Efficient Horizontal Ground Heat Exchanger Simulation with Zone Heat Balance Integration

Abstract

For horizontal heat exchangers buried near a building slab or basement, interaction between the heat exchanger and the zone can be significant. Thermal interference effects can also be significant for heat exchangers with multiple pipes in close proximity. Previous simulation methodologies have not modeled these phenomena in a general manner and have lacked integration with other simulation domains, including zone heat balance calculations and fluid loop solvers. A numerical model for horizontal ground heat exchanger applications is presented, featuring a computationally efficient mesh and flexible heat exchanger tube placement. The model integrates the ground with zone heat balance and hydronic system simulations through boundary conditions within a whole-building energy simulation program. Thermal interference between pipes is captured, including circuiting effects of the fluid flow direction in individual pipes. Validation is performed using experimental data from a foundation heat exchanger research facility. Undisturbed ground temperature data is used to estimate ground and boundary properties. The model predicts heat pump entering fluid temperature with mean error of 1.3°C (2.3°F) and basement wall heat flux with mean error of 1.1 W/m² (0.35 btu/hr-ft²). This accuracy is achieved with a coarse grid, ensuring a small computational burden suitable for whole-building energy simulation.

Acknowledgments

Edwin S. Lee, Student Member ASHRAE, is Research Assistant. Daniel E. Fisher, PhD, PE, Fellow ASHRAE, is E. Fisher Professor. Jeffrey D. Spitler, PhD, PE, Fellow ASHRAE, is Regents Professor and C. M. Leonard Professor.