Cruachan Power Station: The Hollow Mountain

But hidden from view, deep inside one of the mountains, is an amazing secret. Cruachan Power Station is a hydroelectric power station built almost entirely within a mountain called Ben Cruachan.

It generates electricity at times of high demand from the energy of water falling from the Cruachan reservoir into Loch Awe. Let’s find out how it works.

Ben Cruachan, at 1126 metres, is the highest mountain in Argyll and Bute. It is part of the Cruachan Horseshoe, a ring of mountains popular among climbers that surrounds the power station reservoir.

Inside the mountain is an enormous, man-made cavern 36 metres long and 90 metres high – large enough to hold a cathedral. Built more than 50 years ago, it was the first power station of its kind.

At 41 kilometres, Loch Awe is Scotland’s longest freshwater loch. As well as the Cruachan Power Station, the loch has a second, more conventional, hydroelectric power station, which generates power from the damming of the River Awe. 

Building work on Cruachan began in 1959, and the power station was formally open by Queen Elizabeth II in 1965. Thousands of workers – known as “Tunnel Tigers” – carried out the construction, living in temporary camps nearby. 

Over the 6 years, a total of 36 workers died while digging out the tunnels and gigantic cavern housing the power station.

Around 220,000 cubic metres of rock had to be excavated to make room for the power station. Access is via this tunnel, which is 1 kilometre long, 7 metres wide and 4 metres high.

Although snow may lie on the surrounding mountains, the temperature inside the power station is always a steady 18°C because it is insulated deep within Ben Cruachan. In fact, visitors are often surprised to see that tropical plants flourish here.

As a safety precaution, 90,000 litres of water are stored to be used in the unlikely event of a fire. The system is tested twice a week, but so far it has remained unused.

In order for Cruachan to work, a reservoir had to be constructed on top of the mountain from which water could flow down into the loch below. It is this downward flow of water, pulled by gravity, that provides the energy Cruachan uses to generate power. An enormous dam was built to create the reservoir.

A natural valley in the mountains formed the reservoir bowl, but in order to contain the water, a dam 316 metres long was built. To protect the appearance of the environment, the operational equipment is housed inside the dam wall.

Once the dam was built, rainwater filled the reservoir, fed into it via a 19-kilometre network of tunnels. It took 9 months to fill the reservoir. Rainwater still contributes around 10% of the power generated at Cruachan. 

The dam is a 200,000-tonne concrete barrier, which is actually a complex moving structure with gates that open and close – pipe maintenance is essential. When power is needed, the dam releases water down a 305-metre slope to the turbines below.

Water from the reservoir above falls for 305 metres and enters 4 giant turbines. These turbines convert the kinetic energy of the water passing through them into electricity. At night, the same turbines are used to push the water back up to the top of the mountain, refilling the reservoir. 

At night, the turbines are powered by excess nuclear and wind-generated energy to push water back up to the reservoir. This is energy that would otherwise go to waste – it is instead converted to potential energy in the reservoir.

The large cylinders are “pony” motors, not the turbines themselves. The motors start and stop the turbines as required, helping the power station to react quickly when power is needed.

As water rushes down, its kinetic energy turns 2-tonne blades within the power station’s 4 turbines. These blades are connected to electricity generators. Transformers step up the 16 kilovolts produced to the 275 kilovolts needed by the National Grid.

With its mountaintop reservoir filled, Cruachan Power Station stores energy in the form of potential energy. At times of high demand – when popular television programmes end, for example – the water is released to provide additional power to ease the burden on other power stations. 

The power station is used to provide extra power only at these peak times. In fact, overall, it uses more energy to pump water up to the reservoir than is generates by the water falling. 

Around 40 people work at the plant, including electrical control and instrumental engineers, whose jobs involve fault finding and resolving, electrical maintenance and keeping equipment up to date. However, the starting and stopping of the power station is remotely controlled.

The advantage of the Cruachan Power Station’s design is that electricity production can be started quickly – from nothing to full production in 30 seconds. When the reservoir is full, it can generate electricity for 22 hours. 

Cruachan is required to always have a 12-hour emergency water supply, in order to be able to supply power to the National Grid when external power resources have failed – known as a “black start”. 

The only signs down by the loch that the power station exists are the number of electricity pylons in the area. At the top of the mountain, the enormous dam is an impressive sight, but most of this amazing construction remains hidden from view. 

However, Cruachan has won architectural and engineering recognition. Conservation group Docomomo listed it as one of the 60 most significant monuments of post-war Scottish architecture. The visitor centre at Cruachan receives 50,000 visitors each year.

Electricity from power stations is delivered to consumers via the National Grid. Step-down transformers convert the 275 kilovolts supplied by power station like Cruachan into voltages safe to use – household mains electricity is around 230 volts. 

Cruachan’s 4 turbines can generate 440 megawatts of electricity – enough to power a city the size of Edinburgh in a pinch. However, the supply is not consistent – power generated at Cruachan usually serves as a supplement to that generated elsewhere. 

Many hill walkers climb Ben Cruachan to visit the dam, which offers spectacular views. The walk involves steep, rocky climbs and – as is common in the Scottish Highlands – boggy ground. A map, compass and waterproof boots and clothing are essential!