Dinorwig pumped-storage hydro-electric power station is one of the largest engineering projects in Britain. Work began on the conversion of the quarry in 1975, when millions of tons of slate had to be removed to create tunnels and machine halls. Dinorwig comprises sixteen kilometres of underground tunnels, deep below Elidir mountain. Its construction required 1 million tonnes of concrete, 200,000 tonnes of cement and 4,500 tonnes of steel.

Externally, all that is visble is a door on the side of the mountain, ensuring that the area of outstanding natural beauty is not impaired. The power station uses two lakes, Marchlyn Mawr and Llyn Peris, for both water power and storage. When power is required, water from Marchlyn Mawr is released down a 3.2 kilometre long tunnel through a series of inlet valves driving six pump-turbines as it passes through the generating chamber on its way to Llyn Peris 500 metres below.

At the end of the cycle, surplus power available during the low-demand period overnight is used to drive the turbines as pumps to force the water back to the top of the lake. The site was fully commissioned in 1984 and it is the third largest pumped storage scheme in the world.

Reference:
Cental Office of Information photographs and notes, dated 1975-1982.
David Gwyn & Merfyn Williams (1996) `A Guide to the Industrial Archaeology of North West Wales?. Association for Industrial Archaeology

RCAHMW, 2011.