Hydroelectric Power

What do you think of when you hear the word 'water'? Maybe you're thinking of a nice refreshing drink on a hot sunny day, a stream in the middle of the woods or a nice hot bath in winter. But did you know that a river flowing downhill has the potential to provide power to an entire city? Hydroelectric power is a form of renewable energy that utilizes the natural processes of the water cycle. Hydroelectric energy has actually been around for thousands of years. For example, romans used wheels turned by flowing water to grind grains for flour and bread. 

Hydroelectric Power Hydroelectric Power

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Table of contents

    Hydroelectric Power: Definition

    Hydroelectric power (HEP for short) is a form of electricity produced by converting the potential energy of fast-flowing water.

    As mentioned earlier, the energy from fast-flowing water has been utilized for centuries. Over 2000 years ago, residents of Ancient Greece used flowing water to turn mill wheels to grind wheat into flour. Water mills are large wheels on the banks of rivers used to create energy for a variety of functions, such as cutting or grinding, and they were very common up to the industrial revolution.

    Hydroelectric power is the most common source of renewable electricity worldwide - in fact, 71% of renewable energy generated on the planet is from hydropower.

    Did you know that China is the largest producer of hydroelectricity?

    Hydroelectric Power Hoover Dam StudySmarterFig. 1 – A large hydroelectric dam in the US. Source: unsplash.com

    Hydroelectric Power Plants

    There are three types of hydroelectric power plants.

    Impoundment Facility

    By far, the most common type of plant is an impoundment facility, where a dam is used to control the flow of water downhill. The aim is to raise the level of water behind the dam, creating as large a head as possible (this will be explained in more depth later!).

    When more energy is required, water is released from the dam. The water flows downwards due to the force of gravity, travelling through turbine blades, which power a generator.

    The Hoover Dam impounds Lake Mead on the Arizona-Nevada border. The dam, built during the Great Depression, can generate up to 2080 MW of electricity, supplying California, Arizona and Nevada. The Hoover Dam is a popular tourist site, attracting around seven million visitors a year.

    Diversion Facility

    This type of hydroelectric plant does not use a dam. Instead, a series of canals is used to channel water towards turbines where electricity is generated.

    The Grand Valley Diversion Dam was built in a canyon of the Colorado River. The original dam was built between 1913 and 1916, but a small hydroelectric plant with a capacity of 3000 KW was built in 1933.

    Pumped Storage Facility

    This type of hydroelectric plant collects energy from other forms of renewable energy and stores it by pumping water uphill into a reservoir. When electricity demand is high, water in the higher reservoir is released and flows down into the lower reservoir, travelling through a turbine.

    The Drakensberg Pumped Storage Scheme, located in the Drakensberg Mountains of South Africa, is a unique hydroelectric plant. It consists of four dams: Driekloof Dam, Sterkfontein Dam, Kilburn Dam and Woodstock Dam. The total facility is able to generate up to 1 GW of electricity.

    Hydroelectric Power Plant Diagram

    Most hydroelectric power plant and their dams look relatively similar. The terrain on which they are built and the forces they have to withstand, such as water volume and pressure or environmental stressors like earthquakes, will typically modify their characteristics.

    Hydroelectric Power Generation

    No matter the type of power plant, hydroelectric power is generated through water travelling downwards through pipes known as penstocks. Falling water causes turbines to rotate, which drives generators that convert the mechanical energy of the turbines into electricity.

    Transformers convert the alternating voltage suitable for generators into a higher voltage suitable for long-distance transmission.

    The powerhouse is the structure housing turbines and generators in a hydroelectric power plant.


    The difference between the high and low elevations is known as the head, measured in metres.

    The power that a volume of water can generate is proportional to its working head. The larger the head, the higher the water pressure across the turbine, thus the more power the turbine can generate.

    Higher heads are preferable in hydroelectric plants because they generate more power. Additionally, the higher pressure means a higher flow rate can be forced through small turbines, which are cheaper.

    New Turbine Developments

    Rotating wheels and falling water have been used for a very long time to generate power. New types of wheels and their systems can improve their ability to harvest water's kinetic (meaning moving) energy.

    Helical Turbines

    These turbines are installed horizontally, which allows their use in shallow sites. The turbine always rotates in the same direction, independent of water flow. Thus, helical turbines are valuable in reversible tidal flows.

    The three peripheral hydrofoil blades provide lots of free space, enabling safe fish passage through the turbine.

    Low Head Turbines

    Low-head turbines can be used as part of small-scale hydropower systems. Small heads provide minimal environmental impact.

    Advantages of Hydroelectric Power

    Generally, hydroelectric power holds a lot of advantages over fossil fuels.

    • Renewable: hydroelectric power uses the energy of running water to produce electricity, without affecting its volume.
    • Reliability: unlike some other forms of renewable energy, such as wind and solar, the availability of hydroelectricity does not fluctuate.
      • Hydroelectric energy can enter the electricity system faster than any other type of energy, so can quickly re-establish supply after a blackout.
    • Environmentally Friendly: production of hydroelectric power does not emit greenhouse gases or pollution.
    • Cost-effective: once built, power plants have few maintenance costs. Their lifetime can be up to a century.
    • Promotes Development: energy from hydroelectric power promotes infrastructure, develops the economy and increases access to health and education.

    Disadvantages of Hydroelectric Power

    However, hydroelectric power is not a perfect method of electricity generation. The main disadvantages will be discussed below.


    • Creating dams floods river banks, destroying wetland habitats. Plants and aquatic birds are commonly affected.

    • Dams often obstruct fish trying to swim upstream.

    • Furthermore, operating the plant may increase the temperature of the reservoir water. Nearby plants and animals either have to adjust or migrate.

    Construction Costs

    • Hydroelectric power has the highest construction cost of all types of renewable energy - at $5312 per kilowatt of electricity generated. Building power plants can take between two and four years to complete.

    • Building the dams themselves causes flooding, which displaces locals and destroys farmland.


    Normally, silt is carried by the flowing river, but its path is obstructed by the dam. As silt builds up on the bottom of the reservoir, the amount of impounded water slowly reduces. A decrease in water causes a decrease in energy flowing through the turbines. Some power plants only provide electricity for 20 or 30 years due to silt build-up.

    Greenhouse Gas Emissions

    Despite the belief that hydroelectricity is a carbon-free energy source, research has shown that reservoirs emit greenhouse gases. There are a few ways that this can happen.


    When new land is flooded after building a dam, carbon from the flooded soil can be transformed into greenhouse gases.

    Water Accumulation

    As water accumulates in the reservoir, aquatic bacteria can transform carbon into greenhouse gases.


    If water stays in a reservoir for a long time, the surface is warmed, but the bottom remains cold. This temperature gradient creates a thermocline, which acts as a physical barrier for small molecules such as carbon dioxide and methane.

    A thermocline is the transition area below the water surface, where warm water mixes with cold water and gradually cools.

    Below the thermocline, the environment could be anoxic. Carbon is transformed into methane, instead of carbon dioxide. If this happens, methane could be released downstream through the turbines - known as degassing.

    Anoxic waters contain little to no oxygen.

    I hope that this clarified hydroelectric power for you. Remember that hydroelectric power is generated by the conversion of potential energy from fast-flowing water. Despite its drawbacks, hydroelectricity is the most common renewable source of electricity worldwide.

    Hydroelectric Power - Key takeaways

    • Hydroelectric power is a form of electricity produced by converting the potential energy of fast-flowing water.
    • There are three types of hydroelectric power plants: impoundment facilities, diversion facilities and pumped storage facilities.
    • Hydroelectric power is generated by water travelling downwards through turbines, which drive generators that convert the mechanical energy of the turbines into electricity.
    • The head (the distance between high and low elevations in a power plant) is proportional to the amount of power produced. Larger heads produce more power, and often make electricity production cheaper.
    • Advantages of hydroelectric power: renewable, reliable, cost-effective, environmentally friendly, promotes development.
    • Disadvantages of hydroelectric power: destroys habitats, high construction costs, flooding, siltation, greenhouse gas emissions.

    1. Alain Kilajian, Carbon emissions from hydropower reservoirs: facts and myths, 2021

    2. Alexander Gorlov, Helical Turbine and Fish Safety, 2010

    3. Britannica, Hydroelectric Power, 2021

    4. Britannica, Hoover Dam, 2019

    5. Drax, Pumping power: pumped storage stations around the world, 2020

    6. Hydro Review, Hydro has highest average construction cost of any generating technology in the U.S., 2018

    7. Linquip, What is Low Head Turbines?, 2021

    8. National Geographic, Hydroelectric Energy, 2022

    9. National Geographic, Hydroelectric Energy: The Power of Running Water, 2022

    10. Renewables First, Head and flow detailed review, 2022

    11. Renewables First, How long will a hydro project take?, 2022

    12. US Bureau of Reclamation, Bureau of Reclamation projects on the National Register of Historical Places in Colorado, 2015

    13. Water Science School, Hydroelectric Power: Advantages of Production and Usage, 2018


    1. Fig. 1: Hydroelectric dam (https://unsplash.com/photos/U7-N8pnKrqQ) by Gary Yost (https://unsplash.com/@gyostimages) is licensed by CC0 1.0 Universal (https://creativecommons.org/publicdomain/zero/1.0/)
    Frequently Asked Questions about Hydroelectric Power

    What is hydroelectric power?

    Hydroelectric power is a form of electricity produced by converting the potential energy of fast-flowing water.

    How does hydroelectric power work?

    Hydroelectric power is generated through falling water travelling through turbines, causing them to rotate. The rotation drives generators that convert the turbine's mechanical energy into electricity.

    Hydroelectric Power: Pros and Cons?

    Pros: renewable, reliable, cost-effective, environmentally friendly, promotes infrastructure and development.

    Cons: destroys habitats, high construction cost, flooding, siltation, greenhouse gas emissions.

    Is hydroelectric power renewable?

    Hydroelectric power is a reliable source of renewable energy.

    How is hydroelectric power produced?

    Hydroelectric power is produced by converting the potential energy of fast flowing water

    Test your knowledge with multiple choice flashcards

    Hydroelectric power is the most commonly used source of renewable energy worldwide: true or false?

    This type of hydroelectric power plant uses a series of canals to channel water towards turbines.

    The larger the head, the less power the turbine can generate. True or false?


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