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On 30 November, the IMechE will present an Engineering Heritage Award to Cruachan Power Station, the world’s first high-head reversible pumped-storage power station. Built between 1959 and 1965, Cruachan is one of only four pumped hydroelectric facilities in Britain.
Pumped hydroelectric plants comprise two bodies of water with a large height difference. When demand for electricity is low, cheap power is used to pump water from the lower to the higher reservoir. This allows a large body of water to be held, ready to generate electricity at very short notice by being released through turbines and back into the lower reservoir.
Pumped storage was first used in Italy and Switzerland in the 1890s, but it was not until the 1930s that reversible hydroelectric turbines were developed. This meant that the same plant could be used for both pumping and electricity storage.
In the mid-20th century, the UK became increasingly reliant on electricity as the national grid grew in scope and complexity. As a result, there was more demand for back-up systems able to respond quickly to either peaks in demand or the failure of a baseload plant.
Large, pumped hydroelectric schemes such as Cruachan were an obvious solution. To this day, pumped storage remains the largest-capacity means of energy storage available to the grid, and accounts for 99% of bulk energy storage capacity worldwide. In order to store as much energy as possible, pumped hydro relies on having two large bodies of water with a large height difference, as at Cruachan.
At 1,126m tall, Ben Cruachan is the highest mountain in Argyll and Bute. At the foot of the mountain is Loch Awe, Scotland’s third-largest freshwater loch. To take advantage of this height difference, a reservoir was formed on the slopes of Ben Cruachan, 396m above Loch Awe, through the construction of a dam 316m long and 46m high.
To divert water from streams around the mountain and maximise the collection of rainfall from all sides of Ben Cruachan, 19km of tunnels were also constructed.
The catchment area for the reservoir was increased from 8km² to 23km², and 10% of the electricity generated comes from natural rainfall, supplementing the water pumped up from Loch Awe.
The reservoir is large enough to allow the plant to operate for around 20 hours before the supply of water is exhausted.
Deep within the mountain, almost 400m below the upper reservoir, 229,000m³ of rock and spoil were excavated to form a huge cavern 90m long and 36m high. This houses the four dual-purpose turbine/pumps which are capable of either pumping 120 tonnes of water per second or generating up to 440MW, enough electricity to power a city the size of Edinburgh. They are capable of going from “spinning reverse”, where they are turning in air awaiting the rush of water, to generating full load within 30 seconds. From rest, they can reach full load within two minutes.
Water from the upper reservoir is released through concrete-lined shafts known as penstocks, which pass into steel pipes terminating at the main inlet valves. These valves are opened when electricity is required and water rushes into the turbines. After passing through the turbines and into a surge chamber, the water is released into Loch Awe along a tailrace 7m in diameter and 975m long.
If you would like to see Cruachan Power Station, you are welcome to come to the Engineering Heritage Award ceremony on 30 November. For information, please email: r_campbell@imeche.org.