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Dinorwig Pumped Storage Scheme
Llyn Peris, Pass of Llanberis, Snowdonia, Gwynedd, Wales, UK
Dinorwig Pumped Storage Scheme
associated engineer
Central Electricity Generating Board
James Williamson & Partners
Binnie & Partners
date  1975 - 1983, opened 9th May 1984
UK era  Modern  |  category  Power Generation  |  reference  SH560610
ICE reference number  HEW 1236
photo  PHEW, courtesy ICE
The hydroelectric power station at Dinorwig was the largest pumped storage scheme in Europe and the third largest in the world when completed. Its location in Snowdonia National Park led to most of the construction taking place underground, inside an 'Electric Mountain', to minimise environmental impacts. The scheme remains in continual use.
As the name implies, pumped storage schemes are designed to store energy. Excess power generated from base-load power stations at times of low demand is used to pump water from one reservoir to another at a higher level. During periods of peak demand, the water is released back into the lower reservoir, passing through turbines on its way, generating electricity for the National Grid. Hydroelectric power stations can be brought online in seconds to satisfy rising demand.
The Dinorwig Pumped Storage Scheme has a generating capacity of 1,728MW. It is located in a former slate quarry, once the second largest in the world (closed 1969). Most of the works are hidden from view, buried deep within the mountain of Elidir Fawr. When the contract was let in 1975, the underground works had the highest value of any civil engineering project in the UK (£425m).
The scheme’s lower reservoir is the pre-existing lake of Llyn Peris (SH591593), below the south east flank of the mountain. Its water level was raised, and its area increased from 36 to 55.4 hectares, by dumping spoil into the deepest water and constructing a 3m high embankment along part of the north shore. Downstream (north west) of Llyn Peris is the natural lake of Llyn Padarn (SH574610).
The upper reservoir is Llyn Marchlyn Mawr (SH616619), also a pre-existing lake, to the north east of and 503m above Llyn Peris. A rock fill dam 36m high was constructed on its north side to increase capacity. The dam is notable for the use of asphaltic concrete on its face.
The station’s machinery is housed in nine manmade caverns, hollowed out of the rock some 750m inside the mountain. The photograph above shows the main machine hall under construction. Around 1 million cu m of slate were removed to form a network of linked subterranean chambers and passages.
The largest of the caverns is the machine hall at 180m long, 23m wide and up to 51m high. It lies 71m below the water level of Llyn Peris. The nearby transformer hall is 160m long, 23m wide and 17m high. There are also massive shafts and galleries for hydraulic and electrical control equipment, and 16km of tunnels. The tunnels carry two-lane roads into the power station and are lined with concrete except where they have less than 305m of rock cover, in which case the lining is of steel.
Dinorwig’s construction also required 1 million tonnes of concrete, 200,000 tonnes of cement and 4,500 tonnes of steel. A total of some 12 million tonnes of rock and slate were excavated to complete the scheme.
When power is required, water is released from Marchlyn Mawr. It flows through a 1.7km long 10.5m diameter low pressure tunnel, on a slight downhill gradient, to a 10m diameter vertical shaft 450m deep. At the foot of the shaft, a 10m diameter high pressure tunnel leads to the power station about 670m away. It divides into six smaller tunnels, from 2.4m to 4m in diameter, before reaching the six vertical Francis turbines.
The turbines and control gear are housed in the underground machine halls. They can also work in reverse as pumps to return water to Marchlyn Mawr, using their alternators as motors. Each is rated at 313MW when generating or 281MW when pumping. Three pairs of 3.65m diameter tunnels lead away from the turbines, connecting to three 8.2m diameter tailrace tunnels discharging into Llyn Peris.
In general, pumping takes seven hours each night and generation lasts five hours every day, with an average output of 1,680MW at 18kV. The turbines are able to run without load to provide an immediate reaction to demand. A load of up to 1,320MW can be picked up in 10 seconds — claimed to be the fastest response in the world — and full load can be achieved in 12 seconds.
Dinorwig requires a constant volume of water within its twin reservoir system to operate at optimum performance. To reduce excess water in the system from natural inflows into Llyn Peris, water from three rivers — the Afon Hwch, Afon Nant Peris and Afon Dudodyn — was diverted into Llyn Padarn. At times of high flow the Afon Nant Peris and Afon Dudodyn overspill into Llyn Peris.
A surge pond (SH601608) is situated midway between Llyn Peris and Marchlyn Mawr. It measures 80m by 40m and is 14m deep with a 30m diameter, 65m deep, surge shaft positioned centrally in the base of the pond.
Great care was taken to minimise impacts on the local environment, which is within a national park and a geological site of special scientific interest. The outgoing 400KV transmission cables are buried underground for the first 9.65km, joining the National Grid at Pentir. The few new buildings above ground are of local stone, much of it reclaimed from derelict quarry buildings. Extensive landscaping included clearing the surroundings of existing waste slate and rock tips, and placing slate around the banks of Llyn Peris to mask the water level mark. The remaining stone and debris arising from the works was tipped into old quarries and into Llyn Peris.
The Dinorwig scheme was fully commissioned in 1983, and formally opened by the Prince of Wales on 9th May 1984. It is now owned by First Hydro Company and has become a popular visitor attraction, with tours of the underground power station arranged from the ‘Electric Mountain’ visitor centre (SH581600) in Llanberis.
Resident engineer: Ithon Jones, James Williamson & Partners of Glasgow
Contractor (underground works): Sir Alfred McAlpine & Son/Charles Brand & Son/Conrad Zschokke joint venture
Dam construction: Gleeson Civil Engineering Ltd
Research: ECPK
"Dinorwig Pumped Storage Scheme" by J.A. Baines et al, ICE Proceedings, London, Vol.74, November 1983
"Water management at Dinorwig pumped-storage power station" by Mark I. Bailes and Owen P. Williams, in Improvements in reservoir construction, operation and maintenance, Thomas Telford Ltd, London, 2006
reference sources   CEH Wales

Dinorwig Pumped Storage Scheme