The caves that will be a giant thermos flask

There are vast caverns beneath the Swedish city of Västerås. A gigantic project plans to fill them with hot water, up to 95 °C. These are the caverns that will be a giant thermos.

The caves have the capacity to store roughly the equivalent of 120 Olympic-size swimming pools. This type of thermal storage is one of several ways to store heat in the ground for later use. They argue that more use should be made of subway heat storage systems.

The caverns that will be a giant thermos are in Sweden.
The caverns that will be a giant thermos are in Sweden.

Heat underground

The heat from the caverns will be sent through pipes to a district heating network. This supplies 98% of the homes in the city of 130,000 inhabitants. It is intended to start filling the caverns with water by the end of the year. The facility will provide 500MW of district heating energy.

The hot water reservoir will allow Mälarenergi to continue heating homes on cold winter days. Storing heat underground tends to work well because it is very difficult for heat to escape. The ground itself acts as a great insulator. Caverns that will be a giant thermos will retain heat for several weeks.

The project in Västerås is not the first of its kind. In Finland, the energy company Helen started filling a slightly smaller cavern system with hot water. It was done on the island of Mustikkamaa in 2021. The installation is now operational and supplies heat to 25,000 one-room apartments year-round. According to the UK Coal Authority, a quarter of the UK population lives above abandoned coal mines.

This would provide natural energy for the city's homes.
This would provide natural energy for homes in the city.


A significant number of these mines are flooded. Naturally, they maintain relatively warm temperatures. This mine water could be further heated, perhaps by a heat pump system. And then distributed through pipes to nearby houses.

Aquifers, porous bodies of rock subway that naturally hold water, could also be used. It is possible to pump heat, or cold, into large areas of these “sponges.” And then draw the heat or cold back out through a fluid when needed. It would be more efficient than hot water reservoirs in caves.

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