Designing Dinorwig: celebrating our work at Electric Mountain

Forty years since the opening of the Dinorwig pumped storage hydropower station in north Wales it has found new purpose, supporting grid decarbonising by storing renewable energy.

The story of the 1800MW Dinorwig pumped storage hydropower station shows that high quality design stands the test of time. This vital piece of national infrastructure, located in the mountains of north Wales, UK, is just as important now as it was 40 years ago when it was first connected into the UK grid.

Nicknamed ‘Electric Mountain’, Dinorwig was designed and construction was supervised by James Williamson & Partners, one of the companies that joined to create Mott MacDonald as it is today. Client was the Central Electricity Generating Board (CEGB).

In our latest heritage podcast we tracked down several of the people involved in creating Electric Mountain.

In 1974 Electric Mountain was the largest civil engineering contract ever awarded by the UK government. It became the biggest construction project in Europe. The hydraulic tunnels of around 10m in diameter that convey water from the upper reservoir to the lower reservoir and back again, were the largest that civil engineers had ever designed. The generating capacity of the six 300MW reversible turbines was over four times greater than any previous pumped storage project in the UK. Their speed of generation, bringing electricity onto the grid within 10seconds, was the fastest ramping power station in the world. The 180m long, 24m wide and 60m tall machine hall was the largest cavern ever excavated in slate and it was just one of nine caverns that required removal of 3Mt of rock.

Its scale and complexity pushed the limits of design and construction with engineers developing state of the art practices for rock reinforcement and concrete design, and pioneering new construction methods. The civil infrastructure was recently inspected under its maximum load case when the tunnels were dewatered for the planned replacement of two inlet valves. They were found to be in fantastic condition.

Originally designed to absorb surplus electricity generated by the nearby Wylfa nuclear power station, Dinorwig has more recently found new purpose as a ‘battery’ capable of smoothing variable output from renewable sources: It stores electricity when an excess is generated and releases it when wind and solar output falls, and demand rises. Over the next few years, the mechanical and electrical equipment is being replaced as it reaches the end of its design life ensuring that the Dinorwig can contribute to the UK grid long into the future.

Hear more about our work on this amazing project from members of the former James Williamson & Partners team including supervising civil engineer Kin Almond, senior engineer James Arthur, senior engineer and associate John Cowie, assistant engineer Dyfrig Roberts, civil engineering technician Robin Thomas (who still works at the site today), project surveyor Ian Waite and former CEGB commissioning engineer Chris Roberts.