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The Electric Mountain - Dinorwig, Gwynedd, UK

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Located in the heart of Snowdonia, in one of the most beautiful landscapes in Great Britain, Dinorwig (or Dinorwic) power station is quite an oddity. The location was carefully chosen from hundreds of different areas and, consequently, the power station was built near Llanberis, North Wales. Except this was an area of such astounding natural beauty that the entire power station had to be built underground, inside the mountain.


The station became operational in 1984, but construction work had started ten years previously. It was a huge process, 12,000,000 tonnes of earth1 had to be removed and thousands of jobs were created in an area much in need of employment after the collapse of Wales' slate and coal industries. In total 6,000 man years of labour were used.

Why Go to Such Lengths?

In England and Wales electrical power is distributed throughout the country by a network of power lines owned and controlled by The National Grid. It is their job to ensure that electricity is available when and where it is needed. In the UK, most of the electrical energy is generated by large fossil fuel power plants or by nuclear power stations.

The demand for electricity varies through the day - far less power is used during the night hours than during the day. While it is possible to match this variation in demand by altering the output of the large power plants, to do so is uneconomical. It would be much better if the excess electricity generated at night could somehow be stored for use during the day.

Dinorwig is an example of such a 'pumped storage hydroelectric plant' (there is another at Ffestiniog, also in Snowdonia). During the day Dinorwig's six turbines supply energy to the National Grid and during the night power is fed from the grid to recharge the reservoirs.

In addition, Dinorwig can be brought from standby to peak capacity in approximately 12 seconds, making it ideal for coping with sudden unexpected 'spikes' in demand.

How Does it Work?

The basic idea behind Dinorwig is quite simple. The power station consists of two lakes, Marchlyn Mawr, high in the hills behind Elidir Fawr, and Llyn Peris on the valley floor. The lakes are one-and-a-half kilometres from the other, with a height drop of 500 metres separating them. The two are connected by a series of pipelines. Inside the mountain between the two lakes is the largest man-made cave in Europe, the inside of which looks remarkably like the set for a James Bond movie. Inside this cavern are the turbines.

When there is a need for power, water is allowed to flow out of the upper lake, down through the pipes into the turbines, at immense pressures. Gravity acting on the water forces the turbines round and once the water has passed through, it drains down into the lower lake. This in itself is an excellent system, but can only be used once; if the top lake is empty then no more power can be produced. The solution is simple. During the night when there is an excess of power in the National Grid, the turbines are reversed and are used to pump water from the lower lake back to the top lake, ready for the next peak in demand.

Take a Tour

The power station has an accompanying museum in Llanberis. The museum is excellent and from there - for a small fee - a minibus will take you inside the power station and you will be given a guided tour through the insides of the mountain. The tour is well worth it: the main cavern where the turbines are is quite spectacular, but be warned, take a jumper as you will go a long way underground and it gets quite cold down there, even in summer.

Dinorwig is owned and run by First Hydro Company, a subsidiary of Edison Mission Energy. Further information, including pictures and statistics for both Dinorwig and Ffestiniog can be found at the FHC website.

1Approximately 4 million cubic metres, equivalent to two Millennium Domes or over 36,000 double-decker buses.

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