Thermal conductivity distributed from a Thermal Response Test (TRT) in a borehole heat exchanger (BHE)

The Thermal Response Test (TRT) is the most versatile tool to determine the thermal propriety of the underground for designing and sizing of the borehole heat exchangers (BHE). The TRT permits to get the average thermal conductivity (λ) across the whole stratigraphy, the thermal resistance (Rb) of the borehole / grout / rocks and the undisturbed temperature of the soil (Tg). The ground temperature is influenced by climate, topographical, geological and hydrological factors. Vertical temperature changes allows to get the relationships with the lithology and especially with the groundwater. Vertical temperature log, acquired during and after the TRT, permits to calculate the distributed thermal conductivity over each stratigraphic interval. This method permits to verify how the different lithologies and the groundwater contribute to the heat exchange in the borehole/ground system, so called geoexchange. The experimental site test indicates that the marls and clayed-marls levels show a higher thermal inertia than the sandstone ones and then lower values of thermal conducivity. The sandstones have a higher thermal conductivity with a rapid cooling and they provide the main contribution to the ground heat exchange. The distributed thermal conductivity is an useful tool for designing the BHE with the best performance, a better economic return and with low environmental impacts.