Numerical modelling of coupled groundwater flow and heat extraction from subsoil.

The subsoil can be used to store or extract heat for cooling or heating buildings, respectively, via a ground-coupled heat pump (GCHP) and the Ground heat exchanger (GHE) commonly used consist of HPDE pipes. The present contribution deals with the role of groundwater flow in the performance of GHEs installed in vertical boreholes. A mathematical model in 2D space is introduced, comprising a pair of partial differential equations of parabolic and elliptic type for soil temperature and ground water velocity field, respectively. The equations have been solved numerically by mean of Difference Finite method and Finite Element Method. The numerical model has been validated by comparing the results with the solutions of Moving infinite line source (MILS) model. The soil temperatures established in a sand around GHE operating in heat mode are calculated either assuming a uniform velocity field and by taking into account the correct seepage around the GHEs to investigate how much the proper modelling of groundwater flow around the borehole affects the results.