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The design length of ground heat exchangers for geother-mal heat pump systems is strongly dependent on the borehole thermal resistance. The borehole thermal resistance is defined by the thermal properties of the materials of construction and the arrangement of flow channels of the ground heat exchanger and strongly influences the performance of a geothermal heat pump system. Simulation studies are presented that examine and compare the thermal performance of some of the more common types of ground heat exchangers, such as single U-tube, double U-tube, concentric tube, and standing column well (with no groundwater bleed), using a mathematical description for the thermal resistance of ground heat exchangers. The system simulations are based on a short time step ground heat exchanger model and are conducted under an equivalent set of conditions considering a small office building. Results show that the single U-tube requires the greatest length of bore for the conditions simulated. Theoretical reductions in total bore length of approximately 22%, 33%, and 36% are seen for the double U-tube, the concentric tube, and the standing column well with no bleed, respectively. A comparative life-cycle cost analysis is also conducted considering a 20-year system operation.

Units: Dual

 

Citation: Symposium Papers, Atlantic City, 2002