The idea of using pumped storage as a way of coping with rapid, short-term changes in the demand for electricity in England and Wales was first considered in 1948, and having found a suitable site, the Ffestiniog station was authorised by Act of Parliament, specifically the North Wales Hydro Electric Power Act 1955.
The first task facing engineers was to provide access roads to the sites, and work on constructing these started in 1957.
This enabled work to commence on the two dams and the power station shortly afterwards.
A pumped storage scheme requires two reservoirs at different levels, and the upper reservoir was created by enlarging Llyn Stwlan, high up on Moelwyn Mawr.
Llyn Stwlan was located in a cirque basin, formed by glacial erosion.

The lake had previously been enlarged in 1898 by the Yale Electricity Company, who built a dam and used the water to generate power at Dolwen, located near Tanygrisiau, below the lower reservoir.
The company was formed by the Votty Quarry company, and direct current was transmitted by overhead line to Dolgarregddu, near the centre of Blaenau Ffestiniog, where there was a 100 hp (75 kW) gas engine and two semi-diesel engines, which generated an additional 240 kW. This was used to supply power to the town when it was not required by the quarry.
Because of the voltage drop in the system, a final 48 kW booster set was needed at the quarry.
The Dolwen station continued to operate after the new scheme was finished, although it was subsequently decommissioned and the building was converted into two semi-detached houses, and then an 8-bedroom detached house.

For the new scheme, the cirque basin was dammed at its eastern end by a concrete gravity buttress dam, 800 feet (240 m) long, and a maximum of 110 feet (34 m) high above its foundations.
There is a spillway at the southern end of the dam, with an overflow level at 1,650 feet (500 m) above ordnance datum (AOD).
The dam contains 6,000,000 cubic feet (170,000 m³) of water, and the water level rises and falls by 65.5 feet (20.0 m) as the station operates.
The catchment area for Llyn Stwlan is 160 acres (65 ha), and the average yearly rainfall over this area is 120 inches (3,000 mm).
The access roadway runs up to the northern side of the dam, and along the top to reach a bridge, which provides access to the intake towers.

The lower reservoir was formed by building another concrete gravity dam across the natural basin of Afon Ystradau.
The dam is around 1,800 feet (550 m) long, with a maximum height of 83 feet (25 m) above its foundations, although most of it is well concealed, as the spoil from the foundations was used to cover the downstream face of the structure.
Because of the larger area of the lower reservoir, it rises and falls by 18 feet (5.5 m) during the daily operation of the station.
Several streams flow off the surrounding land into the reservoir, and these flows are monitored, so that an equivalent amount of water can be discharged into the Afon Barlwyd through regulators in the dam.
The levels of Tanygrisiau reservoir are carefully monitored, to ensure that there is always sufficient spare capacity to hold the balance of the water left in Llyn Stwlan.
The catchment area for Tanygrisiau reservoir is 2,370 acres (9.6 km²) and the average yearly rainfall is 85 inches (2,200 mm).
The reservoir is designed for a maximum water level of 616 feet (188 m) AOD, and there is a spillway set at a height of 2.75 feet (0.84 m) above this.

The main power station building is 235 feet (72 m) long and 72 feet (22 m) wide.
In order to accommodate the vertical shafts of the machines, 140,000 tons on rock were excavated, and the building extends 110 feet (34 m) below the operating floor, which is at ground level.
There are four lower floors, and working downwards, these are the alternator floor, the turbine floor, the pump floor and the pump basement.
The height of the building above ground level is 65 feet (20 m).
The main building is flanked by two single storey blocks, the southern one used for the control room and the northern one for administr
The idea of using pumped storage as a way of coping with rapid, short-term changes in the demand for electricity in England and Wales was first considered in 1948, and having found a suitable site, the Ffestiniog station was authorised by Act of Parliament, specifically the North Wales Hydro Electric Power Act 1955. The first task facing engineers was to provide access roads to the sites, and work on constructing these started in 1957. This enabled work to commence on the two dams and the power station shortly afterwards. A pumped storage scheme requires two reservoirs at different levels, and the upper reservoir was created by enlarging Llyn Stwlan, high up on Moelwyn Mawr. Llyn Stwlan was located in a cirque basin, formed by glacial erosion.
The lake had previously been enlarged in 1898 by the Yale Electricity Company, who built a dam and used the water to generate power at Dolwen, located near Tanygrisiau, below the lower reservoir. The company was formed by the Votty Quarry company, and direct current was transmitted by overhead line to Dolgarregddu, near the centre of Blaenau Ffestiniog, where there was a 100 hp (75 kW) gas engine and two semi-diesel engines, which generated an additional 240 kW. This was used to supply power to the town when it was not required by the quarry. Because of the voltage drop in the system, a final 48 kW booster set was needed at the quarry. The Dolwen station continued to operate after the new scheme was finished, although it was subsequently decommissioned and the building was converted into two semi-detached houses, and then an 8-bedroom detached house.
For the new scheme, the cirque basin was dammed at its eastern end by a concrete gravity buttress dam, 800 feet (240 m) long, and a maximum of 110 feet (34 m) high above its foundations. There is a spillway at the southern end of the dam, with an overflow level at 1,650 feet (500 m) above ordnance datum (AOD). The dam contains 6,000,000 cubic feet (170,000 m³) of water, and the water level rises and falls by 65.5 feet (20.0 m) as the station operates. The catchment area for Llyn Stwlan is 160 acres (65 ha), and the average yearly rainfall over this area is 120 inches (3,000 mm). The access roadway runs up to the northern side of the dam, and along the top to reach a bridge, which provides access to the intake towers.
The lower reservoir was formed by building another concrete gravity dam across the natural basin of Afon Ystradau. The dam is around 1,800 feet (550 m) long, with a maximum height of 83 feet (25 m) above its foundations, although most of it is well concealed, as the spoil from the foundations was used to cover the downstream face of the structure. Because of the larger area of the lower reservoir, it rises and falls by 18 feet (5.5 m) during the daily operation of the station. Several streams flow off the surrounding land into the reservoir, and these flows are monitored, so that an equivalent amount of water can be discharged into the Afon Barlwyd through regulators in the dam. The levels of Tanygrisiau reservoir are carefully monitored, to ensure that there is always sufficient spare capacity to hold the balance of the water left in Llyn Stwlan. The catchment area for Tanygrisiau reservoir is 2,370 acres (9.6 km²) and the average yearly rainfall is 85 inches (2,200 mm). The reservoir is designed for a maximum water level of 616 feet (188 m) AOD, and there is a spillway set at a height of 2.75 feet (0.84 m) above this.
The main power station building is 235 feet (72 m) long and 72 feet (22 m) wide. In order to accommodate the vertical shafts of the machines, 140,000 tons on rock were excavated, and the building extends 110 feet (34 m) below the operating floor, which is at ground level. There are four lower floors, and working downwards, these are the alternator floor, the turbine floor, the pump floor and the pump basement. The height of the building above ground level is 65 feet (20 m). The main building is flanked by two single storey blocks, the southern one used for the control room and the northern one for administr
Ffestiniog Power Station