Water world: The picturesque setting
Snowdonia’s steep-sided valleys and wet climate have long been exploited for hydro-electricity. The biggest hydro plant, however, is almost invisible to the millions who visit the beautiful national park, because it was built underground to minimise visual intrusion.
Dinorwig power station sits deep inside a mountain near Llanberis. Its water comes from an upland lake in a cirque – a circular hollow left by a glacier. The lake would be far too small to sustain a 1,728MW power station using traditional technology, but Dinorwig needs only enough water for a day’s operation. Each night, when electricity demand is low and tariffs are cheap, the generators are reversed to pump water back up from the lower reservoir, Llyn Peris.
The pumped-storage station cost £450 million and took less than 10 years to build, being officially opened in 1984.
The station’s owner, First Hydro, runs popular tours for the public to view the underground infrastructure. Booking ahead is essential. Tours start and finish at the Electric Mountain visitor centre (free admission) in Llanberis.
It’s worth arriving early at the centre to look at a clinker-built boat on display there. The vessel, thought to date from the 16th century, was discovered when
Llyn Peris was drained in the 1970s for adaptations for use as a reservoir. And upstairs, where the tour participants gather, there’s a fine model by Bassett-Lowke of one of Dinorwig’s six turbine and generator assemblies.
A bus takes the tour group from the visitor centre on to the foot of the former Dinorwig quarry, once the world’s second-largest slate quarry. From there, the tour bus descends along a tunnel to the chamber housing the main inlet valves.
This chamber has the proportions of a cathedral nave. In place of angels, up above are maintenance cranes, which can run along hefty concrete beams. The eye is immediately drawn to a large yellow counterweight lever, actuated by an hydraulic cylinder below.
There are two such levers for each of the six valves. When they’re raised, the valve is open and you can hear water rushing through from left to right. The water has descended 600m – much of the fall being vertical – by this point.
The valves must withstand considerable water pressure when closed. Even so, they can close off the flow (418,000 litres per second) in 20 seconds. It takes only six seconds to open a valve and begin generating power.
Next on the tour are the turbines, in another cavern. If you’re lucky, the nearest turbine will be at work and the vertical shaft connecting it to the generator above spinning at 500 revolutions per minute. After leaving through the valve, the water flows into a spiral of decreasing diameter, which further increases pressure as the water reaches the turbine. The flow at that point is regulated with guide vanes, all connected to an outer ring that ensures they act in unison.
The final stop is the top level of the same chamber. Blue shaft caps denote the locations of the six generators below, but here the civil engineering steals the limelight. Stretching 179m before you is Europe’s largest man-made cavern, which is up to 51m in height.
The mountain’s rock is mostly slate, which fractures along its ‘grain’, so excavating Dinorwig’s caverns and 16km of tunnels required special techniques – the walls had to be nailed and coated with cement and pulverised ash.
Rhodri Clark
» For more details, see: electricmountain.co.uk
The main inlet valve chamber
5 things to see
1. Main inlet valves: Huge machinery allows each generator to produce electricity within six seconds of starting up.
2. Turbines: You might want to cover your ears if the nearest of the six is rotating.
3. Main chamber: It’s the biggest man-made cavern in Europe.
4. Tudor boat: Probably carried livestock, people and cargo across the lake.
5. The view: Take the short walk to Dolbadarn Castle to appreciate how little the power station intrudes into the splendid landscape.