A quiet field near Aalborg could become the launchpad for Denmark’s next big green energy breakthrough. The goal: tap heat more than 4 kilometres underground and turn it into reliable, fossil-free district heating – using technology adapted from the oil and gas sector.
Behind the project is former oil geologist Jørgen Peter Rasmussen and his company Green Therma.
Their claim is bold: geothermal could supply 25% of Denmark’s district heating within 25 years.
8 January 2026 | Original article in Danish by Lars Henrik Aagaard | Berlingske
From Earth science pioneer to geothermal frontrunner
Seismologist Inge Lehmann was the first, in 1936, to describe the Earth’s hot core. The discovery earned her international recognition — just not in Denmark. Photo: Even Neuhaus (6.2.1863-20.4.1946), Public domain, via Wikimedia Commons
Eighty-nine years ago, Danish seismologist Inge Lehmann became the first to map the Earth’s inner core, more than 6,000 kilometres beneath us – a world of extreme heat and pressure.
Now, Denmark may once again become a pioneer underground. This time with a climate focus.
The goal is simple: tap the vast heat stored beneath our feet and turn it into clean district heating. A quiet field in Storvorde, east of Aalborg, could be one of the first sites.
Behind the project stands Jørgen Peter Rasmussen, founder and CEO of Green Therma and a veter-an of the oil and gas sector. “Within 25 years, geothermal could supply 25% of Denmark’s district heating,” he says.
Geothermal gains global momentum
The outlook is shared by Innargi, the Maersk-founded company already delivering geothermal heat to thousands of residents in Aarhus from 2.5 km deep wells. Internationally, expectations are rising fast.
According to the International Energy Agency (IEA), geothermal potential is almost comparable to solar power. It could even overtake wind and hydropower as a stable renewable source.
Researchers at Princeton University estimate geothermal could supply three times more energy to Americans by 2050 than today’s nuclear fleet.
Yet today it remains marginal, accounting for less than 1% of global energy production. In Den-mark, the share is barely measurable.
Earlier Danish projects closed due to corrosion and technical problems. Only Thisted’s small plant has operated reliably for decades.
Why traditional geothermal struggles
Conventional geothermal systems often fail. The design typically uses two vertical wells: one brings hot water up, the other pumps cooled water back down. In practice, pipes corrode, clog, or lose pressure. Flow patterns underground become unstable. At one Danish plant, even radioactive deposits built up inside the pipes. Globally, failure rates are estimated at up to 75%.
A closed-loop design: “a giant thermos flask”
Green Therma’s solution is its patented Heat4ever technology. “It’s basically a giant thermos flask,” Rasmussen explains. Instead of open wells, the company drills 4.5 km down and then 3 km horizontally through hot rock. Water circulates inside sealed pipes with no contact with groundwater.
No salt. No corrosion. No scaling.
At that depth near Aalborg, temperatures reach around 120°C.
About 4,000 litres of water circulate continuously, heating underground before returning to the sur-face to supply district heating – initially for around 1,000 households – then flowing back down again.
The same water is reused in a closed loop.
The entire plant will fit inside a container-sized footprint and operate almost silently. It can also be throttled down in summer when demand drops.
Rasmussen expects the first demonstration plant to be running by 2027 and operating for decades with minimal maintenance.
Oil and gas technology repurposed
The drilling method is not new or experimental.
It builds on techniques long used in North Sea oil fields, where horizontal wells are standard. That reduces risk and keeps costs predictable.
Tests in Norway have already validated the concept. Similar projects are underway near Berlin, with plans in the Netherlands and the US.
Germany alone aims to expand district heating to half of all households — creating strong demand for reliable geothermal supply.
Just a few kilometres beneath our feet, temperatures typically exceed 100°C, and in the Earth’s core, 6,000 kilometres down, they reach nearly 6,000°C. It is an almost inexhaustible source of energy.
An almost inexhaustible resource
Denmark is geologically well suited for geothermal.
As a rule of thumb, temperatures rise 25–30°C per kilometre. Just a few kilometres down, the heat is already sufficient for district heating.
Globally, the IEA estimates geothermal resources could cover the world’s electricity demand many times over.
“The heat is always there – 24/7, all year,” Rasmussen says. “It doesn’t depend on wind or sun. You just bring it up and use it.”
From heat to power
In Denmark and Germany, geothermal is mainly used for heating. But in the US and Iceland, ambitions go deeper.
Very deep wells can reach around 400°C, producing supercritical water capable of driving turbines for electricity generation. Such wells could deliver up to ten times more energy.
The challenge is drilling that deep through hard rock. New technologies, including laser-based systems, aim to make it faster and cheaper.
A Danish geothermal revival
For Denmark, geothermal may be on the verge of a comeback – perhaps even a quiet revolution.
If projects like Green Therma succeed, underground heat could become a core part of future district energy systems.
The resource has always been there.
Now the technology may finally be ready.
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This article was translated into English by DBDH with the assistance of AI tools. The content has been reviewed and edited by the editorial team.
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