Geothermal energy and transport infrastructures: an unexplored potential? The example of the metro in Rennes

Energy geostructures: what are they?

Let’s start by reminding ourselves what «geothermal energy» is. As all wine lovers know, the temperature of a cellar can vary slightly over the year. Geothermal power uses the temperature of the subsoil, which is more constant than at the surface, for thermal energy exchange that provides heating or cooling for buildings.

Energy geostructures, also called thermoactive geostructures, are part of a larger family of shallow geothermal systems installed in the subsurface soil at a depth of just a few dozen metres. These geostructures are equipped with heat pumps that allow them to heat or cool neighbouring buildings.

The principle is to use the foundations of buildings or civil engineering structures (pile foundations, tunnels, metro stations, etc.) as geothermal exchangers by fitting heat exchanger tubes during construction. Pile foundations fitted with such systems are called energy pile foundations.

These necessary structural elements are thus given a second function: that of harnessing energy, which saves having to drill dedicated wells for geothermal energy. In addition, the added costs associated with the work are very low compared with the benefits, and the system helps reduce the carbon footprint of the structure.

Recent scientific developments have also resolved most of the questions around the mechanical behaviour of energy geostructures, particularly during temperature fluctuations, and confirmed their beneficial nature for urban environments.

Geothermal energy has been studied and explored since the 1980s. However, large-scale development of this non-intermittent (unlike solar or wind power, for example), renewable, low-risk, local and low-carbon energy is far below its potential.

The metro, an interesting example

The city of Rennes is a trailblazer in this field. It is the first time that such a large surface area has been exploited (nearly 4,000m2 of plates and 3,600m2 of cast walls) and that the heat generated has been destined for aboveground buildings that have nothing to do with the metro itself.

However, to bring the project to fruition, the city of Rennes had to overcome several obstacles, which were not so much technical as administrative. Despite their desire to include local and renewable energy sources, contracting authorities of public transport infrastructures often struggle to estimate the potential of projects due to a lack of results from previous examples.

Could the success of the Rennes project inspire other municipal authorities and pave the way for similar undertakings by demonstrating the advantageousness of energy geostructure technology? To identify the stumbling blocks that hinder the development of these systems and collect as much information as possible from the example in Rennes, a research project called THERMETRENNES, has been launched, supported by ADEME.

The project involves all those contributing to the design and installation of the exchangers in the metro stations, including the city of Rennes, KEOLIS, EGIS (design office and geothermal system project managers) and AQUASSYS (the installer).

These on-the-ground stakeholders are joined by two research laboratories:

• LGCGM, of the University of Rennes, which conducts experiments on the thermomechanical behaviour of concrete subjected to thermal cycles.

• 3SR of the University of Grenoble-Alpes, which carries out precise energy modelling of the station..

• The project is coordinated by BRGM, the French geological survey, which notably has expertise in shallow geothermal systems and the development of dimensioning methods for geothermal heat exchangers.

As part of the project, the ground around the Cleunay station was fitted with measuring instruments, while laboratory tests and numerical thermal transfer modelling are carried out. The research project aims to provide quantitative arguments for the energy performance of geostructures for prime contractors of future projects.

From construction to control, overcoming organisational challenges

The main issue hindering the development of energy geostructures probably lies in the need for interaction between different stakeholders of the building and construction sector.

In a typical building project, the geotechnical design office, which is in charge of the foundations, is involved at the very start of the works to determine the structural dimensions, while the thermal design office, which is in charge of the thermal design of the structural elements and the aboveground buildings, is involved much later on or even as part of a completely different project.

However, the installation of energy geostructures requires close collaboration between the geotechnical and thermal design offices, whose respective fields of expertise are both essential in defining the structure to be built. The project can therefore only succeed if the architect and/or contracting authority explicitly plans this communication phase from the start. Without such coordination, there is a risk of problems arising such as the exchanger tubes being installed but extending too far beyond the space reserved for the heat pump, for example.

The same is true of the project execution and inauguration phases. Structures such as tunnels and metro stations, which require very little heating or cooling, allow large amounts of energy to be harnessed that must then be redistributed or sold, but this task is not usually one that falls within the transport operators’ remit. The success of such an undertaking therefore depends on the developers. In Rennes, the project owes its success largely to the tenacity of the municipal authorities, who acted as a coordinator during the ten years between the initial feasibility studies to the inauguration of the aboveground buildings.

Lastly, inadequate knowledge of these systems leads to a sense of wariness among technical inspectors, who are in charge of approving the works and are often concerned that the heating and cooling cycles will affect the strength of the structure, even though recent studies show that this is generally not the case. This reticence naturally spreads to investors and insurance companies.

In light of all this, the results of ThermetRennes are awaited with interest. Still underway, the work is due for completion in 2026.

A technique that could also be applied to tunnels and car parks

The Rennes project aims to provide comprehensive results that can be used to inform other initiatives, both in terms of the thermomechanical design of the geostructures and the energy design of the exchangers to adapt production to the surface area.

The use of such systems need not be limited to metro stations. The experience of the Rennes project could also be transposed to rail and road tunnels, deep building foundations and underground car parks.

A 2023 German study estimated that the energy supplied to the groundwater from underground car parks in Berlin could heat nearly 15,000 homes.

Currently, the main constraint lies in the fact that geothermal systems must be fitted to buildings during their construction, but even this is not insurmountable: in 2021, an Italian study, proposed a shallow geothermal system (at a depth of less than five metres) that could be fitted to existing buildings.

For a typical municipal development project, the benefits of using energy piles instead of classic geothermal probes are often outweighed by the organisational complications caused during the works.

Examples do exist, such as the Cité du Design in Saint-Etienne, built on around 100 geothermal piles, and the Silex contemporary music venue in Auxerre, built on 24 geothermal piles, but widespread development of such systems is faltering. And yet, harnessing the energy of larger geostructures (in metros, underground car parks, etc.) could provide substantial benefits that would compensate for the organisational difficulties mentioned above.

Reducing the urban heat island effect

Such structures currently contribute to the urban heat island effect by warming the subsoil. However, if fitted with geothermal exchangers, they could help heat buildings and accelerate the decarbonisation process.

The use of geocooling or two-way heat pumps that can both heat and cool as required, would also help reduce the use of air conditioning and thereby also the urban heat island effect.

The growing urbanisation of our societies and the urgent need for adaptation and global warming mitigation should contribute to the development of low-carbon transport structures in cities. These issues also make the search for renewable energy sources a matter of crucial importance.

In this quest, energy geostructures are not a miracle solution. The exploitable energy available is insufficient to cover all the needs, but it could provide a base supply of non intermittent, energy that could be topped up by power from intermittent sources. In addition, such structures generate little extra cost for infrastructure construction projects.

Large-scale development therefore seems desirable. However, this can only happen if it is backed by local authorities and integrated into the process by all the stakeholders ahead of all development and construction projects.