Seven degrees Celsius, it rarely gets warmer in Esbjerg in mid-November.
The North Sea, reachable on foot from the center of the Danish port city in just a few minutes, is just over nine degrees.
Hardly anyone would think of jumping into the water to warm up.
And yet, in the future, around half of all households connected to the region's district heating network will be used to heat the lake water.
This is made possible by two large heat pumps from Switzerland and electricity from wind turbines off the coast.
Editor in the business sector, technology and engine department
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District heating has a long tradition in Denmark.
Esbjerg laid the first pipes in 1917 to supply a public bathhouse with hot steam, a waste product from the power station.
25,000 households are now connected to the heating network, which extends up to 30 kilometers inland.
Half of the hot water required for this is generated by a waste incineration plant, the other half by a hard coal-fired power plant, which is scheduled to be shut down next year.
“It was already clear in 2016 that we needed an alternative to coal,” explains Esbjerg’s mayor Jesper Frost Rasmussen.
He is an engineer and ran the municipal utility company Din Forsyning until his election.
Initially, they thought about a purely biomass power plant, but then the choice fell on a flexible solution that could use the wind power generated off the coast.
And this is how the concept, approved in 2019, came about: A waste-to-energy plant takes over the base load, which is also required in summer, two large heat pumps that can be controlled independently of each other also heat up a large heat storage facility, and on particularly cold days a biopower plant works, processing wood chips from Northern Europe.
If there is a lot of cheap electricity on offer, the supplier also throws in a kind of immersion heater, meters high and equipped with 40 megawatts of electrical power.
Some diesel gas engines are also available for emergency operation.
Heat from 4000 liters of sea water per second
For Claus Nielsen, project manager at Din Forsyning, the decision began an intensive search for suitable suppliers.
Although there were already suppliers of large heat pumps and heat exchangers back then, no one had ever extracted heat from such large quantities of seawater - 4,000 liters per second - in a system.
In addition, at that time almost all large heat pumps worked with synthetic refrigerants.
However, since Esbjerg is located directly on the Wadden Sea protection zone, no chemicals that are harmful to the environment should be used.
Nielsen finally found what he was looking for in Zurich at a subsidiary of MAN Energy Solutions.
Originally it only built compressors for natural gas pipelines in the sea, which have to work in salt water without maintenance for many years.
The technology can also be used for large heat pumps that use carbon dioxide as a refrigerant.
“Carbon dioxide is perfect for us,” explains Nielsen, “because if it gets into the seawater through an accident, it’s no different than releasing oxygen into the air.” Only some protective measures for workers and visitors are necessary, because in the event In the event of a leak, the gas would displace the oxygen in a closed building in seconds.