Hydroelectric power is a type of electricity made from water energy. In 2023, hydropower provided 15% of the world’s electricity, about 4,210 terawatt-hours, which is more than all other renewable energy sources combined and also more than nuclear power. Hydropower can supply large amounts of electricity with low carbon emissions, helping to create reliable and clean energy systems. A hydroelectric power station with a dam and reservoir can quickly change how much electricity it produces to meet changing needs. Once built, these stations create no direct waste and usually release much less greenhouse gas than power plants that use fossil fuels. However, when built in lowland rainforest areas, some greenhouse gases may be released if parts of the forest are flooded.

Building a hydroelectric project can harm the environment by reducing farmland and moving people from their homes. It also affects river ecosystems, changing habitats, soil movement, and erosion patterns. While dams can help reduce flood risks, dam failures can cause serious damage.

In 2021, the world had about 1,400 gigawatts of hydropower capacity, the highest among all renewable energy technologies. Countries like Brazil, Norway, and China use hydroelectric power heavily. However, there are limits based on geography and environmental concerns. Tidal power is another option available in coastal areas.

In 2022, China added 24 gigawatts of new hydropower capacity, which was about three-quarters of all new global hydropower additions that year. Europe added 2 gigawatts, the largest increase in the region since 1990. Globally, hydropower generation increased by 70 terawatt-hours (a 2% rise) in 2022 and remains the largest source of renewable energy, producing more electricity than all other technologies combined.

History

Hydropower has been used since ancient times to grind flour and perform other tasks. In the late 18th century, hydraulic power provided the energy needed to start the Industrial Revolution. In the mid-1700s, French engineer Bernard Forest de Bélidor published Architecture Hydraulique, which described vertical- and horizontal-axis hydraulic machines. In 1771, Richard Arkwright combined water power, the water frame, and continuous production, which helped develop the factory system and modern employment practices. In the 1840s, hydraulic power networks were created to generate and send hydro power to users.

By the late 19th century, electrical generators were developed and could be connected to hydraulics. The Industrial Revolution’s growing demand for energy drove further development. In 1878, the first hydroelectric power scheme was built at Cragside in Northumberland, England, by William Armstrong. It powered one arc lamp in his art gallery. The old Schoelkopf Power Station No. 1, near Niagara Falls, began producing electricity in 1881. The first Edison hydroelectric power station, the Vulcan Street Plant, started operating on September 30, 1882, in Appleton, Wisconsin, with an output of about 12.5 kilowatts. By 1886, there were 45 hydroelectric power stations in the United States and Canada. By 1889, there were 200 in the United States alone.

At the start of the 20th century, many small hydroelectric power stations were built by companies near cities in mountain areas. Grenoble, France, held an International Exhibition of Hydropower and Tourism in 1925, which attracted over one million visitors. By 1920, when 40% of the power in the United States came from hydropower, the Federal Power Act was passed. This law created the Federal Power Commission to manage hydroelectric power stations on federal land and water. As power stations grew larger, their dams took on other roles, such as flood control, irrigation, and navigation. Federal funding became necessary for large projects, and government-owned companies like the Tennessee Valley Authority (1933) and the Bonneville Power Administration (1937) were formed. The Bureau of Reclamation, which had started irrigation projects in the western United States in the early 20th century, began building large hydroelectric projects, such as the 1928 Hoover Dam. The United States Army Corps of Engineers also helped with hydroelectric development, completing the Bonneville Dam in 1937 and being named the main federal flood control agency by the Flood Control Act of 1936.

Throughout the 20th century, hydroelectric power stations grew larger. Hydropower was called "white coal." In 1936, the Hoover Dam’s power station, with an initial capacity of 1,345 MW, was the world’s largest hydroelectric power station. It was later replaced by the Grand Coulee Dam in 1942, which had a capacity of 6,809 MW. The Itaipu Dam, opened in 1984 in South America, became the largest, producing 14 GW of power, but was later surpassed by the Three Gorges Dam in China in 2008, which produces 22.5 GW. Eventually, hydroelectricity supplied more than 85% of the electricity used in countries like Norway, the Democratic Republic of the Congo, Paraguay, and Brazil.

Future potential

In 2021, the International Energy Agency (IEA) stated that more action is needed to help reduce climate change. Some countries have already used most of their hydropower potential and have limited space for further development: Switzerland uses 88% of its potential, and Mexico uses 80%. In 2022, the IEA predicted that hydropower generation would increase by 141 gigawatts (GW) between 2022 and 2027. This increase is slightly smaller than the growth from 2017 to 2022. Because getting permits for projects and building take a long time, the IEA estimates that hydropower growth will stay limited. Only an additional 40 GW is expected in a faster development scenario. In 2021, the IEA also said that major updates to existing hydropower systems are needed.

Generating methods

Hydroelectric power is created when water stored behind a dam flows through a large pipe to turn a turbine and generator. The amount of electricity produced depends on how much water flows and how high the water falls. This height difference is called the "head." A large pipe, called a "penstock," carries water from the reservoir to the turbine.

This method helps meet high electricity needs by moving water between reservoirs at different heights. When electricity demand is low, extra energy is used to pump water into the higher reservoir, helping manage electricity needs. When demand increases, water is released from the higher reservoir through a turbine to generate power. In 2021, pumped-storage systems provided nearly 85% of the world's 190 gigawatts of grid energy storage, improving the daily efficiency of power systems. Pumped storage does not create energy itself and is shown as a negative number in energy reports.

Run-of-the-river hydroelectric stations have little or no storage, so they use only the water flowing from upstream at any given time. Extra water that cannot be used immediately is not saved for later. A steady water supply from a lake or reservoir upstream is helpful when choosing locations for these stations.

Tidal power stations use the rising and falling of ocean water caused by tides. These sources are predictable and, if reservoirs can be built, can produce electricity during times of high demand. Other less common hydro systems use the movement of water or water sources that are not dammed, such as undershot water wheels. Tidal power is only possible in a limited number of places worldwide.

Conduit hydroelectric stations use the movement of water through man-made channels, such as existing water pipelines used for public water supply, to generate electricity. Some definitions include tunnels, canals, or aqueducts that are primarily used for other water delivery purposes, not just electricity generation.

Sizes, types and capacities of hydroelectric facilities

Hydropower plants are divided into two main groups: small hydropower plants (SHP) and large hydropower plants (LHP). The classification depends on the plant’s maximum power output, which is called its nameplate capacity. The threshold for what is considered large or small varies by country. For example, in China, a plant with less than 25 megawatts (MW) is classified as SHP. In India, the limit is 15 MW, and in most of Europe, it is 10 MW. A plant with 50 MW or more is always considered an LHP.

SHP and LHP are further divided into smaller groups, and these groups are not always separate. For instance, a low-head hydro power plant, which uses a small height difference in water flow (a few meters to tens of meters), can be classified as either SHP or LHP. Another difference is how much the water flow is controlled. SHP plants usually rely on natural water flow with little control, while LHP plants have more regulation. Because of this, SHP is often used to describe run-of-the-river power plants, which use water flow directly without large reservoirs.

Hydroelectric power is the largest source of electricity globally. Some hydroelectric plants can produce more than twice the power of the largest nuclear plants. While there is no official definition for large hydroelectric plants, those with more than a few hundred megawatts are generally considered large.

Currently, only seven hydroelectric plants with more than 10 gigawatts (10,000 MW) are operating worldwide.

Small hydro refers to hydroelectric power that serves small communities or industrial sites. The definition of small hydro varies, but it is usually up to 10 MW. In some countries, like Canada and the United States, the limit may be as high as 25 MW or 30 MW. Small hydro plants can connect to regular power grids or operate in remote areas without access to a grid. They often have smaller reservoirs and less construction, which makes them less harmful to the environment compared to large hydro plants. However, their environmental impact depends on how much water is used for power production.

Micro hydro systems typically generate up to 100 kilowatts (kW) of power. These systems can provide electricity to individual homes or small communities and sometimes connect to larger power networks. They are common in developing countries because they do not require fuel and are cost-effective. Micro hydro systems work well with solar energy because water flow is often highest in winter, when sunlight is limited.

Pico hydro systems produce less than 5 kW of power. They are used in very small, remote communities that need only a little electricity. For example, a 1.1 kW project in Kenya provides power to 57 homes for basic needs like lighting and phone charging. Even smaller systems, using turbines as small as 200–300 watts, can power a few homes with just a small water drop. Pico hydro systems usually do not use dams but instead use pipes to redirect water through turbines before returning it to the stream.

Large hydroelectric plants often use underground power stations. These stations take advantage of natural height differences, such as waterfalls or mountain lakes. Water is moved through tunnels from a high reservoir to a cavern where turbines are located. After passing through the turbines, the water flows out through a tailrace to a lower waterway.

A simple way to estimate the power produced by a hydroelectric plant is using this formula:

Power (P) = Efficiency (η) × (Water Density (ρ) × Flow Rate (V̇) × Gravity (g) × Height Difference (Δh))

• P is the power in watts.
• η is the efficiency, which measures how well the system converts water energy into electricity (ranges from 0 to 1).
• ρ is the density of water (about 1,000 kg/m³).
• V̇ is the volume of water flowing per second (in m³/s).
• Δh is the height difference between the water source and the turbine (in meters).
• g is the acceleration due to gravity (9.8 m/s²).

Larger and newer turbines are usually more efficient. The amount of electricity produced each year depends on the water supply. In some places, the water flow can change by as much as 10 times over the course of a year.

Properties

Hydropower is a flexible source of electricity because power stations can increase or decrease their output quickly to match changes in energy needs. Hydro turbines can start operating within a few minutes. While battery power is faster, it has much less storage capacity than hydropower. Most hydropower units can reach full power in under 10 minutes, which is faster than nuclear and most fossil fuel power plants. Hydropower can also reduce its output quickly when there is extra electricity available. Because of this, hydropower is not usually used for continuous base power, except in cases like managing floodwater or meeting needs downstream. Instead, it often acts as backup for other types of power generators.

A major benefit of traditional hydropower dams with reservoirs is their ability to store water at low cost for later use as clean electricity. In 2021, the International Energy Agency (IEA) estimated that all existing conventional hydropower reservoirs combined could store 1,500 terawatt-hours (TWh) of electricity in one full cycle, which is about 170 times more than all pumped storage hydropower plants globally. Battery storage is not expected to surpass pumped storage in capacity during the 2020s. When used to meet peak energy demand, hydropower has higher value than baseload power and much higher value than intermittent sources like wind and solar.

Hydroelectric power plants often have long lifetimes, with some operating for 50 to 100 years. Operating costs are usually low because plants are automated and require few workers during normal operation.

When a dam serves multiple purposes, adding a hydroelectric station can be built at a lower cost, creating a revenue stream to help cover dam operation expenses. For example, electricity sales from the Three Gorges Dam are expected to cover construction costs within 5 to 8 years of full operation. However, in many countries, large hydropower projects may be too costly or take too long to build to provide a good return on investment unless proper risk management is used.

Some hydropower projects supply electricity to public networks, while others are built to serve specific industries. For example, dedicated hydropower projects often provide large amounts of electricity needed for aluminum production. The Grand Coulee Dam originally supplied power to an aluminum plant in the United States during World War II before later supporting irrigation and public use. Similarly, the Brokopondo Reservoir in Suriname and the Manapouri Power Station in New Zealand were built to supply electricity for aluminum production.

Hydropower does not use fuel, so it does not produce carbon dioxide during electricity generation. While some carbon dioxide is released during construction and methane is produced annually by reservoirs, hydropower has one of the lowest greenhouse gas emissions among electricity sources. This is especially true in temperate climates, where emissions are lower compared to tropical regions. In tropical areas, reservoirs often produce more methane because flooded plant material decays in oxygen-free environments, releasing greenhouse gases.

Like other non-fossil fuel sources, hydropower does not emit sulfur dioxide, nitrogen oxides, or other harmful particles.

Reservoirs from hydropower projects often support activities like water sports and become tourist attractions. In some regions, aquaculture (raising fish or other aquatic life) is common in reservoirs. Multi-purpose dams used for irrigation help provide a steady water supply for farming. Large hydropower dams can also reduce flood risks for communities downstream. However, managing dams with multiple uses, such as irrigation, can be complex.

In 2021, the IEA called for clear sustainability standards and simpler rules for all hydropower projects.

Large hydropower reservoirs can flood large areas upstream, sometimes destroying forests, wetlands, and grasslands. Dams can disrupt river flows, harm ecosystems, and displace people and wildlife. The loss of land is often worsened by habitat fragmentation caused by reservoirs.

Hydropower projects can harm aquatic ecosystems both upstream and downstream. Water released from turbines often carries little sediment, which can erode riverbeds and riverbanks. Turbines can also kill many aquatic animals, such as eels, with up to 70% dying after passing through. Fluctuating water levels caused by opening turbine gates can create sudden changes in river flow.

Droughts and seasonal rainfall changes can greatly reduce hydropower output. Water can also be lost through evaporation.

When water flows, it can carry heavy particles downstream, which can damage dams and power stations, especially in areas with high siltation. Silt can fill reservoirs, reduce flood control capacity, and increase pressure on dams. Over time, reservoirs may become unusable or even fail during floods.

The amount of electricity produced by a dam depends on river flow. Lower river flows reduce reservoir storage, limiting the water available for hydropower. This can lead to power shortages in areas relying heavily on hydropower. Climate change may increase the risk of reduced river flows. For example, a study on the Colorado River found that a 2°C temperature rise and 10% drop in rainfall could reduce river runoff by up to 40%. Brazil, which depends heavily on hydropower, could see a 7% annual drop in energy production by the end of the century due to rising temperatures, lower water flow, and changing rainfall patterns.

In tropical regions, hydropower projects often have lower positive impacts. Reservoirs in lowland rainforests produce significant methane emissions because flooded plant material decays without oxygen. According to the World Commission on Dams, reservoirs with large surface areas compared to their generating capacity and no prior forest clearing may emit more greenhouse gases than traditional oil-fired power plants.

In contrast, boreal reservoirs in Canada and Northern Europe typically emit only 2% to 8% of the greenhouse gases produced by fossil fuel power plants. A new type of underwater logging may help reduce these emissions further.

Hydro power by country

In 2022, hydroelectric power produced 4,289 terawatt-hours, which is 15% of all electricity generated worldwide and half of all renewable energy sources. Of the total global hydroelectric power, China produced 30%, which is the highest among all countries, followed by Brazil (10%), Canada (9.2%), the United States (5.8%), and Russia (4.6%).

Paraguay generates almost all of its electricity from hydroelectric power and exports more energy than it uses. Larger hydroelectric plants are usually built and managed by national governments, so 70% of the total hydroelectric power capacity is owned by the public. However, nearly 70% of the actual hydroelectric plants are operated by private companies, according to data from 2021.

The following table shows information for each country, including:
– Total hydroelectric power generation in terawatt-hours,
– The percentage of each country's electricity that comes from hydroelectric power,
– Total hydroelectric power capacity in gigawatts,
– The percentage increase in hydroelectric power capacity, and
– The efficiency of hydroelectric power plants for that year.

The data comes from Ember, with updates as of 2023. Only countries that produced more than 1 terawatt-hour of electricity are included. Links to more information about each country's hydroelectric power are provided when available.