Hydropower: Renewable Hydroelectricity

Hydropower is an established source of renewable energy which provides almost one-fifth of the world's electricity. We outline the main types of hydro technologies including dams, run-of-river and pumped storage. Read about the contribution of hydroelectricity to a sustainable energy system, as well as the challenges it faces, including its life-cycle greenhouse gas emissions, and the ecological problems it causes for the environment.
Three Gorges Dam China
The Three Gorges Dam, China. Photo: Le Grand Portage/CC BY 2.0

One of the root causes of climate change is our huge energy consumption, which will increase substantially during the 21st century. With fossil fuels discredited due to their greenhouse gas emissions, renewable energy like hydropower has become our only hope.

In fact, hydropower was around when global warming first started and has since become one of the most reliable solutions to help slow it down. This is because advancements in hydroelectricity, especially in the area of pumped storage, could support other renewables like wind energy and solar power in supplying electricity in a more dependable and sustainable way.

What Is Hydropower?

Hydropower or water power is the use of running water to power machinery or make electricity. Since ancient times, hydropower has been used to turn water mills and operate mechanical lifts and cranes. In more modern times it has been used to rotate a turbine in order to create electricity. This electricity is called hydroelectric power or hydroelectricity, and is fed directly into power grids to supply communities with power.

The water cycle is a constantly moving feature of Planet Earth: water evaporates from oceans and lakes, forms clouds and then falls as rain or snow, before returning via rivers to the ocean.

The water cycle is driven by energy from the sun which evaporates water into the troposphere. This means that hydropower – which is derived from water – is a renewable source of energy.

One of the main benefits of renewable energy is the fact that it produces very few greenhouse gas emissions compared to fossil fuels. Thus hydroelectricity is essentially clean power – unlike that produced by conventional power plants, which use coal, oil or gas to generate electricity.

For this reason, hydroelectric power – along with other renewables like solar, wind, organic biomass and geothermal energy – plays a vital role in helping us to resolve our climate crisis by dialing down our emissions.

There are several different types of hydropower plants. Some use dams, others don’t. Facilities also range in size, from large utility power plants that supply towns and communities, to small installations that provide enough power for a single household, with possibly a small surplus to sell to utility companies.

A large hydropower has a capacity of 30 megawatts (MW), a small plant generates roughly less than 10 MW and a micro hydropower plant has a capacity of up to 100 kilowatts. A micro-hydroelectric power system can produce enough to power a home, farm or village. On the opposite end of the scale – the Hoover Dam in Colorado, USA is a whopping 2,074 MW which is enough to serve 1.3 million people.

Note: Other forms of hydropower include tidal power and wave power, since both these marine energies convert the kinetic energy of flowing water into electricity.

How Much Hydroelectricity Does Hydropower Generate?

Hydropower is currently the world’s primary source of renewable power, providing about 16 percent of the world’s electricity. The biggest producers are China, Brazil, Canada, the United States and Russia.

Graph of hydropower generation growth.  Top 5 countries.
Source: Our World in Data. 2020

Although the global energy system remains dominated by coal, oil and gas, it is projected to undergo a dramatic change by 2050. Although hydropower is still on course to make a significant contribution, it is projected to be overtaken by solar PV and wind energy. Geothermal power and bioenergy will also increase. 1 2

Hydroelectricity Targets Not Being Met

In 2019 installed global hydropower capacity was 1307 GW. 3 According to the International Renewable Energy Agency (IRENA), this figure needs to grow by about 60 per cent by 2050 to help limit the rise in global temperature temperatures below 2°C above pre-industrial levels. We are not on track to achieving this target, because to do so requires an average annual increase of 3 percent a year until 2030. In 2019, for instance, hydroelectricity increased by just over 2 percent. 4

In truth, we are not on track with most renewable targets, which begs the question, could it be that our climate plan can’t cope and is no longer fit for purpose? For more on this, see: What is the Emissions Gap?

According to energy experts at IRENA, reaching the necessary 60 percent increase in hydropower would require an investment of US$1.7 trillion. It would also require support in fast-tracking planning approvals, and introducing tax relief or low-interest loans for potential investors.

On the plus side, additional capacity does not always have to come through building new hydropower plants, but significant opportunities exist to upgrade and modernize older plants. For instance, some 600 GW of existing plants are older than 30 years, allowing plenty of scope to upgrade the sector. 5

Top 10 Biggest Hydropower Plants In The World

DamInstalled CapacityCountryRiver
Three Gorges Dam22,500 MWChinaYangtze
Itaipu14,000 MWBrazil / ParaguayParaná
Xiluodu13,860 MWChinaJinsha
Belo Monte11,233 MWBrazilXingu
Guri10,235 MWVenezuelaCaroní
Tucuruí8,370 MWBrazilTocantins
Grand Coulee6,809 MWUnited StatesColumbia
Xiangjiaba6,448 MWChinaJinsha
Longtan Dam6,426 MWChinaHongshui
Sayano-Shushenskaya6,400 MWRussiaYenisei
Listed by installed capacity. Source: wikipedia

Note: Installed capacity (also called nameplate capacity) refers to the maximum output of a plant. Plants rarely operate at maximum capacity, all the time.

In terms of installed capacity, the world’s largest hydroelectric plant is Three Gorges in China which is 2.3 km wide (1.4 miles) and 185 meters (607 feet) high. However, the Itaipu plant situated between Brazil and Paraguay actually generates the most electricity annually.

The biggest hydropower plant in the United States is at the Grand Coulee Dam on the Columbia River in Washington, a state that gets about two-thirds of its electricity from hydropower.

Types of Hydropower Plants

There are three types of hydropower facilities: (1) Impoundment (2) Diversion and (3) Pumped Storage.

1. Impoundment Hydropower

This is the most common type of hydroelectric power plant. Impoundments use a dam to store river water in a reservoir. They are large-scale projects built by governments and operated in partnership with utility companies.

A dam is built across a waterway to hold back the flow of water and create a reservoir. The water is impounded. When that reservoir is full, it becomes in effect stored energy. Now engineers can raise the headgate and release the water at various paces to meet changing electricity demand – baseload and peak load demands. Water may also be released at times of high rainfall and flooding to relieve pressure on the dam wall.

How do hydroelectric dams work?
The idea is to build a dam on a large river that has a large drop in elevation – that is why there are not many hydroelectric plants in Kansas or Florida. Credit: Environment Canada

When the sluice gate is opened, water travels through a trash rack, which is a type of metal structure that prevents debris from entering the dam. It moves down a pipeline called a penstock, to the turbines.

The turbine is a machine which is turned by the fast moving force of water. In this way, turbines convert kinetic energy to mechanical energy. There are two main types of hydro turbines: impulse and reaction turbines. Which type is used, depends on the height the water falls from, referred to as the head; as well as the volume of water. (Turbine names include: pelton, turgo wheel, cross-flow, reaction turbine, straflo, propeller, bulb turbine, kaplan, tube turbine, francis turbine. Kinetic energy turbines are also called free-flow turbines.)

The turbine is connected to a generator which rotates and produces electricity. The amount of electricity generated depends on how far the water drops and how much water moves through the system.

A transformer converts electricity from the generator into usable voltage electricity. Transmission lines then conduct electricity from the plant to the grid.

2. Diversion or Run-of-River

Run-of-River (ROR) hydropower still uses turbines and generators – but no dam is built to impound water. Instead, it relies on natural flows of rivers and waterways, diverting just a portion of that water. Because it is subject to natural water variability it is more intermittent than energy from a dammed reservoir. Diversions may have no water storage at all, or a limited storage reservoir which is referred to as pondage. The Chief Joseph Dam in Washington, USA, is a major run-of-the-river station which means the lake behind the station is not able to store large amounts of water.

Chief Joseph Hydropower Dam, Washington State.
Chief Joseph Dam on the Columbia River, Washington State, USA. It has an installed capacity of 2,620 MW with a capacity factor of 42 percent. Capacity factor refers to the ratio of net electricity actually generated compared to what could have been generated at continuous full-power operation.

ROR hydroelectricity is considered ideal where streams or rivers can sustain a minimum flow or where they are regulated by a reservoir or lake upstream. This sort of plant is radically different in design to an impoundment and so usually has less environmental impact.

3. Pumped Storage Hydroelectric

Another type of hydropower is pumped storage (PS) or pumped hydroelectric energy storage (PHES). It’s a way of storing hydropower much like a battery. It is a sort of water battery!

Note: compare hydropower’s system of pumped storage with that of hydrogen energy in underground spaces. Both systems offer huge potential benefits to intermittent renewables like solar and wind.

Pumped storage is most often used in combination with wind and solar power as a hybrid solution. This is known as a hybrid hydro-wind-solar solution. 6 The future may in fact be an electric grid primarily fed by wind and solar technologies, with pumped-storage hydropower schemes. Here’s how it works.

How pumped storage works: animation diagram
How Pumped Storage (PS) works. Image: Creative Commons.

Pumped storage (PS) require two reservoirs linked by conduits. Generally, the lower reservoir is underground. When too much solar or wind energy is created, it could go to waste. To avoid this, the excess solar/wind power is used to pump water to an upper reservoir in a PS plant. The upper reservoir becomes a giant natural battery, ready to be used again when needed. When energy is needed, the water is released from the upper reservoir and it flows back down through a turbine to generate electricity.

Pumped storage is the most dominant form of storage on the electricity grid today. 7 It is estimated that global PS capacity will grow by 78 GW by 2030, considerably more than other forms of energy storage technologies. 5

Storage is vital for the large-scale adoption of solar and wind. For example, solar plants generate the most electricity in the middle of the day, while demand for electricity is often highest in the evening. Wind can die down, or there can be a gale. Pumped storage of hydropower can help smooth out the edges. For this reason, there is growing interest in pumped storage globally, in particular the aspect of retrofitting underground caverns, disused mines and hydro plants as storage reservoirs.

Like all storage systems, there is energy loss. In the case of pumped storage, you lose about 20 percent of the original input – because of friction in the pipes and turbine as well as in the generator. In comparison, lithium-ion batteries lose about 10 percent and hydrogen energy storage about 50 percent. 8

What Are The Advantages of Hydropower?

Renewable Energy

Hydroelectricity is a renewable energy source. Hydroelectricity uses the energy of running water, without the risk of depletion, to produce electricity.

Cheap Electricity

Hydropower is the cheapest way to generate electricity today. That’s because once a dam has been built and the equipment installed, the ‘fuel’ – flowing water – is free. The global weighted average levelized cost of electricity (LCOE) from new projects commissioned in 2019 was US$0.05/kWh for hydropower. Around nine-tenths of the capacity commissioned had costs lower than the cheapest new source of fossil fuel-fired electricity.

LCOE of utility-scale solar PV plants was $0.7/kWh, onshore wind $0.6/kWh and $0.07 for bioenergy and geothermal projects. 9.

Hydropower Is Flexible

Hydropower is always available which is why it can be used a baseload energy. Once a dam is constructed, electricity can be produced at a constant rate. If electricity is not needed, the sluice gates can be shut and generation stops. The water can simply be saved for another time when demand is higher. Hydroelectricity also has the flexibility to be used as a water battery, as we have seen with pumped storage. This capacity makes it possible to pair with other renewable sources like wind and solar.

Hydroelectricity Doesn’t Cause Air Pollution

Hydroelectricity plants don’t cause air pollution, like conventional power plants. Nor do they generate toxic by-products like black carbon associated with fossil fuel combustion, wood-burning and some types of biomass.

By using hydropower instead of coal, we avoid the production of around 150 million tonnes of air polluting particulate matter, 60 million tonnes of sulphur dioxide and 8 million tonnes of nitrogen oxide. 5

Furthermore, according to the International Hydropower Association, if current hydropower capacity was replaced with coal-fired power stations, then around 3.5 to 4.0 billion metric tonnes of additional greenhouse gases would be emitted annually and global emissions from power plants and industry would be around 10 per cent higher.

Helps Climate Mitigation

The UN’s Water report on World Water Day 2020 stated that ‘Hydropower will continue playing a role in climate change mitigation and the climate-proofing of the energy sector.’ Hydropower can help mitigate some of the impacts of extreme weather events such as floods and drought by controlling the flow of water. In addition, reservoirs can offer recreational opportunities, such as swimming and boating.

Hydropower can also help businesses reduce their carbon footprint by sourcing clean energy from local hydropower sources. For example, Microsoft has switched the fuel source for its headquarters in Washington state, to hydropower in order to limit emissions.

China’s Hongqiao Group, the world’s largest aluminium producer, is moving provinces in order to take advantage of cleaner hydropower. More companies are embracing climate action to save the planet, while at the same time securing green investment.

On the downside, there are only a limited number of suitable locations where hydroelectric power plants can be built and we are near full capacity in many developed nations.

What Are The Disadvantages?

Hydropower is generally viewed as a renewable resource of clean energy, although with major economic, environmental and social limitations.

Environmental Damage

The biggest disadvantage of hydropower is the disruption posed to local ecosystems. Damming water and constructing roads and power lines changes the flow, depth and velocity of water and can impact fish habitats, as well as their mating and feeding patterns.

Dams prevent fish such as salmon from swimming upstream – known as a salmon run – to spawn. For example, the Klamath River, once home to the third-largest salmon runs in the U.S, now has runs at a fraction of their original numbers. The spring-run Chinook salmon, which historically numbered in the hundreds of thousands, has almost entirely been wiped out. While the Coho salmon has been designated ‘threatened’ under the US’s Endangered Species Act. The extinction has been linked to the eight dams built on the river between the 1900s and 1962. 10

Dams impact whole biomes, causing low dissolved oxygen levels in the water, known as deoxygenation and species invasion. Decaying organic material in reservoirs leads to a release of methane, a powerful greenhouse gas that contributes to global warming.

Dam construction is also linked to deforestation as reservoir land is flooded. For example, the installation of dams in Brazil has been linked to deforestation in the Amazon basin. The excess of dead trees in the reservoirs has caused eutrophication and species invasion.

The most notable example is the controversial Belo Monte Dam, the world’s fourth-largest hydroelectric project completed in 2019. The dam blocked the 1,000-mile Xingu River, a major tributary of the Amazon. The dam’s reservoir, flooded 260 square miles of lowlands and forest, displaced more than 20,000 people, and caused extensive damage to the river ecosystem – home to more than 50 species of fish found nowhere else. Already, experts say its capacity has been massively miscalculated and does not take into account fluctuations between wet and dry season or for reductions in flow due to deforestation and climate change induced droughts. 11

Dams Uproot People and Communities

People living in villages and towns that are in the valley to be flooded, must move out. This means they lose their homes, farms and businesses.

The building of large dams can cause serious geological damage. For example, the building of the Hoover Dam in the USA triggered a number of earthquakes and depressed the earth’s surface at its location. Although modern dam design is much better, some dams have breached and led to flooding and deaths.

In 2018, following a period of heavy rain, the Xe-Pian Xe-Namnoy hydroelectric power plant in Laos collapsed, sending a wall of water into neighboring villages. 12 At least 49 people were killed, and 7,000 more lost their homes. The plant was nearing construction completion at the time and sub-standard building was blamed.

High Start Up Costs

While hydroelectricity is cheap to produce, dams are very expensive to build. Most dams will need to operate for decades before becoming profitable. On the other hand, plants do not require a lot of workers to operate and maintenance costs are usually low.

Vulnerable To Changing Weather Patterns

One of the many effects of climate change, is changing weather patterns. And this poses a problem for hydroelectric plants.

Africa is becoming increasingly reliant on hydropower for electricity production. It is also experiencing a decrease in rainfall. For example, Malawi relies heavily on hydropower and generation has dropped suddenly at times of drought resulting in widespread power outages across the country.

If all the large dams in planning phase are constructed by 2030, 70 percent of total hydropower generating capacity in eastern Africa will be dependent on areas with similar patterns of rainfall. 13 In the western United States, droughts have been increasing since the early 2000’s, and rivers are shrivelling. 14

Greenhouse Gas Emissions

Anthropogenic greenhouse gas emissions are the primary cause of global warming – but even renewables must be scrutinized for potential emissions.

Large hydro projects are a source of emissions, but mainly in the construction phase and also the emission of methane from the reservoir area.

One study of four regular sized hydropower stations in China, assessed the carbon footprint of those dams using a Life Cycle Assessment. 15 It concluded that more than 70 percent of emissions came from the construction stage, in particular the use of concrete to build dams. ( Cement emissions are bad for climate change, and the industry accounts for huge unsustainable carbon dioxide emissions. )

Greenhouse emissions fall when lifetime of a plant increases from 10 to 40 years and then drops slightly when lifetime becomes longer. That is 13.60 tCO2e/GWh for 50 years and 8.13 tCO2e/GWh for 100 years on average. Electricity generated by coal in China, in comparison was 822 tCO2e/GWh, making hydroelectricity almost 100 times more efficient and cleaner.

Other studies have examined the emission balance of run-of river stations in countries such as Norway, Japan and Switzerland and the results appears to be a decreasing trend of emissions per unit of electricity generated when installed capacity grows.

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Future Prospects

Despite the environmental issues surrounding hydropower – issues which lead many conservationists to reject it as a source of sustainable energy – proponents argue that its impacts can be mitigated. Moreover, they claim the impacts remain low compared with the environmental effects of fossil fuels. Special adaptions, for example, can be made to turbines to keep water better aerated to avoid deoxgenation. Dams can be placed more strategically for safe fish passage.

At the same time, thousands of dams are being decommissioned because they are no longer safe or fit for purpose. The U.S. Army Corps of Engineers has cataloged at least 90,000 dams that are preventing fish from accessing historic habitat. Some 17,000 have already been removed. In 2014, two large dams on the Elwha River in Washington state were removed, restoring 75 percent of previously inaccessible spawning habitat. An even bigger project slated for 2023 will remove four dams on the Klamath River.

Power companies are in favor of dam removal, as it is often a cheaper option than paying for upgrades to ensure compliance with new legislation (including the installation of costly fish ladders at each dam that would enable migration).

Even still, hydropower will remain an important renewable resource in the decades ahead. It is estimated, for example that by 2050 hydropower could reduce cumulative greenhouse gas emissions in the United States by 5.6 gigatonnes – equivalent to nearly 1.2 billion passenger vehicles driven in a year. It could also save $58 billion from avoided healthcare costs and economic damages from air pollution. 16


  1. Energy Transition Outlook Report 2020 – DNV GL Group 2019 []
  2. IRENA Global Energy Transformation. Roadmap to 2050 Report. []
  3. IEA Tracking Report 2020 []
  4. IRENA’s Global Renewables Outlook 2020 – Energy Transformation 2050. []
  5. Hydropower Status Report Sector trends and insights 2020. See PDF: https://hydropower-assets.s3.eu-west-2.amazonaws.com/publications-docs/2020_hydropower_status_report.pdf [][][]
  6. “Hybrid Pumped Hydro Storage Energy Solutions towards Wind and PV Integration: Improvement on Flexibility, Reliability and Energy Costs.” September 2020 Mariana Simão and Helena M. Ramos. []
  7. Office of Energy Efficiency & Renewable Energy: energy.gov/ []
  8. The Conversation 2019: “Five gifs that explain how pumped hydro actually works.” []
  9. Renewable Power Generation Costs in 2019. Irena. June 2020 []
  10. The Largest Dam Removal Project in History. BBC, Future planet ecology. 2020. []
  11. “Belo Monte boondoggle: Brazil’s biggest, costliest dam may be unviable.” January 2020. Mongabay []
  12. An estimated 175 billion cubic feet of water was released, from a tributary of the Mekong River. []
  13. “Hydropower in Africa: Plans for new dams could increase the risk of disruption to electricity supply.” London School of Economics 2017. []
  14. An unexpected side effect of drought: Higher carbon emissions.” National Geographic 2018. []
  15. “Carbon Footprint Assessment of Four Normal Size Hydropower Stations in China.” Ting Jiang et al. 2018. []
  16. “Hydropower Vision 2050: A New Chapter for America’s 1st Renewable Electricity Source.” Office of Energy Efficiency and Renewable Energy. []
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