The various options geothermal energy offers to heat and cool our metro stations were looked at when the metro line 14 extension project was launched back in 2014. There are two main options: energy harnessing using vertical geothermal probes and geothermal harnessing from thermo-active foundations.
Energy harnessing consists of drilling up to 100 m into the ground to pump out the hot water found there. This requires major works that would be difficult to carry out in a highly urbanised area. This is why this option was not selected.
We opted for thermo-active foundations. A closed water circuit needs to be installed along the framework that supports the moulded foundation walls in the metro stations. The aim is not to collect hot water from the ground, but to use the ground’s warmth, which is at a constant temperature of roughly 12 °C, to heat the glycol heat-transfer fluid in the circuit.
Depending on the ambient temperature, geothermal piping exchanges heat from or to the ground to regulate the temperature in the station. As well as for passenger comfort, this is vital in ensuring that technical rooms remain sufficiently cooled for all equipment to work properly.
The other advantage of geothermal piping is its low maintenance and lifespan of 50 years. This was the obvious solution for the Porte de Clichy and Mairie de Saint-Ouen stations.
Energy supply in line with the stations' energy needs
At Porte de Clichy station, the annual heating requirements are 188 MWh, with a power demand (energy consumed at a specific moment) of 155 kW to cover the consumption peak, and 130 MWh for cooling requirements with a power demand of 117 kW. By way of comparison, the annual heating requirements for a 75 m² flat are about 8 MWh!
Geothermal piping, inserted into 1.5 m thick moulded walls, has been installed over a total surface of 3,750 m², at a height of 50 metres, 19 m of which are situated under the metro’s track foundations, and covering 120 linear metres.
The requirements for Mairie de Saint-Ouen station are not as high: 75 MWh with a power demand of 71 kW for heating, and 101 MWh per year for a power demand of 68 kWh for cooling. The station’s geothermal potential is also enough to cover between 45% to 65% of the heating requirements of a building programme including 80 housing units.
Did you know?
The system covers a surface of 5,775 m², equivalent to the size of a football pitch. It is installed at a depth of 45 metres, 19.5 m of which is under the track’s foundations, covering a distance of 196 linear metres. It is inserted into 1.2 metre-thick moulded walls.
Although these two station’s thermodynamic walls are more expensive, they will ultimately save a substantial amount of energy and reduce greenhouse gas emissions. This will reduce CO2 emissions by 50%, sulphur dioxide (SO2) by 20% to 40%, and nitrogen oxide (NOx) by 40%.
These two projects are financially viable as the initial investment will recouped in the reduced running costs over 17 years.