Dinorwig Power Station

Purpose

The original purpose of the scheme was to deal with the difficulty that the National Grid would have had if the large numbers of nuclear power stations then planned had been built. Nuclear power stations must be run at close to full output all of the time so storage capacity was needed for excess power generated at times of low demand. Electric night storage heaters and the Economy 7 tariff fulfill a similar purpose.

The stall in the UK nuclear power programme after privatisation and the coincident "dash for gas" increased the network's ability to respond to changes in demand, making the use of pumped storage for day/night load balancing less attractive. As a result, a similar facility planned for Exmoor was never built. With the increase of renewables such as wind and solar power, the need for storage is expected to increase.

Today, Dinorwig is operated not to help meet peak loads but as a STOR (Short Term Operating Reserve), providing a fast response to short-term rapid changes in power demand or sudden loss of power stations. In a common scenario (known as TV pickup), the end of a popular national television programme or advertising breaks in commercial television programmes send millions of consumers to switch on electric kettles in the space of a few minutes, leading to overall demand increases of up to 2,800 megawatts (3,800,000 hp). In anticipation of this surge, an appropriate number of units at Dinorwig (or other services competing for National Grid Reserve Service duty) may be brought on line as the closing credits start to roll. The monitoring of popular television channels is an important factor in electricity grid control centres.

Construction

The scheme was constructed in the abandoned Dinorwic slate quarry. To preserve the natural beauty of Snowdonia National Park, the power station itself is located deep inside the mountain Elidir Fawr, inside tunnels and caverns. The project – begun in 1974 and costing £425 million and taking ten years to complete – was the largest civil engineering contract ever awarded by the UK government at the time. The work was undertaken by an Alfred McAlpine / Brand / Zschokke consortium. 12,000,000 tonnes (12,000,000 long tons; 13,000,000 short tons) of rock had to be moved from inside the mountain, creating tunnels wide enough for two lorries to pass comfortably, and an enormous cavern 51 metres (167 ft) tall, 180 metres (590 ft) long, and 23 metres (75 ft) wide known as "the concert hall". This has also given rise to the station's alternative name of Electric Mountain. The power station comprises 16 kilometres (9.9 mi) of tunnels, 1 million tons of concrete, 200,000 tons of cement and 4,500 tons of steel.

The power station is connected to the National Grid substation at Pentir by 400 kV cables that are buried for approximately 10 kilometres (6 mi), rather than using transmission towers or pylons to transmit the electricity across an area of outstanding natural beauty.

A 50MW pumped storage facility at nearby Glyn Rhonwy in disused slate pits was approved in 2013, with an expected cost of £120m and an opening date in 2018. It is planned to hold about 500 MWh of energy, with a round-trip efficiency of 75%.

Operation

Water is stored at a high altitude in Marchlyn Mawr reservoir and is discharged into Llyn Peris through the turbines during times of peak electricity demand. It is pumped back from Llyn Peris to Marchlyn Mawr during off-peak times. Although it uses more electricity to pump the water up than it generates on the way down, pumping is generally done at periods of low demand, when the energy is cheaper to consume.

The power station comprises six 300MW GEC generator/motors coupled to Francis-type reversible turbines. The generators are vertical shaft, salient pole, air cooled units each having 12 electromagnetic poles weighing 10 tonnes each, producing a terminal voltage of 18 kV, synchronous speed is 500 rpm. From standstill, a single 450-tonne generator can synchronise and achieve full load in approximately 75 seconds. With all six units synchronised and spinning-in-air (Water is dispelled by compressed air and the unit draws a small amount of power to spin the shaft at full speed), 0 MW to 1800 MW load can be achieved in approximately 16 seconds. Once running, the station can provide power for up to 6 hours before running out of water.

Another important feature of Dinorwig is that it has been designed to assist restarting the National Grid on the occasion of a complete power failure (a black start). It includes diesel generators and large batteries which would allow the plant to restart even in the event of a complete shutdown of the grid (Dinorwig is not unique in this respect as some fossil-fuel plants are also able to self-start).

The plant runs on average at 74–76% efficiency. It fills an important need in the system by responding to sudden surges in electricity demand because of its rapid ability to deliver power on load spikes. One of the alternatives would be to have spare capacity from conventional power stations running part loaded ("spinning reserve"), hence at lower efficiency than otherwise, and thus capable of being rapidly run up to full load. This would mean generating an additional 1330–1590 GWh of power each year and releasing more than 140,000 tonnes of CO₂ into the atmosphere each year.. Other forms of power plant compete in this market for reserve power such as gas turbines and diesel generators for the National Grid Reserve Service.

Excess water overflows to Llyn Padarn and is lost from the Reservoir system. Both Llyn Peris and Llyn Padarn were ancestral homes to the Arctic char, a rare fish in the UK. When the scheme was commissioned, a fish rescue was undertaken to transfer the char from Llyn Peris to other local suitable lakes and it is believed that due to the very variable water levels in Llyn Peris, Arctic char are now absent from the lake.

The power station is also promoted as a tourist attraction, with visitors able to take a minibus trip to see the workings inside the so-called "Electric Mountain"; 132,000 people visited the attraction in 2015.