Thermophysical Properties of Wisconsin Rocks for Application in Geothermal Energy

Abstract

Limited information exists about the geothermal gradient and heat capacity of the geologic formations in Wisconsin. The viability of deep (? 300 m) geothermal exchange systems in Wisconsin depends on the thermal properties (e.g., thermal gradient, thermal conductivity, thermal diffusivity, specific heat, and heat production) of the rock formations. To aid the evaluation of Wisconsin?s heat capacity, thermal conductivity and specific heat tests were conducted in the laboratory to determine the thermal properties of a sample of Wisconsin rocks. Thermal property tests were conducted three to six times to ensure repeatability of results. In addition, quantitative X-ray diffraction (XRD) analysis was completed to determine relative mineral abundance and the corresponding calculated thermal property values for each specimen. These values were then compared to the experimental results. In the field, a novel ground source heat pump (GSHP) system, the Deep, Insulated, Single Hole geothermal heat pump (DISH) system was instrumented and installed to evaluate the viability of deep geothermal exchange systems in Wisconsin. However, collection and analysis of data from the DISH system must still be completed to determine system performance, efficiency, economic return and life cycle. Samples were obtained from Wisconsin?s Research Collections and Education Center though collaboration with the Wisconsin Geological and Natural History Survey (WGNHS) as well as from field sites located in central Wisconsin. Cylindrical specimens were trimmed to a diameter of 2.54 cm and length of 2.03 cm using a drill press, rock saw and surface grinder. Using the guarded-comparative-longitudinal heat flow apparatus, thermal conductivities between 2.30 W m-1 K-1 and 6.71 W m-1 K-1 were measured. The thermal conductivity for the majority of the specimens (omitting the Barron Quartzite and St. Lawrence Dolomite) ranged between 2.30 W m-1 K-1 and 3.86 W m-1 K-1. The Barron Quartzite had the highest thermal conductivity (6.71 W m-1 K-1), and the St. Lawrence Dolomite had the second highest thermal conductivity (4.67 W m-1 K-1). The thermal conductivity of the Precambrian Granites ranged from 3.10 W m-1 K-1 to 3.69 W m-1 K-1. Sandstones ranged between 2.59 W m-1 K-1 and 3.86 W m-1 K-1. Using the ?coffee-cup? calorimeter, specific heat values between 713 J kg-1 K-1 and 891 J kg-1 K-1 were obtained. The Tunnel City Sandstone had the highest specific heat (891 J kg-1 K-1), and the St. Lawrence Dolomite had the second highest specific heat (872 J kg-1 K-1). The specific heat of the Precambrian Granites ranged between 824 J kg-1 K-1 and 729 J kg-1 K-1. Sandstones ranged between 818 J kg-1 K-1 and 891 J kg-1 K-1. Thermal property values obtained in the laboratory were compared to mixture-based calculated values from weighted averages based on XRD analysis and to literature. The majority of the thermal conductivity values obtained fall within the estimated range from literature and the calculated thermal conductivities values. Deviations from the expected or calculated values can be attributed to relative mineral abundances, density or porosity of the specimens. The thermal property data was reliable based on a small-scale statistical analysis, although a thorough analysis of saturated rock thermal properties is recommended. In addition, a parametric study should be completed to determine how strongly rock thermal properties control the ground source heat pump system design, efficiency and cost since the variation of thermal properties with rock type is small. Due to effects of anisotropies across microscopic, laboratory, and field scales, in situ thermal property testing methods warrant further research. Many variables affect the thermal properties of a given rock. Thus, simply selecting a value based on rock type, mineralogy, porosity or density may yield inaccurate analysis of a GSHP system.