Numerical Modelling of Multiple Inclined Borehole Heat Exchangers

Abstract

This research describes the development and application of a numerical modelling method for angled borehole heat exchangers in ground-source heat pump systems. Inclining the boreholes relative to the vertical axis presents an opportunity to reduce the ground level footprint of the borehole field thus allowing for the installation of geothermal systems in retrofit applications or under buildings with small footprints. The commercial code COMSOL was used to develop the computational model. A series of validation and verification studies were performed to ensure the accuracy of the modelling approach. Simulations were conducted under constant and transient heat injection, where the effect of energy load imbalance is analyzed. Additionally, the effect of discontinuous loading with natural and forced recovery cycles is investigated. When exposed to a constant heat injection rate, configurations of angled borehole heat exchangers initially perform the same as vertical borehole heat exchangers. Then, there is a relatively short period where the angled configurations have slightly decreased performance due to increased thermal interaction in the near surface. At longer times, however, there is a significant benefit in using angled borehole heat exchangers as a result of the increased ground volume in the lower portion of the field. Under transient loading conditions, the conclusions were the same as constant heat injection, although the differences were smaller when the energy loading was balanced. However, when the loading was cooling dominated, by year 10 there was a significantly better performance observed for the angled boreholes. This indicates that the configurations of angled borehole heat exchangers can withstand a higher intensity of imbalanced energy loads compared to vertical configurations. Discontinuous loading was investigated by varying the length of time heat injection would occur on a daily basis. These daily perturbations led to small performance losses in the angled boreholes due to the increased thermal interaction in the near surface. Furthermore, imposing a forced recovery on the system by circulating fluid while heat injection was off did not significantly affect the fluid temperature or ground temperature.