Multilayer analytical model for vertical ground heat exchanger with groundwater flow

The vertical ground heat exchangers (GHE) are the most common application of the ground source heat pump (GSHP) systems. Due to ground heterogeneity and length of the boreholes, the heat exchangers cross usually several geological layers. However, in most of the current analytical models for GHEs, the restrictive assumption of ground homogeneity is considered. In this paper, a finite line-source model is proposed for GHEs that takes into account not only thermal conduction but also advection and dispersion mechanisms, induced by ground water flow, in a multilayer porous medium. Firstly, the anisotropy is added to the moving finite line-source (MFLS) model, and an existing composite model approach is modified. The temperature comparison with the numerical model results demonstrates the suitability of the approach. The proposed model provides faster solution than typical 3D numerical methods Furthermore, the homogeneous and multilayer assumptions are analyzed in dimensionless form to check the convenience of both of the approaches. The results demonstrate that, in case of high groundwater velocity in one layer, the thermal interaction with the neighboring layers decreases due to strong groundwater flow suppressing the thermal flux interaction. In that case, the prediction of homogeneous assumption is slightly sufficient in the middle of the layer. Otherwise, the multilayer approach is more appropriate in transient conditions, particularly, at the interface of layers.