Effect of the geothermal heat flux on vertical ground heat exchanger performance

Ground heat exchangers (GHE) are typically modeled using a simplifying assumption: the mean ground and atmospheric temperatures are assumed to be equal to the ground temperature halfway down the length of the borehole. This paper examines the impact of this assumption on vertical ground heat exchanger performance. A numerical model using a finite volume method is developed to simulate the performance of ground heat exchangers. The model takes into consideration the effect of the geothermal gradient, varying surface temperatures, inlet fluid temperatures or energy extraction loads over long periods of time. Results from the first comparison show that the geothermal heat flux provides more energy to the GHE than the outside environment and that over long periods of operation, the geothermal heat flux establishes itself as the main energy source. Moreover, the existence of a geothermal heat flux diminishes the effect of continuous heat extraction on the natural ground heat flux profile and therefore, infers a more sustainable exploitation of the energy resource. Results from the second comparison show that inadequate GHE dimensioning in both cooling and heating can occur if the geothermal heat flux is not considered. However, using simple approximations to the ground temperature profile that consider the geothermal heat flux can lead to accurate heat exchange calculations.