Dinorwig: A unique power plant in the north of Wales

The UK’s fastest source of electricity is hidden away in the mountains of north Wales, poised to spring into action in the event of energy shortages. Dinorwig is a pumped hydroelectric power plant, and a forbearer of modern hydropower. But how does it work?


Dinorwig, in the picturesque Snowdonia National Park in north Wales, provides rapid-response power for the UK. Most famously for occasions when a large number of the British public simultaneously make a cup of tea during the ad break of a popular TV programme, putting an unusually high strain on the grid. For this, the UK can rely on one of the fastest power supplies in the world.

A predecessor of modern hydropower, Dinorwig was built in an old slate quarry in Elidir Mountain in the 1970s. The pumped hydroplant is able to create energy by opening its six valves, the ball mechanisms for which are the largest ever built, allowing the water from its top reservoir to flow through turbines to its bottom.

It’s been over thirty years since Dinorwig opened, yet it continues to play a crucial role in our energy network.

Wales’s old slate works

Prior to becoming a hydropower plant, Dinorwig was a slate quarry. The 18th century saw its boom in production, with the mine supporting more than 3,000 jobs as slate roofs became popular. At the mine’s peak, it was the second-biggest producer of slate in the world. However, as trade slowed Dinorwig was abandoned, leaving a cavern on the eastern side of beautiful Snowdonia.

During the 1970s, as the electricity makeup of the UK began to change, a rapid-response source of energy became a necessity. First Hydro’s Ffestiniog pumped storage plant had been built in the 1960s and was proving successful, but something bigger was necessary. Dinorwig’s naturally elevated position, and the excavation which had already taken place, made Elidir a natural choice.

The construction of Dinorwig hydropower plant was an extensive project, with 2,000 locals employed in its build. “The power station is comprised of 16km of underground tunnels below Elidir Mountain,” says First Hydro station manager John Armstrong. “Its construction took ten years to complete, and required one million tonnes of concrete, 200,000t of cement and 4,500t of steel.”

Dinorwig was finally opened by Prince Charles in 1984. “When it was built, Dinorwig Power Station was regarded as one of the world's most imaginative engineering and environmental projects,” says Armstrong. Dinorwig’s design was particularly lauded, as the plant is almost entirely concealed within the mountain, making it invisible from outside. 

Fast response times of energy supply

Pumped storage works in a similar fashion to conventional hydro, with water turning the turbines to generate electricity. The plant uses the different altitudes of its two reservoirs created by the mountain to move the water. “When the water is released from the upper reservoir energy is created by the downflow, which is directed through high-pressure shafts linked to turbines,” explains Armstrong. By the time the valves are halfway open, 65m3 of water per second will already be coursing through the system. This is the equivalent of one and half million cups of tea a second. The force of this water turns the turbines at 500rpm.

"The six generating units can achieve maximum output of 1,728MW, from zero, within 16 seconds."

“The six generating units can achieve maximum output of 1,728MW, from zero, within 16 seconds,” says Armstrong. The units are fixed vertically within the mountain, where the colossal generators – weighing at 445t each – can maintain maximum output for up to five hours. These units are situated within the main cavern, the largest man-made cavern in Europe, which is big enough to contain St Paul’s Cathedral.

Turning on the power generation system is streamlined by the ‘spinning-in-air’ function. This allows the plant to be placed on standby; a small amount of energy is then used to pump compressed air into the turbine casing. This ensures that when the inlet valves are opened, the water flow can take over from the air in just five seconds. “There are people working on-site and in our trading team in London 24/7, 365 days a year,” says Armstrong. “We work with National Grid to ensure that, where possible, our supply is planned in advance of anticipated peaks in demand.”

The same system of turbines and generators is later used to move the water back from Llyn Peris, the lower reservoir, to Marchlyn Mawr, the upper. This occurs when there is surplus energy in the grid, usually at night, using the generators as motors to reverse the plant’s turbines so they function as pumps.

Hydropower still has a lot to give

Dinorwig remains an essential part of the UK’s electricity pool due to its impressive response times. Nuclear can take days and coal power plants take hours to reach the necessary temperatures for energy generation, which is too slow to address unexpected or rapid power shortages. “Pump storage generation offers a critical back-up facility during periods of unexpected peak demand or sudden shortfalls in supply on the National Grid system,” explains Armstrong.

Electricity demand can peak for a number of reasons, the most notorious of which is halftime at major sporting events. In the 2006 World Cup England versus Paraguay match, electricity usage increased by 1,500MW – the equivalent of 600,000 kettles being switched on at once – as thousands of people rushed to use the toilet and fetch a drink during halftime. But, “it is also an important asset for security of supply at times of increased supply volatility, such as unplanned outages or intermittent generation from renewables”, says Armstrong.

"In the UK there is currently 1,676MW of installed hydropower capacity, generating over 5,885GWh/year."

Dinorwig was one of the first and most ambitious pumped storage plants, which pushed our understanding of hydropower’s benefits. In the UK there is currently 1,676MW of installed hydropower capacity, generating over 5,885GWh/year. On top of this pumped storage adds an additional 2,800MW capacity to the grid. The success of Dinorwig, and its smaller sister plant, Ffestiniog (the first pumped hydroplant in the UK), were essential for the technology’s continued support and development.

Armstrong believes that, although technology has improved, there is still a need for pumped hydro, and there will be for many years to come. “Technology around other power storage capabilities, such as battery storage, is evolving over time but the pumped storage capabilities of Dinorwig are still at a scale and capacity to be of strategic importance to the UK energy market,” he says. “Dinorwig remains one of the largest and fastest-acting pumped storage plants in Europe.”