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Marchyn MawrElectric Mountain is the Visitors’ Centre that introduces you to an underground tour deep into Eliden Mountain where, in Europe’s largest man-made cavern, operate some of the world’s most powerful hydro-generators.

At the Centre the split level complex contains a fascinating range of interactive displays, models and exhibits. Here you see the history and development of hydro-electricity and how power is fed into the National Grid. A natural science exhibition introduces the natural history of Snowdonia and gives an insight into how the Ice Age sculpted the landscape into the hills and valleys that are here today.

Starting with a sound and vision show the Underground tour then takes you by bus from Electric Mountain into the centre of the Dinorwig power station. After driving through the labyrinth of tunnels you alight at several places to see the generators close up and find out how they can go from zero output to full power within seconds.

The pump storage system of producing electricity at Dinorwig relies on falling water turning turbines. To put it simply, high-pressure water enters the system from a high source (Marchyn Mawr), turns the turbines and exits under low pressure into a retaining reservoir (Llyn Peris).

The whole generating system is buried deep inside Eldin Mountain in a cavern large enough to house St Paul’s Cathedral. More than 16 kilometres of tunnels were drilled or blasted into the heart of the mountain and further tunnels constructed to Marchyn Mawr and Llyn Peris. In order to accommodate the amount of water needed to operate the turbines both lakes needed to be enlarged. The construction of a dam at Marchyn Mawr took four years to build and increased the holding capacity to 7 million cubic metres (1,540 million gallons) of water. During full operation the water level will fall and rise 34 metres daily. Llyn Peris also needed to be enlarged but this was done largely through removing large quantities of slate quarry waste from the lake rather than by constructing dams at both ends of the lake. The stream, Nant Peris, entering the lake was diverted around it to enter the lower Llyn Padarn. When the valves are opened and water falls from Marchyn Mawr there is an enormous surge in pressure. To counteract this another shaft was built top a surge pond high upon the mountain.

Dinorwig can produce electricity (1320 megawatts of power) in 12 seconds when there is a sudden surge in demand that cannot be met by power stations already connected to the grid. Most conventional power stations take 12 hours to start up from cold and 45 minutes to switch to the grid if they are on ‘hot standby’ but not synchronised to the grid. If necessary, Dinorwig can generate 1,680 megawatts for 5 hours.

When operating as much water passes through the tunnels at Dinorwig as London uses in a whole day. When the extra power is not needed the turbines can be reversed and water is then pumped back to Marchyn Mawr where it is ready to use again when the need arises.

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