Impact of soil heterogeneity on the functioning of horizontal ground heat exchangers

• Assumption of homogeneity shown to be appropriate for HGHEs.
• Uncertainty minimized by increased pipe spacing and use of multiple trenches.
• Error associated with homogeneity assumption shown to be within <2%.

The impact of heterogeneity in soil thermal properties on the performance of horizontal ground heat exchangers (HGHE) is closely examined using a custom-made finite element model. Ensembles of heterogeneous soil fields were generated with spatial correlation parameters derived from similar studies of heterogeneity in hydraulic parameters. Within these soil fields, a single loop HGHE was modelled as discrete pipes placed in a heterogeneous soil continuum. The effect of heterogeneity was found to be minimal relative to uncertainty of the mean soil thermal conductivity, supporting the continued use of the assumption of homogeneity when modelling and designing these systems. Multiple design techniques were identified which would allow HGHE designers to further mitigate any deleterious effects, such as preferential heat transfer between adjacent pipes which may act as a ‘short-circuit’ mechanism, reducing HGHE effectiveness.