This paper investigates the possibility of thermal activation of future tunnels to heat and/or cool buildings or underground infrastructures such as subway stations in the context of Grand Paris Express, more than 200km of automatic underground railway for public transport in the region Île-de-France. The principle is to insert pipes in the tunnel lining, which are connected to a geothermal pump. Heat transfer fluid antifreeze circulates inside the pipes to exchange heat with the surrounding environment (tunnel air and surrounding rock). A 3D finite element model is devoted to study the mechanisms of the thermo-hydraulic problem with the equations of mass and heat transfer in porous medium, concrete (solid), and fluid in pipes, and to predict the thermal potential of the rock according to thermal properties. This model is employed for comparative studies to analyze the influence of fluid properties, rock thermal properties and mode of functioning, on the effectiveness of the exchange system and the thermal equilibrium of the rock. The influence of groundwater flow velocity is also studied in order to identify the range in which heat storage within the surrounding rock is possible from the summer season to the winter when needs for easy available energy are greater.

1.1 Context and objectives

To face the climate threat confirmed by numerous scientific studies, France has pledged to the international community to multiply by 6 the contribution of geothermal energy in its energy mix by 2020. In the context of increasing needs for exploitation of renewable energy sources, sensible heat storage in the subsoil appears as an opportunity to seize by disseminating heat pumps in civil works.

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