Hydroelectric Energy Guide: what is it and how does it work?

20 January 2021 | Celia Topping

hydro electricity hydro power

The power of water has been a crucial source of energy to humans for thousands of years. Records of water mills being used across the ancient world date back to the 3rd Century BC. In fact, the word “hydro” comes from the Greek word for water. 

Medieval England got in on the act, too, and water energy has been harnessed in the UK ever since. Picture that bucolic English scene, with a water mill sitting prettily by a flowing river.

Then fast-forward a few hundred years to the opening scenes of Goldeneye, with James Bond bungee-jumping off a huge, vertigo-inducing dam. That’s the scale of hydroelectric power stations we’re seeing nowadays. Not quite so pretty as in days of old – but much more effective. 

What is hydroelectric energy or hydro power?

Put simply, hydro power, or hydroelectricity, is a form of renewable energy generated by the movement of water. The flow of water is used to spin a turbine, which is connected to an electric generator. The electricity is then fed into the National Grid, and into our homes. 

To find out more about how OVO is fighting climate change by supplying renewable energy to around 500,000 UK homes, read our comprehensive blogs

Why is hydro energy renewable?

contra dam hydro electricity hydro powerThe Contra Dam in Switzerland, used as a film location in Goldeneye

Hydro energy is considered renewable because water is in infinite supply, and the water cycle is constant. Water falls to Earth in the form of rain, filling our rivers, lakes, reservoirs and oceans. The water then evaporates, forms clouds, and the whole cycle begins again. Water is as readily available and as free to use as solar or wind power. In our ever-increasingly climate-conscious world, water is a powerful and abundant alternative low-carbon source of energy. 

Would you like to know more about the impact of fossil fuels and how we’re moving towards becoming coal-free? Read our guide to fossil fuels, and then find out more about the future of renewables.

And to find out more about solar and wind energy, check out our relevant blogs. 

How does hydroelectric energy work?

The basics of hydroelectricity depend on water flowing from one place to another. We can rely on the force of gravity to ensure water always flows downwards. That kinetic downward movement of water can be harnessed in various ways. 

What are the different types of hydroelectric energy plants?

There are 3 different types of hydroelectricity power plants:

An impoundment facility

This is the “reservoir and dam” system we’re most familiar with. A concrete dam is built to hold back a river and create a man-made reservoir. The dam is used to control the flow of water from the reservoir.

When energy’s needed, water is released through the dam, flowing downwards to spin a turbine. As the turbine blades spin, they power a generator. This in turn generates electricity. 

A diversion facility

Also known as “run-of-river”. This type doesn’t use a dam, but instead harnesses the natural flow of water falling from a greater to a lesser height. The water is channelled, via a series of canals, towards the generator-powering turbines. One good example of this is the power stations at Niagara Falls. 

A pumped-storage facility

pumped storage hydro electricity hydro power

An example of a pumped storage facility in Australia

This works as a kind of battery, storing energy generated from other sources such as solar, wind and nuclear. It stores the energy by pumping water from a reservoir at a low level, up to a reservoir at higher level. When there’s a demand for electricity, the water is released back down to the lower reservoir and on to that all important turbine.  

Advantages of hydro power

Billions of people the world over depend daily on hydroelectricity – it powers homes, offices, factories, hospitals and schools. So why is it so popular?

  • Renewable – as long as water exists, hydroelectricity is possible. (And if water didn’t exist, we wouldn’t be around very long to need electricity!) 
  • Low emissions – the biggest appeal of any renewable energy is that it creates no emissions whilst in service. Water is clean and carbon-free. So hydroelectric power lowers greenhouse gas emissions.
  • Reliable – the sun sets, and sometimes the wind doesn’t blow, but water can always be made to flow.
  • Flexible – a hydro plant can easily control the flow of water. In times of demand, they can hit capacity in just 2 minutes. And stopping the flow can be done just as quickly when demand is low.
  • Recreation – the lakes or reservoirs created by a dam can be used for recreational purposes such as fishing, water sports and swimming.  
  • Land development – dams can only be built in certain locations, and they take a long time to build. So building them can also help develop the surrounding area with transport links and infrastructure. 
  • Employment – many countries use hydropower to bring affordable energy to rural areas. It creates employment opportunities for the local communities. The Three Gorges Dam in China created jobs for around 250,000 people. 
  • Low operational cost – hydro stations need little maintenance, so they’re cheaper to manage than other types of power station.    
  • Long-lasting – hydroelectric power stations are built to last. Many are still operational 50 to100 years after they’re built. 

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Disadvantages of hydro energy 

Despite being a clean, renewable source of energy, with many benefits, it’s not all good news. Particularly for the environment:

  • Suitability – only certain locations and conditions are suitable for power plant construction. Sites also need to be relatively close to major cities where the energy is needed, to minimise infrastructure costs. 
  • Cost – building costs for hydroelectric plants can be much higher than other sources of renewable energy, with huge investment needed.
  • Permissions – there’s often a lot of red tape involved – which can slow, or even stop development altogether.
  • Drought – hydropower is the most reliable renewable energy source available. But with global warming heating up the planet, and causing increasing droughts, water may become more scarce.  
  • Human cost – large-scale dams create huge reservoirs that flood entire valleys. Whole communities sometimes have to be relocated as dam construction begins. This means lives are disrupted, businesses are lost and homes destroyed. 
  • Flood risk – dams are generally built to be very strong and incredibly safe. But there is always some risk to towns further downstream, with tragic consequences. The Banqiao Dam failure (China 1975) was caused by typhoon rainfall, and caused the death of 171,000 people.
  • Environmental concern –  usually, to create a hydro power plant, a river must be dammed. This causes riverside habitats to be destroyed, with serious loss of freshwater biodiversity. Andreas Baumüller, Head of Natural Resources at WWF’s European Policy Office, says,“Freshwater species populations are declining at twice the rate of terrestrial or marine species – and hydropower dams are a major pressure.”
  • Carbon and methane emissions – as we know, there are no greenhouse gas emissions from the electricity generated by hydro power plants. But when a reservoir is created, flooded plants and trees begin to decompose, creating vast amounts of carbon and methane.   
  • Fish – when a river’s dammed, fish are blocked from swimming upstream and returning to their breeding ground. This also affects any animals that rely on those fish for food. In answer to this problem, some dams, such as the Bonneville Dam, have installed fish ladders to help fish migrate. This “ladder”, which is actually a series of wide steps, allows the fish to slowly swim back to their breeding grounds. 
  • Siltation – silt, carried by a flowing river, usually ends up in a delta or mouth of the river. But because the flow of the river has been blocked by a dam, this silt builds up on the reservoir bed. This process is called siltation. Hundreds of metres-worth of silt can build up, reducing the amount of water that can gather in the reservoir. And less water means less powerful energy to flow through the turbines. Most dams spend a considerable amount of money preventing siltation. But it’s still a problem – and some power plants can only create electricity for 20 or 30 years because of it. 

How much electricity is generated with hydro energy around the world?

Currently, 20% of the world’s electricity and 74% of the planet’s renewable energy, is generated by hydropower. It’s the most commonly-used renewable energy source in the world, with China being the largest producer. Other top generators of hydropower around the world include the United States, Brazil, Canada, India, and Russia. 

What’s the largest hydroelectric power plant in the world?

three gorges dam china hydropower hydro electricity

The Three Gorges Dam in Yichang, China

The Yangtze River in China is held back by the Three Gorges Dam. This gigantic dam is a whopping 2,335 metres long and 185 metres high. And it can produce a staggering 22,500 megawatts of power at capacity. 

How much electricity is generated with hydro in the UK?

In 2019, the UK generated 7.77 TWh of hydropower – that’s 7,700,000 Mwh. The average annual consumption per home in the UK is 3.7kWh, which means hydro can power around 2 million homes a year.  Most of this energy is generated in the wet and mountainous regions of Wales and northwest Scotland. 

Can I use hydropower to generate my own electricity?

Generating your own electricity with hydropower is possible – but it’s not for everyone. Because hydroelectric power is very site-specific, you’d have to go through some very careful assessments.

A certified installer would need to take into account not only your location, but also the impact of the system before you could start building. Not many homes in the UK have access to a suitable water resource. Remember, you’d need to consider:

  • How much water is flowing per second – in both winter and summer
  • The difference in height over a short distance

And building a micro-hydro system isn’t cheap. If you think you might have a suitable site, you can find out more about permissions and licenses from the Energy Saving Trust. 

If you’re keen on building your own private hydro station, check out how you could earn money by putting energy back onto the Grid through our Smart Export Guarantee

In the meantime, make sure you’re being as efficient as possible with the electricity you’re using in your home. Check out our simple guides on how to make a real difference to your bills, as well as reduce your carbon footprint.

What does the future look like for hydro energy?

Hydropower is one of the most effective renewable sources of energy. But environmental concerns and limitations can affect its global reach. In spite of that, according to the International Energy Agency (IEA), electricity generated by hydropower could be tripled by 2050, if all available resources are harnessed.