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Empire State-sized underground energy storage project is 'ten times bigger than nearest competitor'.

World’s largest energy storage facility will ‘work like a hybrid car,’ while holding as much power as 1.3 million electric vehicles

An energy storage facility bigger than the Empire State Building is being built under a Finnish city to save summer sun for winter – and there could be a “global opportunity” for systems like it in the future, claimed the chief of the company behind the plan.

When it is delivered – scheduled as early as 2028 at a cost of €200m ($217m) – the Varanto project is planned to hold up to 90GWh of power, a scale that appears to be unprecedented for energy storage facilities.

“As far as I know, it’s about ten times bigger than the next one,” Jukka Toivonen, CEO of Vantaan Energia, the Finnish energy company behind Varanto, told Recharge as the project prepares to begin construction work this summer.

A lack of definitive lists of energy storage facilities makes it hard to corroborate such claims, but NASA’s Earth Observatory has said the largest battery energy storage system is 3.3GWh. And Guinness World Records says the largest thermal energy storage plant is 5.9GW.

Pumped hydro facilities are typically in a league of their own when it comes to capacity, and often not directly compared to other types of energy storage facilities. But even including these, a monster 40GWh facility in China named the world’s largest is dwarfed by Varanto.

Varanto is therefore “unique,” said Toivonen, both for its size and capacity.

Varanto will be built in three caverns together measuring 1.1 million cubic metres – enough to comfortably fit the Empire State Building (1 million cubic metres) and squeeze 40 Olympic-sized swimming pools around the edges.

These caverns, 100 metres underground, will be filled with hot water. Pressure will be created within the space, allowing the water to reach temperatures of up to 140 degrees without boiling or evaporating.

At 90GWh, it can pack in as much power as a typical 1GW nuclear plant generates in nearly four days. Or around 1.3 million electric car batteries.

‘How can we save summer heat for winter?’

Such a facility might seem befitting of powering a huge global metropolis, like London or Tokyo. But it is in fact being built under Vantaa – an unassuming city of around 240,000 people that borders Finland’s capital Helsinki.

Varanto will link up to Vantaa’s 600km underground district heating network – which pumps hot water into heat exchangers in most of the city’s buildings – to keep homes and offices warm in the winter.

The city already has smaller-scale energy storage facilities, said Toivonen, that provide a “buffer” between swings in wind and solar power generation on an “hour-by-hour” level.

“But when we come to seasonal stories then there’s a big difference,” said Toivonen.

“Because heat in summertime is really cheap,” whereas, when energy demand peaks in the winter, “the cost is really high.”

“So the challenge is really, 'how can we transfer that summertime heat to winter time?'” His answer, naturally, is Varanto.

The vast thermal battery will slowly charge using waste heat from the district heating network during those warm summer months. It will also have two 60MW electric boilers that can heat water using excess renewable energy when it is cheap to do so. All coordinated by artificial intelligence.

“It’s like a hybrid car,” said Jukka, that is powered both by its internal combustion engine and by a battery. “This is the core of the whole system.”

In July, Toivonen said that Vantaa needs around 70MW of power to heat homes at peak times. In January or February, it’s 700MW.

So Varanto, which can pump out 200MW of power at any one moment, “has a really big impact on those cold winter days and nights.”

Thermal storage much cheaper than batteries

Running from capacity on full throttle, Varanto could pump out power for almost 19 days straight.

This will also be a boon for energy security, as Finland continues to adjust from its hard exit from Russian gas following the invasion of Ukraine.

In reality, running on full power flat out for three weeks is unlikely, and it is claimed Varanto could “heat a medium-sized Finnish city or town for as long as a year.”

Looking ahead, Toivonen said there could be a “global opportunity” for energy storage systems like this.

Using batteries for energy storage is still, relatively speaking, “really expensive,” he said. Building Varanto at the same capacity using batteries would cost more than €1bn ($1.08bn) – “a huge difference.”

“We shall see how it proceeds,” whether it sticks to budget and how it will “work in practice,” said Toivonen. If in 2028 they can say that “this was a great success,” he said they can look at creating “other kinds of success” elsewhere.


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