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Energy Resources:
Pumped storage reservoirs


Pumped Storage Reservoirs:

Storing energy to cope with big demands

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See a French translation of this page


Pumped storage reservoirs aren't really a means of generating electrical power. They're a way of storing energy so that we can release it quickly when we need it.

Demand for electrical power changes throughout the day. For example, when a popular TV programme finishes, a huge number of people go out to the kitchen to put the kettle on, causing a sudden peak in demand. This is called a "TV pickup".

If power stations don't generate more power immediately, there'll be power cuts around the country - traffic lights will go out, causing accidents, and all sorts of other trouble will occur.

The top reservoir at Dinorwic. From First Hydro photo library

The problem is that most of our power is generated by fossil fuel power stations, which take half an hour or so to crank themselves up to full power. Nuclear power stations take much longer.

We need something that can go from nothing to full power immediately, and keep us supplied for around half an hour or so until the other power stations catch up. Pumped storage reservoirs are the answer we've chosen.

The UK has one in North Wales, at Dinorwic. There's an older one at Ffestiniog, also in North Wales.

This video clip from "Britain from Above" explains about "TV pickups" - how the national grid copes with surges in demand (4min 31 sec)  

How it works

Between 1976 and 1982 at Dinorwig, in North Wales, a huge project was built. Yet there's little to see as you drive past, as most of it is deep inside a mountain.

Water is pumped up to the top reservoir at night, when demand for power across the country is low.

When there's a sudden demand for power, the "headgates" (huge taps) are opened, and water rushes down the tunnels to drive the turbines, which drive the powerful generators.

The water then collects in the bottom reservoir, ready to be pumped back up later.

Dinorwig has the fastest "response time" of any pumped storage plant in the world - it can provide 1320 MegaWatts in 12 seconds. That's a lot of cups of tea!

a pumped storage plant

More about Dinorwig

When water is pumped up to the top reservoir (called "Marchlyn Mawr") we are storing gravitational potential energy in it. The greater the height, the more energy is stored.

This is one of the reasons that the Dinorwig site was chosen - there was a big height difference between two existing lakes, so less work was needed to build the station.

The water falls 600 metres on its way to the turbines, so it's under a great deal of pressure when it arrives. For this reason, the tunnels are lined with steel at the bottom end.

Each of the six generators is capable of producing 288 MegaWatts of power at 18,000 Volts, which is stepped up to 400,000 Volts by transformers and sent along underground cables to be fed into the "supergrid", which is the long-distance network of the National Grid.

From First Hydro's image gallery

Dinorwig has "pump/turbines", which can be used both as pumps for getting water from the lower to the upper reservoirs, and as turbines for generating electrical power.

This video clip explains how it all works

There is a complex system of gutters in the roof of the caves, to collect water that drips down through the rock. Carol Vordeman worked on this part of the station - helping to design this was one of her first engineering jobs before she moved into television.

You can find out more about the Dinorwig station from First Hydro's web site

  • Without some means of storing energy for quick release, we'd be in trouble.

  • Little effect on the landscape.

  • No pollution or waste


  • Expensive to build.

  • Once it's used, you can't use it again until you've pumped the water back up.
    But the industry is very good at predicting when the surges in power demand will happen, so good planning can get around this problem

Is it renewable?

It's not really a power station, but a means of storing energy from other power stations.
So the question doesn't apply.


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