In-Situ Testing of Shallow Depth Helical Heat Exchangers for Ground Source Heat Pump Systems

This paper explores the performance of shallow depth helical heat exchangers coupled with ground source heat pumps (GSHP) for residential HVAC applications. These vertical helical heat exchangers occupy considerably less land when compared to horizontal configurations, and are less influenced by outdoor temperature and weather conditions. When compared to traditional vertical deep probes, savings on drilling costs can be significant. However, performance data and design information is limited for this type of heat exchangers, which has limited their adoption amongst GSHP system designers and installers. In-situ heating and cooling tests were performed at a residence located in Bozeman, Montana, with a system containing three 12 ft (3.7 m) long helical ground heat exchangers installed to a depth of 6 to 18 ft (1.8 to 5.5 m) below the surface. The heat exchangers were coupled to a GSHP with variable capacity compressors. Moreover, a recently developed numerical model was used to compare the experimental results with the simulated performance. Heat exchanger outlet temperature as predicted by the CaRM-He model were compared with experimental data, resulting in a good agreement, especially in cooling mode. The advantages of this ground coupling method and the possibility to predict its performance make these systems an interesting option for the residential geothermal designer.