Implementing a New Effective Finite Difference Formulation for Borehole Heat Exchangers into a Heat Transport Code

Abstract

We present an effective finite difference formulation for implementing multiple ground heat exchangers (GHE) in the 3-D coupled heat and flow transport model SHEMAT (Simulator of Heat and Mass Transport, Clauser, 2003) The GHE with arbitrary length can be coaxial as well as U-shaped.

The new approach does not require the fine discretization of the GHE assemblage since it considers heat transport between fluid and the soil through pipes and grout via thermal resistances. Therefore, the simulation time can be significantly reduced.

The coupling with SHEMAT is realized by introducing an effective heat generation. Due to this connection, it is possible to consider heterogeneous geological models, as well as the influence of groundwater flow. This is particularly interesting when studying the long term behavior of a single GHE or a GHE field. The model is validated against the existing GHE modeling codes EED and EWS. A comparison with monitoring data from a deep GHE in Switzerland shows a good agreement. A synthetic example demonstrates the usages and field of application of this new numerical tool.