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Cruachan Power Station

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Tropical Plants
Underground tropical plants

Cruachan Power Station is a pumped-storage hydro-electric power station which has a visitor centre at the side of Loch Awe, located on the A85 about 19 miles east of Oban. Opened by the Queen in October 1965, it is one of four pumped storage power schemes in the UK, and has the distinction of being the world's first high head reversible pumped-storage hydro scheme. The station has also been used as the setting for several films, including the James Bond film The World is not Enough.

Nicknamed The Hollow Mountain, Cruachan was constructed between 1959 and 1964 and is unique in that the station is concealed within the hollowed out rock of Ben Cruachan, 3,694 feet (1,126 m), which overlooks the visitor centre. It was conceived and designed by Sir Edward MacColl, a Scottish engineer and a pioneer of hydro electricity in Scotland, with numerous projects to his credit. The only visible features are the dam, which is 1,000 feet (316 m) wide and located 1,300 feet (390 m) up the mountain, and the station offices and visitor centre, which lie on the loch side below the dam, and next to the road.

Buried in the mountain and accessed by a tunnel 23 feet (7 m) wide and 13 feet (4 m) high, the main cavern houses four motor-generator sets capable of generating a total output of 440 MW. Construction of the main cavern, similar in size to a football pitch, required the excavation of 220,000 cubic metres of rock and soil.

The tour is unique, beginning with a trip in an electric coach from the visitor centre into the heart of the mountain over half a mile (1 km) underground. The natural heat at this depth, supplemented by artificial lighting, is sufficient to allow tropical plants to be cultivated.

Pumped storage

Although Cruachan can operate like a conventional hydro-electric station, using run-off water collected by a network of tunnels driven through the hills above Ben Cruachan and draining into the reservoir, the normal operating mode is pumped storage. In this mode, the generators are used as motors to drive the reversible turbines to pump water uphill from Loch Awe to the reservoir, using the relatively cheap electricity available during periods of low energy demand, usually overnight. This ensures the station as always ready to meet peak demand during the following day.

The station can operate for 22 hours before the supply of water in the reservoir is exhausted, and is required to maintain a 12 hour emergency supply in reserve. Figures given for the Foyers scheme show that the system can raise 167 tonnes of water (167 cubic metres) per second when pumping at full power, with the flow rate increasing to 200 tonnes per second when generating at full power.

Cruachan can go from standby to full power within two minutes, or just 30 seconds if the turbines are being operated as spinning reserve in anticipation of being called on to supply power during a period of peak demand on the grid. In this mode, compressed air is is used to spin the turbines in readiness for use, so they do not have to start from rest when called for.

UK pumped storage schemes

  • Ben Cruachan, Scotland (1965), 440 MW
  • Foyers, Scotland (1975), 300 MW (2 x 150 MW units)
  • Dinorwig, Wales (1984), 1728 MW (6 x 288 MW units)
  • Ffestiniog, Wales (1963), 360 MW (4 x 90 MW units)

Engineering Heritage Award 2012

In November 2012, the station received the Institution of Mechanical Engineers' Engineering Heritage Award.[1] Previous recipients of the honour include the Titan Crane at Clydebank, the paddle steamer Waverley, and the Falkirk Wheel.

2014 study could see output double

In February 2014, Scottish Power announced the start of a two-year feasibility study into plans to more than double the output of Cruachan, which followed news that rival SSE had already announced plans for the Coire Glas hydro electricity scheme in Lochaber.[2]

The expansion project could see Cruachan's output rise to 1,040 MW (1.04 GW), cost hundred of millions and take years to complete.

2015 Book commemorates the project and workers

In 2015, a book about the project, the 3,000 workers who carried out the construction and the 36 who died, was published on the 50th anniversary of its completion. [3]

50th anniversary of Cruachan

Widely referred to as The Hollow Mountain, Cruachan was devised by Sir Edward MacColl and was built by a workforce of 4000. Over 220,000 cubic metres of rock was drilled by explosives experts to create a cavern 1km inside the mountain to house the turbines and a network of pipes and tunnels.

The power station powers more than 200,000 homes at full capacity, and is used to support the National Grid during peak hours.

Hugh Finlay, director at ScottishPower, said; "It was a prodigiously complex feat of engineering to design a power station buried deep inside a mountain, and it was a herculean task to construct it.

"The genius of Sir Edward MacColl’s initial design, coupled with the skill and determination over six years from the 4,000 strong workforce, ensured that Cruachan Power Station was built to last. It is as important today as it was 50 years ago.

"The station has also been a major presence in the local community. Generations of families have worked there over the years, and Cruachan has only been a success because of their efforts.

A plaque on the banks of Loch Awe pays tribute to 36 people who died working during the construction of Cruachan.

- Cruachan power station built inside mountain marks 50th anniversary.[4]

2016 plans could see output double or more

In February 2016, ScottishPower said a feasibility study has shown that Cruachan could be expanded, increasing its current 440 MW capacity by between 400 MW and 600 MW, together with the creation around 800 construction jobs.

However, the company also stated that the £400 million project would only be possible if ScottishPower could secure a guaranteed floor price for its use from the Government in return for a cap on profits.[5]

The move towards renewable energy sources means that pumped storage hydro has become one of the most cost-effective solution to large scale electricity storage - which recalls its original reason, to store excess generation from nearly nuclear power station as these could not be started and stopped quickly, so needed storage to absorb their output when demand fell.


Cruachan dam, 2005
Cruachan dam
© J M Briscoe
Cruachan dam, 2004
Cruachan dam
© Richard Webb
Cruachan reservoir, 2005
Cruachan reservoir
© James Hearton
12.007 Tunnel entrance, water outlet, NN 0827 2948, 1989
12.07 Tunnel entrance, water outlet
© Chris Heaton

Aerial views of Cruachan


1 Engineering award for Cruachan power station's 'hidden' hydro scheme Retrieved February 05, 2013.

2 Scottish government approves £800m Lochaber hydro scheme Retrieved 25/02/2014.

3 Story of Ben Cruachan ‘tunnel tigers’ to be told - The Scotsman Retrieved July 20, 2015.

4 Cruachan power station built inside mountain marks 50th anniversary | Highlands & Islands | News Retrieved October 19, 2015.

5 Plans to double size of power station built inside mountain | Glasgow & West | News Retrieved March 02, 2016.

External links

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Aerial views



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