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Ffestiniog Pumped Storage Scheme
Tanygrisiau Reservoir, Snowdonia National Park, Gwynedd, Wales, UK
Ffestiniog Pumped Storage Scheme
associated engineer
Central Electricity Generating Board
Freeman Fox & Partners
date  1957 - March 1963, opened 10th August 1963
era  Modern  |  category  Power Generation  |  reference  SH679445
ICE reference number  HEW 1238
photo  © David Medcalf and licensed for reuse under this Creative Commons Licence
Ffestiniog’s pumped storage hydroelectric power station was the first of its kind in Britain. It generates electricity on demand by releasing water under pressure to drive turbines. Still in continual use, the station has sufficient capacity to meet the power needs of north Wales for several hours. The UK now has four pumped storage schemes. The others are Dinorwig, also in Snowdonia, and Cruachan and Foyers in Scotland.
Pumped storage schemes work by using two reservoirs, one higher than the other. Water is discharged from the upper reservoir at peak times to generate hydroelectricity. It is then stored in the lower reservoir and pumped back to the higher reservoir at times of low demand, usually overnight, ready to begin the cycle again.
In the Ffestiniog scheme, the upper reservoir was created by enlarging an existing glacial tarn — Llyn Stwlan — with a new dam (SH665444). The mass concrete buttress gravity dam is 381m long, 33.5m high and a maximum of 4m thick. The reservoir has a capacity of 2 million cubic metres of water.
The lower reservoir, 315m below, was formed by damming the Afon Ystradau south of the village of Tan y Grisiau. The mass concrete gravity dam (SH683447) is some 565m long and 15.2m high. The lake it created is known as Tanygrisiau Reservoir or Llyn Ystradau and it too has a capacity of 2 million cubic metres. A winding private road of 3.2km climbs the hillside to link the two reservoirs.
Twin intake/discharge towers in the upper reservoir lead into two vertical concrete shafts of 4.4m internal diameter that fall 195m to the four pressure tunnels 1.1km long. These slope down towards the power station at a gradient of 1 in 40. The tunnels are lined with concrete (3.25m internal diameter) and steel (2.95m), with the transition halfway along their lengths. They are connected to four 2.3m diameter 213m long steel penstock pipelines with prestressed reinforced concrete surrounds. These convey water to the turbines and pumps.
The station’s four alternators are each of 90MW output when driven by the turbines and attain full power in under 60 seconds. Electricity is produced at 16kV and stepped up to 400kV to be fed into the National Grid. In the first year after commissioning (1963-4), the scheme was operating at 72.2 percent overall efficiency — very close to its design figure of 72.5 percent.
The alternators also operate as motors to drive the pumps, which are uncoupled when the station is generating hydroelectric power. For pumping, the turbines start the pumps and run them until frequency synchronisation with the system is reached, at which point the alternators become driving motors and water to the turbines is cut off. The pumps each require 75MW of electricity to return water to Llyn Stwlan.
The power station building on the west side of Tanygrisiau Reservoir is steel frame, faced with local stone. At 83.2m long, 21.9m wide and 20.1m high, it’s probably the largest stone building constructed in Wales since the Medieval castles of Harlech and Criccieth.
As with the much larger Dinorwig project (1974-84), great care was taken with the landscaping of the Ffestiniog Pumped Storage Scheme because it lies within Snowdonia National Park. The spoil from constructing Stwlan Dam was placed in the upper reservoir and the downstream face of Tan y Grisiau Dam has been camouflaged using rock taken from the excavations.
Construction took place between early 1957 and March 1963. Queen Elizabeth II opened the scheme on 10th August 1963. Its current owner is First Hydro.
The scheme obstructed and submerged part of the Ffestiniog Railway between the 1844 Moelwyn Tunnel (SH679428) and Tan y Grisiau Station (SH683449). The railway was awarded compensation and, in the 1970s, volunteers built a new tunnel and realigned section of narrow gauge track.
Designed in association with: James Williamson & Partners, Kennedy & Donkin
Main contractors: Cementation Co Ltd, Sir Alfred McAlpine & Son
RCAHMW_NPRN 401209, 306315
Research: ECPK
"Ffestiniog Pumped-Storage Scheme" by J.C.A. Roseveare, ICE Proceedings, London, Vol.28, pp.1-30, May 1964
reference sources   CEH Wales

Ffestiniog Pumped Storage Scheme