Europe's largest hot water storage facility - an interview with Stefan Winkler

Hi there, Stefan Winkler. Right now, you are working in an old oil storage cavern in Västerås.
What are you doing there?

“We are preparing the sanitation work in what will be one of the largest hot water storage facilities for district heating in Europe. There are a total of three rock caverns of 100,000 cubic meters each, and they were built in the early 1970s for heavy crude oil for Mälarenergi’s CHP plant. The last oil from the rock caverns was used in 1985 and they have been empty since then, but they will now be used again in Mälarenergi’s district heating system.” 

How is the storage supposed to work?

“The old rock caverns will be filled with water that will be heated by the CHP plant’s waste boiler. It will have a storage capacity of approximately 13 GWh and will be used to optimize electricity and heat production or as a reserve. When production is cheap, for example during the summer, the stock is replenished.”

When will the storage facilities be operational?

“The first parts are expected to be up and running in late 2022. The entire storage facility will be used in late 2023 to early 2024, but before the storage facility can be used, it has to be sanitized from remaining oil residues.”

How common is it to have such large-scale storage facilities? 

“This storage facility will be very large and have a capacity of 300,000 cubic meters, which can be compared to Mälarenergi’s new accumulator of approximately 27,000 cubic meters. There is an old rock cavern in Hudiksvall that is now used as energy storage, and they will soon start using rock caverns for energy storage in Helsinki. In order for it to be profitable, you have to have access to an existing rock cavern. Blasting a new one is too expensive, but on the other hand, there are many empty rock caverns in Sweden that were previously used for fuel or oil storage, so we may see more such solutions in the future.”

What is FVB’s role in the project?

“We are involved in several parts and have participated in the feasibility study, where we investigated various system solutions, made stratification calculations, and made simulations of heat loss in the rock caverns. Right now, I lead the sanitation effort and I am also in charge of system design and ventilation. It will be quite hot in the rock caverns’ service spaces with all the surrounding hot water, so we need to dimension the ventilation to create the right conditions for the machines and the staff who will work there.” 

For more information:
Stefan Winkler, 021-81 80 93

Brief facts:
Mälarenergi’s energy storage has a storage capacity of 13 GWh, which is enough to supply Västerås with district heating for 2-4 weeks, depending on the outdoor temperature.


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