Workers are busy transforming a huge, damp underground cavity into a giant “thermos flask”: In Vasteras, Sweden, the Mälarenergi Group is converting a former oil depot into a hot water storage tank to heat the city.
The three underground rooms, dug in the 1970s, represent a volume of 300,000 m3 and served as an oil storage facility until the site was abandoned in 1985.
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“We are currently in the process of converting it into a giant thermos flask to store hot water,” explains Rickard Svensson, project manager at energy and heat group Mälarenergi in Vasteras, 100 km west of Stockholm.
The location “will make it possible to sometimes store excess energy (…) to be used during shortages,” he adds.
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These underground deposits are located near the heat and electricity plant, which supplies electricity and, in particular, heat to the city’s 130,000 residents.
Hundreds of meters of pipes are laid along heat exchangers so that the excess heat can warm the stored water, which is then used in times of scarcity.
In another room, hundreds of steel rods are installed to form a wall and serve as an electrical outlet.
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“It’s about being able to store energy that we sometimes have in excess and use it in the event of a shortage. Use it as a large battery to reduce our climate footprint,” said the manager.
Once the work is completed, the site will be flooded and closed. At the time of closure it had never been fully decontaminated.
The system already has surface heat storage, but with a significantly lower capacity. The new plant should be able to deliver 13 gigawatt hours (GWh), says Mälarenergi.
Store energy
This new heat storage will enable the group to make less use of the power plant’s current reserves, some of which are powered by fossil fuels, during cold seasons.
“Some days we will even be able to stop production and use this cavity,” predicts the company’s vice president, Magnus Eriksson.
Converting underground storage facilities is not a new concept, but Mälarenergi believes it has built the largest site of its kind in the world.
In 2021, Finland’s Helen Group completed a similar project on the island of Mustikkamaa near Helsinki with a capacity of 11.5 GWh.
Vantaa Energy, another Finnish group, is also preparing to build a 1 million cubic meter site capable of producing 90 GWh of electricity from hot water, according to the company.
Energy storage has long been a challenge for energy producers seeking to limit the loss of the energy produced.
“For electricity and heat, energy storage is crucial to adapt to peaks in production and demand,” emphasizes to AFP Filip Johnsson, professor of energy issues at Chalmers University of Technology in Gothenburg.
Such a challenge exists, for example, for wind energy, although several solutions still need to be further developed: storing the electricity generated in batteries or converting it into hydrogen through electrolysis of water, he notes.