THERMAL CONDUCTIVITY OF SURFICIAL MATERIALS: AN AID TO GROUND SOURCE HEAT PUMP DESIGN

Thermal conductivity (K) is a key parameter used to determine the length of borehole needed for proper ground exchange in the design of ground source heat pump (GSHP) systems. Although conducting a thermal response test in the field is the preferred method of determining K, these tests are not always performed, particularly for smaller projects. In such cases, K is estimated. Because GSHP installation costs are sensitive to drilling depth, we question whether estimation is the best way to proceed with design? In this study, an alternative approach is offered. With funding from the U.S. DOE, 250 soil samples from representative surficial materials across MA and CT were collected including glacial till, various coarse-grained stratified deposits, loess and lake-bottom sediments. K was measured using a Decagon KD2 Pro conductivity meter and measured at ambient moisture, dry and saturated conditions. Grain size analyses were also performed on each sample. A thermal dryout curve was then fitted to the data using the model described by Campbell (1985) and Campbell et al. (1994). The thermal dryout curve provides the complete range of K values as a function of moisture content. The concept behind this approach is that a user can look at a surficial geologic map or borehole log, identify the appropriate dryout curves for the surficial materials at their site and then assign a suitable K value for each type of material. One K value is assigned to each deposit (clay, sand and gravel, etc.) and different values assigned depending on whether the materials are above or below the water table. That combined with K measured for different rock types (separate study) provides a means of calculating a depth-averaged K value that can be used for ground exchange design when thermal response test data are unavailable.