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Jérôme Diot
Directeur technique développement durable et énergie
Published on March 21, 2019

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Innovative thermoactive foundations

Have you heard about foundations that can capture the energy in the ground to provide comfortably heated buildings? The technique is new in France and has made its debut in Tours in its urban tramway maintenance centre.

Pieux thermoactifs innovants

Thermal piles contain two 25 mm polyethylene tubes forming a double U shape inside the pile cage. - Crédits : © Egis

Tours Métropole Val de Loire opened its 15-km tram line in 2013. The trams are stabled and maintained in a maintenance depot at the northern end of the line. This building required deep foundations, as the subsoil is made up of successive layers of backfill. 500 concrete piles, 50 m in length were necessary for this. The architect’s practice L’Heudé et L’Heudé (Orleans) and Egis chose to use these foundations to capture energy from the ground and release it indoors in the form of heating or cooling. The technique is still quite new to France, where geothermal sensors are generally installed in aquifers or in the ground.

54 thermal piles

The 54 thermal piles are positioned every 10 m under the 2500 m² of the office zone, which is the part of the building that supports the least weight. The other foundation piles, located under the tram maintenance hall (4000 m²), exposed to much more substantial stress, do not contain any energy capture pipes so as to not jeopardize their solidity. In a thermal pile, the water circulates in a closed loop. It sources its calories from the ground in the winter and returns them in the summer to regenerate the energy charge of the ground. A double loop is required due to the necessity of working simultaneously. In the winter, it may occur that the offices and meeting rooms of this well-insulated building need air conditioning in places.

Cascading heating

The thermal piles are connected to a heat pump which constitutes the first part of a “cascading” heating system. For most of the year, this covers all of the heating needs of the offices and workshops (these needs have been pooled).

In winter, this heat pump only heats the offices, while the rest of the building uses the heat recovery of a water/water heat pump which cools the control centre IT room. Finally, the third level of the cascade is formed by a gas condensing boiler which operates when the heat pump of the IT room is not switched on, and also to supply domestic hot water. The workshop is heated using radiant ceiling panels.

Low-energy certification for the whole building

At the outset, we were aiming to obtain BBC certification (low consumption building, the guarantee of the lowest possible energy bill) for the “offices” part alone. But by pooling the needs and equipment, in particular the heat pump connected to the IT room, we were able to achieve 30 kWh/m²/year for the combined “offices + workshops” facility. The electricity generated by the 1,600 m² of solar panels on the roof also contributes to this positive result.

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