Design of Horizontal Ground Heat Exchangers in Sub-arctic Conditions-Sensitivity to Undisturbed Ground Temperatures

Application of ground source heat pumps (GSHPs) in extreme cold climates can be challenging due to the long heating season even with heat pumps designed and marketed for colder climates. One challenge (of several) is design of the ground heat exchanger under conditions where the desired minimum heat pump entering fluid temperature (EFT) is close to the undisturbed ground temperature (UGT). Furthermore, ground heat exchanger (GHE) models used for design purposes and/or energy calculation purposes don't usually incorporate freezing and thawing of the soil. This is the case whether the freezing/thawing is induced by surface conditions or by heat transfer between the ground heat exchanger and the surrounding soil. A recently developed model (Xing and Spitler in Sci Technol Built Environ 23(5), 809-825, 2017, [1]) implemented in a simulation-based ground heat exchanger design tool (Oklahoma State University in GLHEPro 5.0 for Windows-Users' Guide. Stillwater, 2016, [2]) incorporates the effect of surface-condition-induced freezing/thawing when calculating a 2nd-order harmonic approximation for the undisturbed ground temperature. This paper examines the suitability of this harmonic model in GHE design applications by comparing the predicted GHE design to the actual design of a GHE at the Cold Climate Housing Research Center (CCHRC) in Fairbanks, Alaska. Field measurements of UGT and GHE performance are used to examine the limitations of the harmonic model.