Borehole heat exchangers (BHEs) are increasingly being used to control temperature in buildings. A typical BHE consists of a long U-shaped tube encased in grout, emplaced 100s of meters below the ground. Heat is transferred from the groundwater through the grouting material to the fluid in the tube which is used for temperature control of the building.
MODFLOW-USG is an open source, public domain code released by the US Geological Survey for simulation of groundwater flow. Unstructured grids along with a Connected Linear Network (CLN) package for simulating interconnected conduit features provides the code with exceptional flexibility for simulating subsurface complexities. USG-Transport is an open source, public domain software that includes several enhancements to MODFLOW-USG including capability for simulating density dependent solute transport and heat transport. This is ideally suited for simulation of Borehole Heat Exchangers (BHEs) – unstructured grids provide the capability to focus resolution on the grout and CLNs provide the ability to simulate the BHE tubes. Additional boundary conditions have been implemented for BHE inflow/outflow temperature requirements. The capability has been tested against other solutions and an application example is presented to demonstrate use of USG-Transport for simulation of BHEs.