Thermal Response Test experiments and modelling applied to shallow geothermal piles of different geometry

Geothermal piles are structural elements that encountered interest in ground source heat pump (GCHP) applications thanks to the great potential in cost reduction compared to the traditional solutions based on borehole heat exchanger (BHE). In this paper a series of experimental results related to TRT measurements carried out on geopiles of different geometry is presented. These geostructures are reinforced concrete cylinders with diameter ranging from 0.6 to 1 m and depth of about 13m, with pipes either arranged as coils or multiple vertical U-tubes. The measurements show that the evolution of the fluid temperature in time cannot be properly described by the usual ILS (infinite line source) solution since the experimental temperature profiles do not gather along any line when represented as a function of the logarithm of the time. Starting from the above findings, measurements are here recast by calculating the temperature response factor of the different pipe arrangements by spatial superposition of base FLS (Finite Line Source) solutions. Finally, a recursive technique of parameter estimation based on a 2 resistance model is applied for calculating the effective BHE resistance and the ground/concrete thermal conductivity at minimum deviation between experimental and predicted temperature profiles.