Future of Energy

Is there a proven way to store surplus energy? NCE reports on the potential of pumped storage hydroelectricity in the UK and looks into the proposed Coire Glas scheme.

As the world slowly moves away from fossil fuelled electricity generation and towards renewable alternatives, a major roadblock still has to be surmounted: intermittency.

Solar panels need daylight and wind turbines require a stiff breeze. Without both the national grid could be in trouble on a calm, dark, subzero winter's evening. Other energy sources are needed to stop the lights going out and pumped storage hydroelectricity is one.

In the UK there are only four pumped storage facilities, with capacities ranging from 300MW to 1.8GW. The last one to be built was the 1.8GW Dinorwig Power Station in Wales which opened in 1984.

Nearly four decades later, plans for the 1.5GW Coire Glas pumped storage station in the Scottish Highlands are progressing. In March, energy firm SSE Renewables said it was investing £100M in the next phase of detailed project design and refinement. It says Coire Glas’ construction would more than double UK's total current electricity storage capacity.

Pumped storage schemes have a trouble-free track record dating back the best part of a century.

Their operating principle is simple; when there is a surplus of electricity available, turbines use it to pump water from a lower lake or reservoir up to a higher reservoir.

Should there be a shortage, the turbines are reversed and water flows down through them generating electricity within minutes – sometimes even in seconds.

The need for long duration storage is becoming more pressing to ensure further penetration of renewables

There are around 80 pumped storage stations with a capacity of more than 1GW in the world and another 80 or so are under construction or nearing completion.

Capital costs are high and carbon footprint during construction is significant, but these factors are usually judged to be acceptable given the low running costs and decades-long service life that can be expected. The real problem is the shortage of suitable sites.

A height difference of at least 500m between upper and lower reservoirs is essential if the economic energy recovery efficiency – the difference between energy required to pump the water uphill and the energy recovered when it runs downhill through the turbines – of 80% or more is to be achieved.

This effectively restricts the location of pumped storage schemes to mountainous terrain – and explains why the UK's existing schemes are in Wales and Scotland.

The proposed 1.5GW Coire Glas facility will be on the shore of Loch Lochy between Inverness and Fort William. It will generate enough electricity to supply 3M homes for 24 hours within five minutes of coming onstream.

Construction is expected to require a capital investment of over £1.5bn. The project received planning consent from the Scottish Government in 2020, with construction planning and procurement progressing through 2023 and into early 2024.

SSE Renewables hopes to make a final investment decision on the project in 2024 and commission the pumped storage scheme by 2031. It has appointed Stantec as the overall reference designer, while Cowi is responsible for the underground structures’ reference design.

Around half of the recent £100M investment will be allocated to the pre-construction refinement phase of the Coire Glas project. This includes a comprehensive package of ground investigation work, which has already got underway.

Specialist contractor Strabag has begun to drill and blast a 1km trial adit into the mountainside while Fugro drills down from above for the ground investigation.

The horseshoe-shaped 5m by 5m adit will be rock bolted and shotcreted as it will pass through an area of damaged metamorphic rock known as the Great Glen Fault Zone with up to 700m of rock cover. The thickness of the zone, how damaged and how stable it is at this location is crucial information that will heavily influence SSE Renewable's final investment decision.

If it goes ahead, numerous tunnels and underground structures will be constructed.

Fugro began ground investigations earlier this year to assess potential ground risks

Cowi UK technical director Chris Jack says that there are lessons to be learned from the Glendoe hydroelectric project as the geology of the two sites is similar. There, in August 2009, fallen rock material caused a collapse in one of the tunnels linking the reservoir to the turbines, resulting in three years of construction delays.

Jack adds: "Key underground areas we are particularly interested in are where the tunnels pass through the Great Glen Fault Zone in general, the power station complex generally and the pressure shaft.

"The thickness of the Great Glen Fault Zone is a significant uncertainty driving the need for the exploratory adit. The current best estimate, and it is very much an estimate, is approximately 500m, but this is subject to the findings of the adit."

Loch Lochy will act as the lower reservoir, from where four 324MW turbines are planned to pump water more than 500m up to a natural coire – a gaelic term meaning cauldron. This will be transformed into the upper reservoir by the construction of a 92m high concrete faced rock fill dam.

This will be the UK's tallest dam, topping the Llyn Briane rock fill dam built in the 1970s by just 1m. When full, this upper reservoir would store approximately 25.9Mm3 of water. There is no natural inflow of water into the coire, apart from the abundant precipitation in the region. The rock fill would be recycled spoil from the approximately 18km of tunnels up to 10m in diameter and other underground structures. The turbines themselves will sit deep underground.

Coire Glas’ relatively remote location and vertiginous terrain pose some serious logistical challenges, but it does have one unique advantage. Loch Lochy is part of the 101 year old Caledonian Canal, which can still accommodate vessels up to 45m long with drafts up to 4m. This could be a very convenient transport route for some of the larger power station components, such as the turbines.

"The need for long duration storage is becoming more pressing to ensure further penetration of renewables on to the system and to deliver net zero," says SSE lead design engineer Matthew Macleod.

"Pumped storage hydro is the most cost effective and well-established long duration storage technology available.

"Future developments will only become more relevant as we look to reliable, efficient solutions to decarbonise our electricity system."

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