The Time is Right for Small Pumped Energy Storage, UK Hydropower Developer Says

A plan for a 50-MW capacity pumped storage facility capable of generating up to 500 MWh of energy has been approved by the local council, the developer has announced. The facility will be built in the Glyn Rhonwy cluster of abandoned slate quarries near Llanberis in north Wales.

The plan is significant in that pumped storage, which stores energy in water reservoirs and delivers it on demand, was previously considered only economically viable on a large scale, like the 1800 MW Dinorwig facility in northern Wales, which has been in operation since 1984 and was the last major commercial pumped storage facility built in England and Wales.

The Glyn Rhonwy project’s developer, Quarry Battery Company (QBC), predicts a new generation of pumped storage facilities built on a much smaller scale, arguing that they are economically viable and environmentally more acceptable than another Dinorwig-sized facility. 

QBC’s managing director Dave Holmes said, “Our facility compares well in terms of CAPEX per MWh. The usual scale quoted is £1.5-2.5 million (€1.78-2.96 million)/MW; we estimate that ours will be about £2 million/MW.” Despite the economies of scale associated with large-scale development, Holmes said a small site can be just as cheap to develop as a large one. “The reason,” he said, “is that we’ve been choosy about sites. Often, with a large site, you’re in the armpit of a mountain and you have to build a huge dam. There’s less engineering work in a quarry because there are already holes in the ground.”

Planning and Community Impact

Holmes says QBC looked “all over the place” for the right quarry. If the rock was highly fractured, he explained, there could be leakage issues and the reservoirs would need to be lined with cement; a volcanic or earthquake-prone area would also be problematic. Glyn Rhonwy emerged as the best choice as it featured a typical Welsh slate formation – the same geology as Dinorwig, Holmes said, “without too many geological issues.” QBC will be able to build the majority of the dams from slate excavated from the site, reducing the amount of new material needed.

Also attractive was that the local council “was looking for an innovative way” to dispose of the site, Holmes said. Glyn Rhonwy had been a slate quarry for 200 years when it was acquired by the Ministry of Defence, which used it as a bomb storage facility. “When that [facility] fell down in heavy snow with bombs inside,” Holmes said, “it was turned into a bomb disposal facility. They chucked bombs into the quarries at the back, one of which we’re using.” In the 1960s and ‘70s the facility was cleared out: “They drained the lake, took out every bomb and blew them up with dynamite, then gave the site to the council where it stayed on the books for 30 years with nothing done with it,” he continued.

“At the time we got involved, the new plan was to put a ski dome on the side of the hill and build an international hotel-type resort. None of the locals wanted that,” he said. In response to this plan, the community had entered a rival “dummy” bid designed to showcase the deficiencies of the resort hotel proposal. At that point QBC contacted community representatives and proposed a downhill mountain biking course on the site, “and on the inside we’d do pumped storage,” Holmes said. In 2009 the council agreed to sell the land to QBC if the company received planning permission, which was awarded on 2 September 2013.

The ecological assessment also took some time, Holmes explained. Because multiple tunnels across the site are used by bats, new habitats will need to be built for them when the tunnels are flooded. The quarries are also a breeding ground for the chough (a rare variety of crow), peregrine falcon and kestrel, so noise abatement measures will be necessary during breeding times (although Holmes pointed out that pumped storage facilities are relatively quiet because the turbine is 80 metres underground). And QBC has promised to plant trees and “manage the landscape better than it has been managed,” said Holmes.

Environmental group the Open Spaces Society has said the proposed facility will be an eyesore, and the Snowdonia Society, a charity focused on protecting the Snowdonia national park near the quarry site, has raised environmental concerns about the construction process, but Holmes said the park authorities have agreed to the plan. “In a bigger project where you’d be flooding valleys and building pylons, environmental concerns would be larger,” he said. “Here we’re not damming valleys or flooding land. There’s already slate waste all over, and having another lake isn’t going to change a landscape already dominated by these features.”

QBC has promised to pay £250,000 (€296,000) into a community benefit trust controlled by local representatives, with a £10,000 top-up each year and an annual increase of 2.5 percent in order to keep pace with inflation. The company expects to pay around £500,000 in taxes each year. “This is like having 400 new houses in the community,” Holmes said, “with none of the associated pressure on schools, trash, traffic, roads etc. It will be very good for the community.”

The Technology

Holmes calls the quarry project a “gravity battery” – one that can store 500 MWh of power and, at a generating capacity of 50 MW, will have a 12-hour battery life. It will use 1.1 million tonnes of water, QBC says.

Pumped storage plants use two water reservoirs, one at a higher altitude. The “battery” is charged by pumping water to the upper reservoir for storage; at peak times the water is released, flowing down through a hydro generator turbine. Proposed for Glyn Rhonwy is a reversible, variable speed Francis turbine.

The plant will take off-peak power from the National Grid and use it to pump water up the hill to the second reservoir, where it will be stored until needed. Pumped storage is flexible: it can be turned on and off quickly, so it can absorb power when there’s too much on the grid; when there’s too little, the plant can come online fast, capture the high price and turn off again. “Pump when prices are low and generate when prices are high” is Holmes’ maxim.

But with pumped storage, losses in the pump turbine unit mean that “you don’t get out as much energy as you put in,” says Holmes. For every 4 kWh of power the plant consumes, it will be able to sell 3 kWh back. So to make the plant work economically, it will need to buy power more cheaply than it sells it – about 25 percent cheaper in order to break even, according to hydropower analysts.


Large pumped storage plants typically feature a wide, shallow reservoir, which holds a large volume of water. This means that, as the water empties out, the overall depth of the reservoir does not change significantly during a typical operational cycle and the pressure at the turbine inlet doesn’t change that much. In a plant where the reservoirs are deeper – at Glyn Rhonwy the planned reservoirs will be 50 metres deep – but hold much smaller volumes, one effect of using the stored water is an appreciable impact on the available head as the water level falls, creating what’s called a variable head (defined in metres) as water is pumped from one reservoir to the other.

At Glyn Rhonwy the head will be highly variable according to Holmes – from 270 to 220 metres. This “can pose some operational challenges” in making the water’s output come out predictably at 50 MW. “You might need to choke down [turn the turbine down] at the start and boost up at the end somehow,” Holmes said. “If the head varies the pressure inside the turbine, you’re not running at optimal consideration. You’ll have to slow it down in order to maintain the best operational profile.” QBC is researching technological fixes for sites with very variable heads, but according to hydropower experts no such fixes currently exist.

Holmes believes there is room for a varied mix of energy storage technologies in the UK. Because pumped storage is limited by the availability of suitable locations, even smaller facilities won’t work in urban areas. While he points to chemical batteries as well as flywheel energy storage as more suitable for many densely populated regions, “in terms of proven technology nothing really beats pumped storage,” he says. “It’s fast, reliable, energy-dense and cheap.” But, he says, “all technologies have a place – depending on where the demand is, where the connections are, and how much cheaper those technologies are over time.”

QBC has received a £200,000 (€236,000), one-year R&D grant from the Department of Energy and Climate Change and is looking into different types of pumped storage, such as seawater storage (of which there is currently only one plant in the world, Japan’s Okinawa Yanbaru, but the UK’s Severn Estuary has also been proposed for this purpose) and low-head storage “where you haven’t got a mountain but just a hill – is it still worth doing?”, says Holmes. The company is also investigating the possible use of potable water reservoirs for pumped storage.

The Time is Right for Pumped Storage

According to National Grid’s report Operating the Grid Beyond 2020, as more wind power is connected to the UK’s grid (up to 32 GW according to the report), the amount of uncertainty in demand forecasting is set to almost double, from 3 to 5 percent. With increasing uncertainty, flexible power delivery will be crucial. In addition, electricity market regulator Ofgem is currently seeking industry views on a proposal to mandate charges for its grid balancing mechanism, with the results to be announced this month. Rather than the current method of averaging the costs associated with grid balancing, the highest cost could be charged, affecting energy trading behaviour in the UK and increasing the value of balancing power plants. “This could change the economics for pumped storage in the UK, and could be the start of a renaissance,” Holmes said.

QBC says it is aiming for the £100 million site to be operational by 2017, and while the company was founded to undertake the Glyn Rhonwy project, “we don’t plan to be just a one-hit wonder,” says Holmes. “We’re looking at number of sites, which tend to be brownfield – some bigger and some smaller, some are in Wales and some aren’t. We think the market is big enough for some more.”

For a live discussion of hydropower as an energy storage asset as well as new developments within the North American hydropower market, register for our upcoming webcast, The HydroPower Opportunity, on Wednesday, September 25

Lead image: The Glyn Rhonwy site, courtesy of Quarry Battery Company

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Tildy Bayar is a journalist focusing on the energy sector. She is a former Associate Editor on and Renewable Energy World magazine.

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