|Date||24 November 2012|
|Location||Tanygrisiau, Ffestiniog||SH 67784 44307; 52.97974°N, 3.97070°W|
Construction for the Central Electricity Generating Board of the Ffestiniog Pumped Storage Scheme commenced in 1957 and the power station was commissioned in 1963, making it the UK’s first major pumped-storage system. Stwlan Dam was built to create the upper reservoir by enlarging the small lake, Llyn Stwlan, nestling in a cwm at an elevation of 500m in the Moelwyn mountains; while the lower reservoir, Llyn Ystradau (or Tanygrisiau Reservoir), was formed by damming the river Ystradau just to the south of Tanygrisiau village.
The facility provides power to the National Grid at times of high demand and it has a combined output of 360 MW from its four generators. During periods of low demand, normally overnight, the system operates in reverse: taking electricity from the Grid, the generators are run as motors and the turbines act as pumps to drive water back up from the lower to the upper reservoir.
Ffestiniog Power Station is currently owned by First Hydro, a joint venture between International Power plc and Mitsui & Co. Ltd. Having been acquired in June 2012, International Power is now a wholly owned subsidiary of French multinational electric utility company GDF Suez SA, and has been rebranded as GDF SUEZ Energy International. Mitsui & Co is one of Japan’s largest sogo shosa, or traditional general trading companies, and is itself part of the zaibatsu (corporate conglomerate) Mistui Group.