Performance characterization of shallow helical ground heat exchangers for ground source heat pump applications

Abstract

The use of the ground as heat source or sink medium for heating and cooling the human built environment has the potential of saving energy and reducing greenhouse gas emissions significantly. One of the main components of this system is the heat exchanger that is buried in the ground. This thesis explores the performance of a specific type of ground heat exchanger (GHE): the shallow vertical helical GHE. This type of GHE occupies considerably less land when compared to horizontal configurations and are less influenced by outdoor temperature and weather conditions. When compared to traditional deep vertical U-tube probes, savings on drilling costs can be significant. However, performance data and design information is limited for these types of heat exchangers, which has limited their adoption amongst ground source heat pump (GSHP) system designers and installers. Various in-situ heating and cooling tests were performed at a residence in Bozeman, Montana, with a system containing three helical GHEs. The heat exchangers are coupled with a GSHP with variable capacity compressors. Moreover, a recently developed numerical model (named CaRM-He) was used to compare the experimental results with the simulated performance. The model is based on the analogy between thermal and electrical phenomena, where the domain (comprised of GHE, surrounding ground, grouting material, and heat carrier fluid) is discretized as a linked network of thermal nodes with thermal capacitances and resistances. Heat exchanger outlet temperature as predicted by CaRM-He model was compared with experimental data, resulting in different degrees of accuracy. This research presents a method to characterize the performance of these types of GHEs, and a comprehensive analysis of uncertainties and sources of error inherent to in-situ testing. Also, it is recommended that the model is improved to include extraneous heat inputs and horizontal runs. Still however, the advantages of this ground coupling method and the possibility to predict its performance make the helical GHE an interesting alternative for the geothermal designer.