How Do You Generate Hydroelectricity?

Hydroelectric power, also known as hydropower, is the process of generating electricity by harnessing the power of moving water. It is one of the oldest and most widely used renewable energy sources in the world.

The idea of using the energy from flowing or falling water to produce power has been around for thousands of years. Ancient civilizations used water wheels to grind grains into flour. In the late 19th century, the first hydroelectric power plants began operating in the United States and Europe.

Today, hydropower provides around 16% of the world’s electricity. It’s considered a renewable energy source because it relies on the water cycle – the continuous movement of water between the earth and the atmosphere. As long as water keeps evaporating, forming clouds, precipitating, and flowing back to the ocean, we can use its movement to generate electricity.

Compared to fossil fuels, hydroelectricity produces much lower emissions of greenhouse gases and air pollutants. It’s also relatively affordable and reliable. That’s why it remains an important part of the global renewable energy mix, despite some environmental drawbacks.

Hydroelectric Power Plants

Hydroelectric power plants use the energy of flowing water to produce electricity. The basic components of a hydroelectric plant are the dam, reservoir, penstock, turbine, and generator.

The dam is constructed across a river to create a reservoir, which stores water and controls its flow. Dams are often massive concrete structures that must be carefully engineered to safely withstand the water pressure.

The reservoir stores the water behind the dam. By controlling the flow of water from the reservoir, the power output of the plant can be varied to match electricity demands.

Penstocks are pipes that carry water from the reservoir to the turbines inside the power plant. The downward flow of water in the penstock creates hydraulic head pressure.

The water from the penstock rotates the blades of hydraulic turbines, which spin a shaft connected to the generator. The most common types of turbines used are Kaplan, Francis and Pelton turbines.

Finally, the generator converts the mechanical power from the turbine into electrical power. The electricity is then stepped up to high voltages for transmission over long distances to homes, businesses, and industries.

Dams and Reservoirs

diagram of a hydroelectric dam and power plant

Hydroelectric dams require reservoirs or dams to store the water’s potential energy. The reservoir acts like a battery, storing energy in the form of water held back by the dam. When electricity is needed, valves open and water rushes through turbines to generate power. Afterward, the water flows downstream.

There are two main types of dams used for hydroelectric power. Arch dams are curved structures built against rock walls and narrow gorges. The weight and curve of the dam presses it against the rock, making it stable. Embankment dams are wider dams made of earth, rock or gravel. The width and mass of the embankment provides stability.

Reservoirs serve the vital purpose of providing a water supply that can be controlled and regulated for electricity generation. Their storage capacity allows energy to be saved for times of higher electrical demand.

Hydraulic Turbines

Hydraulic turbines are a critical component of hydroelectric power plants. They are used to convert the energy of flowing or falling water into rotational energy to drive an electrical generator. There are two main types of hydraulic turbines used in hydro plants – impulse and reaction turbines.

Impulse turbines operate by using the velocity of water to move the turbine runner and drive the shaft. Common impulse turbine designs include Pelton wheels and turbo turbines. In a Pelton wheel, one or more water jets hit spoon-shaped buckets arranged around the runner. The kinetic energy of the water jet is transferred to the buckets, causing the wheel to turn. Turgo turbines are a modified version of Pelton wheels that operate with higher heads of water.

Reaction turbines operate by the action of water pressure against the turbine blades, causing them to move. Water flows through the turbine runner and causes a pressure differential. This pressure drop then rotates the shaft. Common reaction turbine designs include Francis turbines and Kaplan turbines. Francis turbines are radial flow reaction turbines that operate in a spiral casing. Adjustable guide vanes can optimize their performance. Propeller-type Kaplan turbines feature adjustable blades for efficiency over a wide range of flow conditions.

Both impulse and reaction turbine designs are engineered to match the water source and hydraulic conditions of a particular hydroelectric plant. The selection depends on factors like head height and water flow rate. Overall, hydraulic turbines are essential components that convert the natural power of water into usable mechanical power to generate electricity.

Electricity Generation

Hydroelectric plants generate electricity through a process that starts with the kinetic energy from falling water. The dam stores water in a reservoir, where hydraulic pressure builds as more water accumulates behind the dam wall. When operators open gates on the dam, water pours through tunnels or pipelines called penstocks. As the water flows through the penstock, it spins a turbine at high speeds.

The turbine is connected to an electrical generator. The fast spinning motion of the turbine causes magnets inside the generator to rotate past wire coils, producing an electrical current through electromagnetic induction. This electricity is then fed into transformers, which increase the voltage, allowing efficient transmission over power lines.

The amount of electricity produced depends on the volume of water flow and the vertical distance the water falls. Larger reservoirs enable more water storage and pressure, while greater dam heights mean increased potential energy that gets converted to kinetic energy to drive the turbines. By optimizing these variables, hydroelectric plants can generate massive amounts of reliable renewable electricity.


Once the kinetic energy from the flowing water has spun the turbines to generate electricity, the electricity must still be transmitted to the end users. Hydroelectric plants are often located far from population centers, so the electricity generated has to travel significant distances. For efficient transmission over long distances, the electricity’s voltage is stepped up to high levels – typically hundreds of kilovolts.

This is done through transformers located at the power plant. Transformers allow AC electricity to be easily converted to higher or lower voltages. Stepping up the voltage reduces the current in the transmission lines, which reduces the energy losses experienced during transmission. When the electricity reaches its destination, transformers are again used to step down the voltage to safer levels for distribution and end use.

Environmental Impacts

While hydroelectric power is a renewable energy source, building dams and reservoirs to generate electricity can have significant environmental impacts.

Flooding land for a reservoir destroys forests, wildlife habitat, agricultural land, and scenic areas. This directly impacts biodiversity by disrupting ecosystems and displacing plant and animal species. Migratory fish like salmon can also be blocked from reaching spawning grounds upstream.

Reservoir water can lack oxygen and contain higher levels of mercury, affecting water quality and harming aquatic life. Methane is also produced by the decomposition of vegetation and carbon in flooded reservoirs, contributing to greenhouse gas emissions.

The changes to river systems and water flows caused by dams alter natural flood cycles and sediment transport downstream, increasing riverbank erosion. This affects the entire watershed area.

Dam operations also impact water temperatures and nutrient levels, changing conditions that local species are adapted to. Lower river levels downstream can discourage fish spawning and eliminate rapids used by certain fish species.

While measures can be taken to mitigate some of these effects, all large hydroelectric projects fundamentally change the local environment. Proper planning is needed to weigh the impacts and attempt to minimize damage to ecosystems.

Advantages of Hydroelectricity

Hydroelectric power offers several key advantages compared to other sources of electricity generation:


Hydroelectricity is considered a renewable energy source because it relies on the water cycle. Rain and snow replenish the water in dams and reservoirs used for hydroelectric generation. As long as there is an adequate water supply, hydroelectric plants can generate clean electricity indefinitely.


Hydroelectric power is extremely reliable. The flow of water in hydroelectric dams is highly predictable and controllable, allowing output to be adjusted to meet demand. Hydroelectric stations have availability factors of 90% or higher, much more reliable than wind or solar power.

Low Operating Costs

Once a hydroelectric plant is constructed, operating costs are relatively low compared to other power plants. No fuel is required for electricity generation. Maintenance costs are also low since the turbines and generators have simple designs and there are minimal emissions or waste byproducts.

Disadvantages of Hydroelectricity

While hydroelectric power has many benefits, it also comes with some drawbacks that need to be considered. Three major disadvantages of hydroelectricity are:

High Upfront Costs

Constructing a hydroelectric plant requires building large dams, reservoirs, turbine systems, and transmission lines. This infrastructure is extremely capital intensive, frequently costing billions of dollars to build. The steep upfront costs can deter investment into new hydroelectric projects.

Geographic Constraints

Hydroelectric dams require specific geography to be built, namely large flowing rivers and mountain reservoirs. Suitable sites that provide the necessary head height and water volume are limited. Additionally, prime locations are often located far from population centers that need the electricity. Long transmission lines are then needed to transport the power.

Drought Impacts

Extended droughts can severely reduce water levels in a hydroelectric dam’s reservoir, limiting electricity generation. For example, hydroelectric output dropped during recent droughts in the western United States. Climate change may increase the frequency and intensity of droughts in some regions, further impacting hydroelectric generation.

Future of Hydroelectricity

Hydroelectricity is expected to continue playing a major role in renewable energy production in the future. While most major river systems in developed countries have already been dammed, there remains potential for growth in developing nations. According to the International Hydropower Association, most of the global hydroelectric growth potential in the coming decades lies in Asia, Latin America and Africa.

China leads the world in hydroelectric generation and still has abundant potential capacity. Major dam projects are planned or underway on rivers like the Nu and Lancang (Upper Mekong) that will supply power to China’s growing cities and industries. Other countries with major hydro expansion plans include India, Brazil, Ethiopia, Turkey, Malaysia,and Laos.

Existing facilities in North America and Europe are also being modernized and upgraded to increase efficiency and capacity. Pumped storage plants that reuse water between reservoirs are growing in popularity to balance intermittent renewables like wind and solar. New small-scale and low-impact hydro projects are being developed that minimize environmental impacts.

Overall, hydroelectricity is poised for moderate growth globally, led by developing countries that still possess untapped river resources. With the ability to provide reliable base load power and support expanding renewable energy, hydro will continue serving a vital role in the world’s diverse energy portfolio.

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