Long-term performance of large borehole heat exchanger fields with unbalanced seasonal loads and groundwater flow

The effects of groundwater flow on the long-term performance of large Borehole Heat Exchanger (BHE) fields with unbalanced winter and summer loads are studied, in a dimensionless form, by finite-element simulations implemented through COMSOL Multiphysics (©COMSOL AB). Peclet numbers in the range [0,0.8] are considered. First, two regular time periodic heat loads, with a period of one year, are considered, with either a partial compensation of winter heating and summer cooling or no compensation. In this part of the study, each BHE is sketched as a cylindrical heat source, and the following BHE field configurations, with a distance of 40 diameters between adjacent BHEs, are analyzed: a single line of infinite BHEs; two staggered lines of infinite BHEs; four staggered lines of infinite BHEs. Then, the effects of hourly peak loads are determined, for double U-tube BHEs, with reference to typical dimensionless daily heat loads, for winter heating and summer cooling. The ground is modelled as a Darcy porous medium. The results show that, in the range of Peclet numbers considered, the groundwater flow does not reduce the effects of hourly peak loads but yields an important improvement of the long-term performance.

► In the absence of groundwater flow, large BHE fields require a compensation of winter heating and summer cooling.
► Groundwater flow, even with a very low speed, ensures the sustainability of BHE fields with unbalanced seasonal loads.
► The groundwater flow, with a usual speed, does not reduce the effects of hourly peak loads.
► Dimensionless tables for the design of large BHE fields with unbalanced seasonal loads and groundwater flow are provided.