What is geothermal energy?

22 September 2026

With geothermal energy, the ground acts as a huge thermal battery. In winter, the heat naturally stored in the ground is extracted via boreholes to heat buildings. In summer, it is ‘recharged’ by returning the heat discharged into the subsoil when buildings are cooled.

La géothermie vise à récupérer la chaleur et la fraicheur du sous-sol, grâce à l’installation de sondes. It is a closed-loop system: the fluid circulating through the probes never comes into contact with the subsoil or groundwater; it merely exchanges heat. In the Maximilian Park, located in the City of Brussels, no less than 104 boreholes will be installed. These will supply the future carbon-neutral heating and cooling network and enable several buildings in the neighbourhood (offices, social housing and private homes) to be heated and cooled.

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The park’s closed-loop geothermal system will be combined with energy recovery from the sewage collectors (riothermy) to form the project’s two local, carbon-free energy sources.

Probes heading deep underground

Two drilling contractors will carry out the drilling and horizontal trenching work required to install the boreholes and then connect them to one another. This work will begin this autumn. At least two drilling rigs will be on site at the same time, completing on average one borehole every two days. The geothermal project foresees seven probe installation zones, taking into account the Park’s current and future constraints.

There will be over a hundred boreholes extending through the geological strata to a depth of 200 metres. The scale of the borehole field is designed to maximise the energy potential of the Park. We are utilising the various layers of the ground to exchange energy in the form of calorie and supply the grid. A probe is a U-shaped pipe placed in the borehole. In contact with the ground, it will absorb or release energy to the earth depending on the needs.

Geothermal energy in Maximilan Park, in figures:

104 boreholes in Maximilian Park
200 meters deep
Approximately 1.1 MW of power and nearly 1,200 MWh of heating and cooling per year (an average household uses 9.5 MWh of heating per year)
Stable underground temperature of around 12°C all year round
First project of that scale in Brussels, in the public sector
1,500 tonnes of CO₂ per year prevented

Geothermal energy that respects the park

This is a discreet and invisible source of energy once the work is complete (the park remains a park), designed to last for decades. The geothermal probe field is located beneath the park’s organic soil layer, in order to preserve the flora and fauna found there. Similarly, the layout of the network has been designed so as not to hinder the growth of the trees.

A major innovation is the use of a local, renewable energy source in the public domain: the subsoil of Maximilian Park. The City of Brussels is making its subsoil available through a public land concession, which allows Karno, Thermal Energy Network Developer, to carry out its work and then use the geothermal energy for the duration of the project’s trial phase, bringing benefits to the neighbourhood and helping to address the energy transition challenges facing the City and the Region.