Cruachan Power Station

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, this is one of four pumped storage power schemes in the UK. 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 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. 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 acessed 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.