Indiana Shallow Geothermal Monitoring Network: A Test Bed for Optimizing Ground-Source Heat Pumps in the Glaciated Midwest

Abstract

Ground-source heat pumps (GSHP) represent an important technology that can be further developed by collecting data sets related to shallow thermal regimes. Computer programs that calculate the required lengths and configurations of GSHP systems use specific input parameters related to the soil properties to enhance the accuracy of models and produce efficient system designs. The thermal conductivity of sediments varies significantly depending on texture, bulk density, and moisture content, and it is therefore necessary to characterize various unconsolidated materials under a wide range of moisture conditions. Regolith texture data are collected during some installations to estimate thermal properties, but soil moisture and temperature gradients within the vadose zone are rarely considered due to the difficulty of collecting sufficient amounts of data. Six monitoring locations were chosen in Indiana to represent unique hydrogeological settings and glacial sediments. Trenches were excavated to a depth of 2 meters (a typical depth for horizontal GSHP installations) and sediment samples were collected at 0.3-meter intervals for a laboratory analysis of thermal conductivity, thermal diffusivity, bulk density, and moisture content. Temperature sensors and water-content reflectometers were installed in 0.3-meter increments to monitor changes in temperature and soil moisture with depth. In-situ thermal conductivity and thermal diffusivity were measured at 1.5-meters using a sensor that detects radial differential temperature around a heating wire. Micrometeorological data were also collected to determine the surface conditions and water budgets that drive fluxes of energy and moisture in the shallow subsurface. Preliminary results indicate that increases in water content can increase thermal conductivity by as much as 30% during wetting front propagation. Although there is a change in temperature associated with the infiltration of wetting fronts, thermal conductivity appears to be independent of soil temperature. By establishing continuous data sets, fluctuations in seasonal energy budgets and unsaturated zone soil moisture can be determined. This information can then be used to establish accurate end members for thermal properties and improve the efficiency of geothermal systems.