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World electricity production 2019 IEA.png
The water cycle

Hydro-electricity is the world's biggest source of clean energy, producing more than twice as much as all other clean renewables combined.

Hydropower derives from the sun's radiation heating water (on land and in the oceans), making it evaporate and rise into the atmosphere where it condenses into clouds and falls as rain (or snow). Some of it falls in hills and mountains where it has the potential to release energy as it descends to lower altitudes (and eventually back to the sea).

The use of terrestrial water as a source of mechanical energy dates back to antiquity, and its more recent use to generate electricity is long-established. Physically the energy which can be obtained from a body of water is proportional to the rate at which it flows and the height it descends: the faster it flows and the greater the height it descends the more energy it can yield in doing so.

1909 Hindu Kush Hydro Power Plant
Archimedes screw

The main ways of exploiting this energy are with water-wheels (as in antique water mills), Archimedes screws (found in some run-of-the-river schemes), turbines and Pelton wheels. Schemes which extract power from a flowing river, often with a drop formed by a weir, are usually described as "run of the river" schemes. The biggest hydro schemes use a dam built to create an artificial lake or reservoir storing a large amount of water. The water can drive turbines as it is released, turning generators to produce electricity. The dam and reservoir may also be used to regulate water flow downstream (e.g. to prevent flooding), or to provide water supplies.

Dam-based hydro-electric power stations can provide power on demand, i.e. dispatchably, and they can be turned on and off (or up and down) very quickly.

Their capacity factor and availability depend on rainfall and tend to be seasonal. They are also liable to be affected by climate change, which can result in rainfall becoming more erratic.

Energy, including electricity, generated from hydropower sources is intrinsically free of greenhouse gas emissions, but reservoirs can be significant sources of GHG emissions through mechanisms such as seasonal submersion of plant matter causing decay and methane release.

Rivers and streams are often delicate ecosystems and damming can seriously impact these with deleterious effects on biodiversity. Damming river systems that cross national boundaries can also result in political tensions between nations. Dams can also pose serious risks to those living downstream.

Costa Rica 100 renewable U30.png
Costa Rica 100 renewable Indy.png

Articles announcing that a country has run for so-many days on renewables alone, such as this and this about Costa Rica, often feature pictures of wind turbines and solar panels when the prime energy source is actually hydro.

See also the EIA's article Hydropower explained.

Pumped hydro

A hydroelectric dam where water released from the dam is collected in a lower dam (or goes to the sea) can be arranged so that water can be pumped back from the lower reservoir (or sea) to the upper reservoir, where it can generate electricity again later. These sorts of pumped storage schemes act like enormous batteries, effectively storing electricity (although there is a trade-off: only about three-quarters of the electricity put into pumping up the storage comes back as electricity generated). Pumped storage schemes such as that at Dinorwig in North Wales and Diablo Canyon in California have long been used to match the constant output from baseload power stations, especially nuclear, to demands which tend to be lower at night, when the pumped system can use surplus electricity to pump water and then release it for extra generation during peak periods in the the daytime.

Pumped storage may also be able to partner with solar to provide electricity 24*7, in certain parts of the world such as the proposed Valhalla project in Northern Chile.

Wikipedia lists pumped storage hydro power stations world wide. The biggest in storage capacity are currently up to 36 GWh, but the Snowy 2.0 scheme in South Australia is planned to have a capacity an order of magnitude higher: around 350 GWh (although a relatively low peak output power relative to its capacity, of 2 GW).


Environmental and Social impacts

Sustainable hydropower in the 21st century Emilio F. Moran, Maria Claudia Lopez, Nathan Moore, Norbert Müller, David W. Hyndman; PNAS; 5 Nov 2018


North American and European countries built many large dams until 1975, after which both started to abandon a significant part of their installed hydropower because of the negative social and environmental impacts. However, there has been a recent trend of new large hydropower dams being built in developing countries, particularly in megabiodiversity river basins, such as the Amazon, the Congo, and the Mekong. The socioeconomic and environmental damages in these river systems are even greater than the early costs in North America and Europe. This paper discusses how the hydropower sector needs to not only focus on energy production but also, include the negative social and environmental externalities caused by dams and recognize the unsustainability of current common practices.


Hydropower has been the leading source of renewable energy across the world, accounting for up to 71% of this supply as of 2016. This capacity was built up in North America and Europe between 1920 and 1970 when thousands of dams were built. Big dams stopped being built in developed nations, because the best sites for dams were already developed and environmental and social concerns made the costs unacceptable. Nowadays, more dams are being removed in North America and Europe than are being built. The hydropower industry moved to building dams in the developing world and since the 1970s, began to build even larger hydropower dams along the Mekong River Basin, the Amazon River Basin, and the Congo River Basin. The same problems are being repeated: disrupting river ecology, deforestation, losing aquatic and terrestrial biodiversity, releasing substantial greenhouse gases, displacing thousands of people, and altering people’s livelihoods plus affecting the food systems, water quality, and agriculture near them. This paper studies the proliferation of large dams in developing countries and the importance of incorporating climate change into considerations of whether to build a dam along with some of the governance and compensation challenges. We also examine the overestimation of benefits and underestimation of costs along with changes that are needed to address the legitimate social and environmental concerns of people living in areas where dams are planned. Finally, we propose innovative solutions that can move hydropower toward sustainable practices together with solar, wind, and other renewable sources.

GHG emissions

Real greenhouse gas footprints of reservoirs revealed Science Daily; 4 Aug 2017

When hydropower reservoirs traps organic matter, it leads to higher local greenhouse gas emissions. But the emissions are not increased but displaced. A new tool calculates the real greenhouse gas footprints of reservoirs.

Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis Bridget R. Deemer, John A. Harrison, Siyue Li, Jake J. Beaulieu, Tonya DelSontro, Nathan Barros, José F. Bezerra-Neto, Stephen M. Powers, Marco A. dos Santos, J. Arie Vonk; BioScience, Volume 66, Issue 11; 05 Oct 2016

Collectively, reservoirs created by dams are thought to be an important source of greenhouse gases (GHGs) to the atmosphere. So far, efforts to quantify, model, and manage these emissions have been limited by data availability and inconsistencies in methodological approach. Here, we synthesize reservoir CH
, CO
, and N
O emission data with three main objectives: (1) to generate a global estimate of GHG emissions from reservoirs, (2) to identify the best predictors of these emissions, and (3) to consider the effect of methodology on emission estimates. We estimate that GHG emissions from reservoir water surfaces account for 0.8 (0.5–1.2) Pg CO
equivalents per year, with the majority of this forcing due to CH
. We then discuss the potential for several alternative pathways such as dam degassing and downstream emissions to contribute significantly to overall emissions. Although prior studies have linked reservoir GHG emissions to reservoir age and latitude, we find that factors related to reservoir productivity are better predictors of emission.

Impact on communities

License Denied To Brazil's Biggest Amazon Dam Sneha Susan John; Natureworldnews; 8 Aug 2016

Brazil's environmental protection agency has denied permission to build a giant hydroelectric dam in the Amazon rainforest because of how it could affect indigenous communities. Building of the dam can affect nearly 10,000 Munduruku people around the river Tapajós. This dam can flood a large area and might also lead to a forced removal of at least some indigenous communities. Removal of the indigenous groups is an act that is strictly prohibited by the Brazilian constitution. If the Sao Luiz do Tapajos (SLT) dam was built it would have been an 8,000-megawatt dam, the sixth-largest hydroelectric dam in the world and the second largest in the country. It was expected to cover a five-mile wide Tapajós river and drowns 376 sq km of the Amazon rainforest that hold thousands of people from indigenous communities.

Safety and environmental

Flood victims on roofs of houses in Laos after dam collapse

Hydro dams are mostly quite safe, but when they do go wrong they can be very dangerous: the world's worst energy disaster (massively worse than Chernobyl) was the collapse of the Banqaio dam which killed around 200,000 people and made about 12 million homeless. Other dams are also at risk, such as Mosul in Iraq, whose collapse could kill around a million people, and the Kariba dam between Zambia and Zimbabwe, which puts the lives of 3.5 million people at risk. In 2018 the collapse of a dam in Laos killed at least 20 people and left 6,000 homeless[1].

Building dams often floods homes and displaces communities, and has large ecological impacts, especially downstream where patterns of river flow and flooding are disrupted by the dam. Silt carried in the river feeding the dam can also deposit in the dam reservoir, reducing its storage capacity. Climate change is likely to alter patterns of rainfall and is already impacting hydroelectric production in some parts of the world.


(AKA Flow-of-river)


Goring Weir hydro-power scheme given go-ahead BBC; 10 Mar 2016

Plans for a hydro-electric scheme on Goring Weir which could power up to 300 homes have been given the go-ahead. Goring And Streatley Community Energy Ltd want to put three Archimedes screws at Goring Lock to generate electricity.


Corwen Electricity Co-operative

We are intending to building a 55kW pelton turbine close to the centre of Corwen, which will be funded by community shares, and owned by the shareholders, who will automatically become members of our co-operative when they buy shares. Shareholders will get their capital returned over the next 20 years, and will get interest payments over this time, and some money will be put into a local community fund.

Osney lock


Share offer document

The total development costs of the project are (or are estimated to be) as follows:
Initial feasibility £14,000
Set-up costs: marketing and legal £10,000
Project development £50,000
Renewable energy project build £550,000 (including fish pass and set-up costs)
There are three proposed sources of income from the scheme:
The Feed-in Tariff (FiT)
This will be paid on the whole amount of energy generated. The starting rate for electricity generated by the hydro scheme is 20.21p/kWh and it rises every year according to the Retail Price Index. The FiT payments will last for 20 years. The FiT for electricity generated by the solar PVs is also payable for 20 years, with a starting rate of 13.99p/kWh.
Direct sale of electricity
The Environment Agency have agreed in principle to buy electricity from the project to supply the Osney Yard and will pay a rate that is the same as the one it pays through its National Contract. We estimate that the Environment Agency will use 80% of the electricity generated.
Exporting electricity to the grid
We plan for electricity not used by the Environment Agency to be sold through a Power Purchase Agreement to an energy supplier. We estimate that 20% of the energy generated will be sold in this way. The combined income from these three sources will start at around £50,000 per year.

Oxford hydro project shines light on electric future Nathalie Thomas; FT; 1 Feb 2020

... Osney Lock Hydro, which since 2015 has been generating an average of 188 megawatt hours a year of electricity on a stretch of the river Thames — enough to power around 60 homes


Justin Bowles' comments in a thread on hydropower (corrected for typos)

The track record of community owned hydroelectric plans is abysmal. Almost none of them have hit their original targets and many of the early ones have a high probability of not being able to pay back their original shareholders and/or could go bankrupt.

The reasons for this are threefold: first the community hydro projects are almost all low river head projects. That is, their economic case is marginal to begin with. In general, the higher head projects have already been snapped up and developed by private land owners.

Second, the hydro consultants generally base generation numbers on far too optimistic assumptions. I am undecided as to whether this is due to a) wilful fraud (they get fat commissions for projects that go ahead, however 'dodgy' they are or b) because they are basing their flow assumptions on historical river flow data that has been rendered unreliable by climate change.

People forget that hydro projects not only suffer from too little water but also too much. When there is a lot of water in a river, the head generally gets smaller. Given that we have been experiencing either feast or famine in river levels over the last decade or so, this is a major problem for projects that assume that most of the time the river is in the sweet spot of not too much but not too little water.

Third, 'shit' happens and this never seems to be conservatively accounted for in the generation numbers. And the 'shit' that happens can be monumental. You would think that the technology is pretty robust with a bloody great Archimedes screw or two and a generator. A common problem is that the intake mesh gets perpetually clogged, so you often need permanent volunteers to clear it of crap like branches or police cones that a passing yob may have thrown into the river. Given these projects have a 20-25 years life, you need pretty dedicated volunteers who are prepared to go out in mid-winter to clear the crap away. Often their enthusiasm falls after a season or two. If you pay someone to do this it plays havoc with your returns. Worse, sometimes stuff gets through the grille and clogs the screw (like a dead sheep, I kid you not). Another problem some of these projects face is that the bearings go on the screw. That costs loads of money to fix.

There was one project, Stockport Hydro, that I thought was actually hitting its numbers and it even paid a dividend (they all promise to do this after a couple of years or so after commissioning but few have actually had the money to afford a dividend). The the concrete plinth cracked holding one of the screws, so putting half of their generating capacity out of commission for a whole year. As I say, shit happens.

Finally, the number of these projects that flounder before build is huge. In the process, hardy bands of volunteers spend multiple years of their life for basically nothing. My heart goes out, for example, to Richard Riggs at Abingdon Hydro. Richard and the team spent five years and even got the share issuance away before eventually throwing in the towel. Here is Richard's sign off titled "Well We Tried".

Vulnerability to Climate Change

Malawi suffers blackouts as drought exposes 98% reliance on hydro power Agence France-Presse; The Guardian; 8 Dec 2017

Large parts of Malawi have been plunged into darkness as water levels at the country’s main hydro power plant fell to critical levels due to a severe drought, according to its electricity company.

The impoverished southern African country which relies on hydroelectricity has been hit by intermitted blackouts since last year, but the outages have recently worsened, lasting up to 25 hours.

The state-owned Electricity Supply Corporation of Malawi (Escom) said on Thursday that power output had been halved as water levels in the Shire river dropped to critical levels.

The water from the river normally generates a total of 300 megawatts of electricity, which is 98% of the country’s supply. “For the past three weeks, the available capacity was 160 megawatts,” said Escom said in a statement.

Affected areas include large parts of the capital Lilongwe and in the second city of Blantyre.

A number of businesses and hospitals in the country had been forced to use diesel-powered generators to keep the lights on.

New Zealand Glacier Retreat will Impact Hydropower Mauri Pelto; From a Glacier's Perspective blog / AGU; 6 Jan 2017

Footnotes and references

  1. "Laos dam collapse: Many feared dead as floods hit villages"; BBC News; 24 Jul 2018