The plan to make a giant hot water bottle underground

During the Cold War the vast caverns beneath the Swedish city of Västerås held a stockpile of oil totalling 300,000 cubic metres.

The oil was there in case World War Three broke out and Sweden found itself cut off from international energy supplies.

In 1985, as geopolitical tensions began to ease, the caverns were emptied and have remained vacant - until now.

Swedish energy company Mälarenergi has embarked on a project to decontaminate the facility and fill it with hot water at temperatures of up to 95C. In essence, they are building a giant underground thermos, which the firm says will be the largest of its kind in Europe.

"It's quite damp," says Lisa Granström, acting head of business unit heat and power, describing her last visit to the tunnels, which are in an undisclosed location. "[The caverns are] a lot warmer than you would expect. It smells a little bit oily, still."

The storage available is roughly equivalent to 120 Olympic-sized swimming pools and 11 times bigger than the largest above-ground hot water tank that Mälarenergi has nearby, adds Mrs Granström.

This type of thermal storage is just one of several ways of caching warmth in the ground for use later. With the rise of renewables and concerns about energy security in Europe following Russia's invasion of Ukraine, some experts argue that we should be making more of below-ground heat storage systems.

In the case of Västerås, warmth from the caverns will be sent via heat exchangers to a district heating network, which supplies 98% of the households in the city of 130,000 people.

Mälarenergi intends to begin filling the caverns with water by the end of the year. The facility will offer 500MW of district heating power.

Where does the heat come from, though? Burning things. The company has a nearby power plant with furnaces for combusting waste or biomass and turning it into electricity or thermal energy. Mrs Granström says that carbon capture technology, which would reduce harmful emissions from the plant, is not yet in place but that her firm is currently considering installing it.

The hot water reservoir will allow Mälarenergi to continue heating homes on cold winter days when demand is high, without reducing electricity production at the power plant.

Storing heat underground tends to work well because it is very hard for the heat to escape - the ground itself acts as one big insulator. Mrs Granström explains that Mälarenergi's caverns will retain heat for multiple weeks and the system ought to be particularly stable once a few years pass and the temperature of the adjacent ground rises.

"Once it's heated, the loss is not that great," she says. "You've heated up the rocks around it."

This may chime with Londoners weary of sweaty journeys to and from work on the tube. For decades, heat from people and trains has been warming the clay that surrounds tunnels on the London Underground. So much so, that this clay now has an ambient temperature of between 20C and 25C, making it very difficult to cool down tube carriages and platforms on the network.

The project in Västerås is not the first of its kind. In Finland, energy firm Helen began filling a slightly smaller cavern system on the island of Mustikkamaa with hot water back in 2021. The facility is now operational, the company says, and supplies heat to 25,000 one-bedroom apartments all year round.

"These cavern solutions being suggested, I think they're great," says Fleur Loveridge at the University of Leeds. "They're just one option, if you like."

According to the UK's Coal Authority, a quarter of the British population lives above abandoned coal mines. Significant numbers of these mines are flooded and naturally maintain relatively warm temperatures, roughly around 15C, for example.

This mine water could be heated up further, perhaps by a heat pump system, before being distributed along pipes to nearby houses, where it would warm radiators or provide hot water. Such a system could use heat exchangers to heat up a closed water loop so that potential contaminants from the mine water are not passed into the home supply.

Prof Loveridge points out that heating accounts for roughly a quarter of the UK's carbon emissions and decarbonizing heating is quite difficult. Millions of households still rely on fossil fuel boilers, for example.

"We should be, as a country, utilizing every option we have for thermal energy sources and stores," she says.

But there's an alternative to giant thermos flasks underground - what about hot rocky sponges? Matthew Jackson at Imperial College London says that in the UK we might be able to make use of aquifers, porous bodies of rock underground that naturally retain water.

It's possible to pump heat - or cold - into large areas of these "sponges" and then take the hot or cold out again via a fluid when required, to warm or cool down dwellings. Such a system could be even more efficient than hot water reservoirs in caverns, Prof Jackson says.

Despite numerous aquifer thermal energy storage installations in Europe - there are thousands in the Netherlands, for instance - it remains rare in the UK.

"That's a technology which is not that prevalent [here]," says Prof Jackson. "We have 11 installations operational at the moment."

One example is the luxury Chelsea Barracks complex in London, which is being redeveloped into a residential and commercial area. Between 2015 and 2018, an aquifer thermal energy storage system was installed by a Belgian company, AGT.

Aquifers suitable for this sort of thing are plentiful around the UK, however, and often conveniently located right under cities, notes Prof Jackson.

And yet, the UK is arguably falling behind on making use of underground thermal energy resources.

"The main barriers to deployment in the UK are not around technical suitability," says Prof Jackson. "They're really around awareness of developers that this is a solution."