Modeling Horizontal Ground Heat Exchangers in Geothermal Heat Pump Systems
Geothermal heat pumps use the earth as a heat source and sink via a ground heat exchanger (GHX) that consists of a network of buried heat exchange pipes, which can either be installed in vertical boreholes or in shallow horizontal trenches or excavations. The main goal in GHX design is to determine the minimum length of pipe needed to provide adequate fluid temperatures to heat pumps over their operating life. In this paper, COMSOL Multiphysics is used to model horizontal GHXs in two-dimensional cross-section with hourly time-varying boundary conditions at the ground surface and at internal heat exchange pipe surfaces. Hourly heat rejection and extraction loads due to heat pump operation are modeled as transient heat fluxes imposed on internal boundaries that represent the inside surface of buried heat exchange pipe. The more popular GHX configurations modeled here include: two-pipe, four-pipe, six-pipe, and slinky coil, and results are compared to quasi-analytical design length equations for a cold, moderate, and hot climate.