants need a relatively long time to change their load and are relatively inefficient if they are used at partial loads, hydroelectric plants can alter the level of their energy output with relative ease.

This gives the network operator the flexibility to meet peak demands and respond to fluctuations in demand across seasons and at different times of the day. If there is an increased electricity demand, plant operators release more water from the dam. On the other hand, when demand drops, the dam simply stores more water for future needs.

Synergetic– The flexibility of hydropower is an asset in power generation mix as it can substitute for a base load plant during maintenance or forced outages. More importantly, it can support the development of intermittent renewable energy sources such as wind and solar power. Storage based hydro plants can provide the required back up energy and ensure electricity supply in times when there is no wind or sun.

Reliable – Hydropower stations have far less trippings, enhancing the system reliability and stability. 

– Hydro plants are more energy efficient than most thermal power plants too. This means they waste less energy to produce electricity. In thermal power plants, a lot of energy is lost as heat. Hydro plants are about 95 percent efficient at converting the kinetic energy of the moving water into electricity.

Economically Attractive

Long Life – While hydropower plants have large up-front capital costs, they have long lifetime with some plants now in service which were built 50 to 100 years ago. The first hydro project in India completed in 1897 is still in operation at Darjeeling.

Low operating costs– Plants are automated and have low operating and maintenance costs.

Low generation costs Hydropower is the cheapest source of electricity today. Cost of generation of electricity is lower than the other sources of energy.

No-fuel cost– Once the hydro plant is set up, the energy source- flowing water- is free. This is a significant advantage considering that fuel accounts for approximately 60% of the total cost of generation in thermoelectric plants.

Inflation-proof– Hydropower being a domestic resource is not subject to the fluctuations and risks of fuel price increases in the international markets. It fosters national energy security.

Carbon Credits– From an economic standpoint, hydropower plants being clean and green can reap the dividends of Carbon Credit Market. The international market of carbon credit is generated as a large number of countries are signatories to United Nations Framework Convention on Climate Change which entails reduction in carbon di-oxide emission in the environment. The developed countries, which have CO2 emission levels well over the stipulated norm, purchase carbon credits for getting sanction for their future projects. Saving of one tonne of CO2 emission gives one carbon credit point. The carbon credits earned can be traded to set up new units and for upgradation.


Multipurpose– Storage based hydropower systems are often part of multipurpose facilities and provide additional benefits like irrigation, flood control, drinking water supply, tourism, navigation, recreation, etc.

Local area development– Hydropower projects support socio-economic development of interior backward areas as the project site is developed. They bring electricity, roads, industry and commerce to communities, develop the economy, improve access to health and education and enhance the quality of life.

Sustainable Energy Hydropower projects that are developed and operated in an economically viable, environmentally sound and socially responsible manner epitomise sustainable development at its best. Sustainable development is meeting the needs of the present generation without compromising the ability of future generations to meet their needs. It is ‘development without cheating our children and grandchildren’.


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Kolkata-700 020, West Bengal, INDIA