Lawrence Berkeley National Laboratory

An efficient means of modeling coupled groundwater flow and heat transport for an entire borefield making up the Ground Heat Exchanger (GHE) for a Ground Source Heat Pump (GSHP) is demonstrated, using the numerical simulator TOUGH, incorporating the “Extra Grid Block” method. A realistic thermal load taken from an EnergyPlus simulation provides inlet temperature and loop flow rate for the borefield. The thermal load is nearly balanced between winter heating demand and summer cooling demand. TOUGH then simulates subsurface heat transport, assuming three different magnitudes of regional groundwater flow: none, small – deemed plausible for Detroit Arsenal, and large – 10 times the small value, to illustrate a more extreme effect. The small groundwater flow has a negligible effect on GHE performance, and the large groundwater flow has only a minor effect. An alternative case using a hypothetical unbalanced thermal load with cooling demand all year showed greater effect of groundwater flow. The implication of the coupled groundwater flow/heat transport model results presented here is that the usual practice of assuming subsurface heat transport occurs by thermal conduction only is valid for the proposed GHE design at Detroit Arsenal. A large hydraulic head gradient, high-permeability strata over most of the borehole length, and a highly unbalanced thermal load are all necessary for groundwater flow to have a significant effect on GHE performance, and none of these is present at Detroit Arsenal.