Monitoring near-surface thermal properties in conjunction with energy and moisture budgets to facilitate the optimization of ground-source heat pumps in the glaciated Midwest
This poster was presented at the American Geophysical Union Fall Meeting 2011, San Francisco, Calif., on December 7, 2011. It was part of IN33C, Geothermal energy research and discovery II posters session.By exploiting the near-surface heat reservoir, ground-source heat pumps (GSHP) represent an important renewable energy technology that can be further developed by establishing
data sets related to shallow (<100m) thermal regimes. Although computer programs are
available for GSHP installers to calculate optimal lengths and configurations of
ground-coupling geothermal systems, uncertainties exist for input parameters that must first
be determined for these models. Input parameters include earth temperatures and thermal
properties of unconsolidated materials. Furthermore, thermal conductivity of sediments
varies significantly depending on texture and moisture content, highlighting the need to
characterize various unconsolidated materials under varying soil moisture regimes.
Regolith texture data can be, and often are, collected for particular installations, and are
then used to estimate thermal properties for system design. However, soil moisture and
temperature gradients within the vadose zone are rarely considered because of the difficulty
associated with collecting a sufficient amount of data to determine predominant moisture
and temperature ranges.
Six monitoring locations were chosen in Indiana to represent unique hydrogeologic settings
and near-surface glacial sediments. The monitoring approach includes excavating trenches
to a depth of 2 meters (a typical depth for horizontal GSHP installations) and collecting
sediment samples at 0.3-meter intervals to determine thermal conductivity, thermal
diffusivity, and heat capacity in the laboratory using the transient line heat source method.
Temperature sensors are installed at 0.3-meter intervals to continuously measure thermal
gradients. Water-content reflectometers are installed at 0.3, 1, and 2 meters to determine
continuous volumetric soil moisture. In-situ thermal conductivity and thermal diffusivity
are measured at 1.5 meters using a differential temperature sensor that measures radial
differential temperature around a heating wire. Micrometeorological data (precipitation,
insolation, ambient air temperature, relative humidity, and wind speed) are also collected to
determine surface energy and water budgets that drive fluxes of energy and moisture in the
shallow subsurface. By establishing continuous, year-round data, fluctuations in seasonal
energy budgets and unsaturated zone soil moisture can be considered such that GSHP
system designers can establish accurate end members for thermal properties, thereby
optimizing the ground-coupling component of GSHPs. These data will also provide
empirical controls such that soil moisture and temperature regimes can be spatially
distributed based on mapped soil units and hydrogeologic settings in Indiana