Geothermal Energy

By nature, heat lies deep under our feet. But how do we unlock its sustainable potential in large-scale district heating systems? The city of Aarhus, Denmark, exemplifies the potential of geothermal energy by integrating 110 MW to revolutionize urban heating, setting a precedent for other cities to follow.

By Søren Berg Lorenzen, Director of Sales, Innargi

Published in Hot Cool, edition no. 3/2025 | ISSN 0904 9681 |

Photo above: Pumping hot geothermal water to the surface to test the characteristics of the reservoir, Spring 2024.

Securing investment and expertise for a geothermal expansion

At the end of 2024, the European Commission concluded that a European strategy for heating and cooling is essential and that this should include the development of a geothermal action plan. From a political perspective, this is a crucial step in speeding up the phase-out of fossil fuels.

One of the reasons geothermal has not been industrialised on a larger scale until now is the significant capital expenditure (CAPEX) required and the inherent risks; this has made geothermal a less attractive commercial proposition in its early stages.

A project in collaboration between Innargi and Denmark’s third-largest city, Aarhus, demonstrates how, by industrializing geothermal heat, it is possible to drive down costs while simultaneously leveraging the learning effects from each project.

Geothermal offers local, stable, and sustainable heating

Geothermal heating stands out as an exceptionally climate-friendly energy source. When combined with renewable power from solar or wind, geothermal energy becomes essentially emissions-free.

Unlike other energy sources, geothermal reservoirs act as their own storage facilities, providing a stable baseload for district heating. The Earth’s core, with temperatures akin to the surface of the sun, continuously reheats the water in the reservoirs.

To harness the heat, a geothermal plant of the hydrothermal type operates with a minimum of two wells: one to extract hot water from the reservoir and another to return the cooled water after heat transfer.

This process, facilitated by heat exchangers and heat pumps, ensures a consistent and sustainable heat supply. The plant will be connected to an existing district heating network.

Figure 1: Two geothermal wells – one producer and one injector.

Figure 2: Geothermal heating plant and distribution of heat to the grid.

Aarhus: Appraisal phase confirms geothermal potential and puts the first heating plant into operation

Figure 3: Drilling the first well in the appraisal phase at the harbour in Aarhus, Winter 2023.

In Aarhus, the local district heating company, Kredsløb, has embarked on an ambitious journey to decarbonize its heating grid by 2030. A key component of this initiative is the transition from burning wood pellets to geothermal heat. Upon completion, the project will constitute the most extensive, coherent geothermal project in the EU.

Aarhus is the second-largest city in Denmark, with more than 330,000 residents. The city is supplied with heat from an extensive district heating grid that covers almost the entire city. The project will deliver 110 MW to this grid, corresponding to the annual heating needs of approximately 36,000 households. The initial plan is to establish geothermal plants across seven sites, directly supplying heat to local distribution networks.

The exploration phase of the project started in 2023 by conducting a seismic survey to map the local subsurface in the area. Additionally, an appraisal phase was developed, including the drilling of three wells and the construction of one heating plant, to ensure that lessons learned could be applied to future site planning and project design.

Figure 4: An illustrated example of the final facility, which will occupy approximately 500 m² of ground space – equivalent to the penalty area on a football field. It is designed to blend into the natural surroundings in Skejby, Aarhus, a field used for dog training.

In November 2023, drilling began on the first well at the harbour of Aarhus. By mid-June 2024, the first pair of wells was completed at the second site, approximately 7.5 km further north.

The drilling involved testing the temperature and productivity (flow) of the geothermal reservoir to ensure that it would deliver the anticipated heat. All results fell within the expected range, with the 70°C geothermal water being pumped to the surface as planned.

Currently, the first heating plant is under construction to harvest the heat from the geothermal water pumped to the surface. The goal is to deliver the first geothermal heat to local citizens before the end of 2025, which also marks the end of the appraisal phase for the project. Afterward, lessons learned will be implemented as the remaining sites are developed across the city.

Engaging with neighbours to gain local support

A geothermal heating plant is much smaller than most other comparable sources of district heating. Once operational, they produce no noise, odour or other emissions. This means neighbours can look forward to a considerate and responsible neighbour over the 30 years the plant will provide heat to their homes.

However, the site can be a source of noise, increased traffic, and lighting during construction – particularly during the 3-4 months of drilling (estimated time for drilling two wells). And for that reason, local outreach activities are essential to gain support from the local community.

In Aarhus, neighbours have been informed directly at every project stage – through flyers, newsletters, and information meetings. The project has also been in a dialogue with local businesses and associations to consider the local use of the area as early as possible.

Adapting to the unpredictable: Geothermal success requires specialized expertise

Drilling for hot water shares several similarities with drilling for oil. The need for thorough data collection and scientific analysis of subsurface conditions is just as critical now as it was during the peak of hydrocarbon drilling in the past.

Geothermal energy is a challenging endeavour: Data provides the best foundation for decision-making, but it is not always possible to predict with complete certainty what will be encountered when drilling into the subsurface.

Therefore, it is important to ally with experts who are used to navigating challenges, adapting quickly to different scenarios, and overcoming unforeseen obstacles, regardless of how unlikely they may seem.

The decision to conduct an appraisal phase also brings the opportunity to implement local learnings and findings quickly, delivering the most optimal design for wells and heating plants across the full project in Aarhus. Every city is different—both above and below the surface.

Geothermal is not yet an option in all cities

As geothermal energy enters the district heating market, it is a viable option in many locations – but not everywhere. To deliver a geothermal project, three key prerequisites must be in place:

• Hot water flowing beneath our feet
  Geothermal energy comes from hot water stored several kilometres beneath the surface, and it’s abundant. The Earth’s core continuously heats this water, enabling it to deliver stable, circular, and local heating. However, subsurface conditions vary greatly, just as the land above ground does. This means that deep geothermal is not feasible everywhere. To ensure the geothermal potential is viable, thorough local exploration is conducted to assess the availability of the required resources.
• Existing district heating network and demand for heat
  The district heating grid is essential for distributing the geothermal energy produced at scale to residents’ homes, providing heat where it is needed.
• Site for geothermal wells and plant above-ground
  The optimal area needed for drilling wells and constructing the heating plant is a little smaller than a football pitch. Once operational, the geothermal plant becomes a considerate neighbour, occupying only the space of the penalty area, delivering heat for up to 30 years, and emitting no noise, smell, or other emissions. It is often also important that the location is close to where the heat is needed—i.e., near the distribution network.
  If the heat must travel far, the cost of exploiting the natural heat beneath our feet will increase. While access is needed to the surrounding area for occasional maintenance of the wells, the space can be used for a variety of other daily purposes, such as parking, dog training, or even playing football.

The above also means that there are places where geothermal is not currently the right solution, either due to insufficient subsurface conditions, limited heat demand, and/or access to suitable sites. The two latter parameters – heat demand and site access – can, of course, develop in the right direction over time.

For further information, please contact: Søren Berg Lorenzen, soren.berg.lorenzen@innargi.com