Thermal Property Measurements of Stratigraphic Units with Modeled Implications for Expected Performance of Vertical Ground Source Heat Pumps

Ground temperature and lithology influence ground source heat pump (GSHP) performance; however, typical design for residential systems uses estimated handbook values that are not necessarily representative of local geology. The incorporation of true ground temperature and thermal property measurements into design models would yield a more optimal design and thus decrease life cycle cost. A two-part study was conducted, first focusing on recording thermal properties of specific lithofacies in a particular region (Wisconsin, USA), then modeling how these measurements could change expected GSHP design and performance in a typical residential setting. Representative sedimentary, igneous, and metamorphic rocks from Wisconsin were characterized using guarded-comparative-longitudinal heat flow experiments (ASTM E1225), calorimetry, and weight-volume assessments. X-ray diffraction was also performed on some samples to assess the effect of mineralogy. Across all rock types, thermal conductivity ranged from 1.84 to 6.71 W m⁻¹ K⁻¹, and specific heat capacity ranged from 713 to 891 J kg⁻¹ K⁻¹. The indexed values were used to construct a hypothetical vertical heat exchange loop penetrating vertically consecutive Paleozoic strata. The hypothetical loop was examined using the rate of system temperature drop under full load as a measure of performance during the heating season. Expected system performance was compared between commonly cited thermal conductivity values of their general lithologies (e.g. sandstone, dolomite, shale) and the laboratory-measured thermal properties of the specific formations. Thermal conductivity indexed by general lithology proved to be insufficient as a design parameter to generate accurate assessments of GSHP performance.