Energiforsk (formerly the Swedish Gas Technology Centre) has commissioned global consultancy FVB to undertake a technical follow-up of Europe's largest hot water reservoir which is under construction in Västerås.
When completed, the reservoir which is being built for Mälarenergi - a power generator owned by the City of Västerås, will hold 300,000 cubic metres of water or sufficient to supply the district heating requirements of the entire city for up to a fortnight.
Its location, a series of rock caverns used for emergency fuel storage until 1985, is currently undergoing a clean-up operation to remove significantly more oil than had been anticipated, which is taking longer than expected.
“The rock caverns consists of three spaces that are similar to three huge accumulator tanks that can be controlled individually,” explains Ulrika Sagebrand from FVB, who is leading the research project. “We will perform measurements in one of the caverns, because we expect the others to behave in a similar way.”
An important part of the project involves investigating how the temperature stratification of the water behaves.
The team will therefore install temperature gauges distanced one metre apart to the cavern's depth of 25 metres.
The caverns also feature a service tunnel allowing personnel from Mälarenergi to access equipment and hot water to be fed in and out - with instrumentation installed within the tunnel measuring the volume of existing water, its temperature and other parameters.
“We do not have a similar service tunnel over our measuring site which will be a challenge as we will need to attach multiple sensors to a line with a sinker anchoring them to the bottom of the cavern whilst buoys keep them afloat.
It’s a practical challenge to get this in place and we are working on it right now.
We will also use Mälarenergi’s measurement data for our analysis,” says Sagebrand.
“In addition, we will find out how quickly the cavern is heated by the hot water.
We will measure the temperature ten meters into the actual rock itself in five different places.
"As part of the project's remit, we will also summarise the current knowledge situation and describe how Mälarenergi has gone about the work of converting the rock cavern for oil into a hot water reservoir."
Another goal of the project was to report results from the initial operation, but there will be changes due to delays in the clean up work.
Mälarenergi had hoped the cavern would be ready to be filled in the summer of 2023 but as the clean-up work took so long, the date to begin filling reservoirs with hot water has been delayed to May - with an additional 12-month wait required because the energy company needs to fill hot water when it has access to cheap energy during the summer months.
“This is unfortunate for our research as we were going to follow up on how the project progressed from a technical standpoint with the storage and layering of the water", says Sagebrand.
"The study will be finalized in autumn 2024, and hopefully we can do that part in a follow-up.”
She does, however, still see major benefits to the project, and adds: "Although we don’t have time to get all the values during the planned research period, we will have all the sensors in place, which is important since the caverns will be off limits in the future.
These sensors will also be able to be used for other follow-up projects in the future.
“There is a renewed interest in large-scale thermal storage in the district heating industry for environmental and economic reasons.
Our study can provide important information for other seasonal storage solutions.
New knowledge is needed because few studies has been done recently.
“A big difference from older studies is that there is now a greater interest in using existing rock caverns.
Previously, new rock caverns were built for storage, but using an existing rock cavern involves some other challenges, and we’re looking at those in more detail in our research project.”
