Oscillatory thermal response tests to estimate the ground thermal diffusivity

Thermal Response Tests (TRT) have been extensively studied in the past decades and is an important area of research belonging to ground heat exchanger design. The most important thermal property inferred in a TRT is the equivalent subsurface thermal conductivity. Previous studies were presented where the thermal diffusivity was estimated but with limited success. In this paper, a TRT with an oscillatory excitation (OTRT) is used to obtain a better uncoupling between thermal conductivity found from constant heat injection and thermal diffusivity. The method was already used in the past with a simplified ground model. It is presented here with a better model that takes into account the ground and the borehole thermal response. The method is based on the concept of concentric composite cylinder where the equivalent radius is used for the U-tube arrangement. Results obtained with this method were successfully correlated to those obtained with numerical simulations and the method was then used to analyze an experimental field test.