An appropriate use of the thermal response test for the design of energy foundation piles with U-tube circuits

The current study investigated a thermal response test on a 20 m long energy foundation pile equipped with a double U-tube circuit. The test measurements were interpreted using both the infinite line source model and inverse numerical analysis by means of a detailed model that was able to consider both the actual geometry of the pile and the axial heat transfer. The results indicated an approximately 50% difference between the two values of ground thermal conductivity. The effect of this evaluation on the calculation of the energy piles' total length was first investigated utilizing a method outlined in the literature. The consequences on the long term energy performance of a ground source heat pump system were then investigated using numerical simulations and taking into consideration the effect of the spacing between the foundation piles. It was found that the total length calculated using the result based on the infinite line source model was about 20% lower than that obtained using the inverse numerical approach. An approximately 10% difference in the heat pump's seasonal energy efficiency was also found.