Numerical evaluation of the effects of groundwater flow on borehole heat exchanger arrays

Abstract

The effects of both direction and rate of groundwater flow on the performance of various types of borehole heat exchanger (BHE) arrays are examined using a two-dimensional coupled heat conduction–advection model. The heating operations were simulated over a period of 15 yrs using three types of arrays: rectangular, L-type, and single line. The results show that the performance of the L-type and single line-type arrays (non-square rectangular arrays) was noticeably influenced by the direction of groundwater flow as well as the flow rate. When the characteristic length of Peclet number was assumed to be a unit value, the Peclet number less than 0.05 (i.e. low flow rates less than 1 m/yr in this study) was found to have negligible effects on the performance of the system, regardless of the array type or flow direction. The cold accumulation in the ground downstream of the groundwater flow seems to be related to the variation of the fluid temperature associated with the flow direction. The comparison of annual heat capacity shows that up to 13% difference can occur depending on the flow direction. This suggests that the consideration of both direction and rate of groundwater flow may be important in designing the optimal BHE arrays, particularly for the non-square rectangular arrays.