HEAT TRANSFER CONTROLS IN SOIL: RECOMMENDATIONS FOR IMPROVED GROUND SOURCE HEAT PUMP SYSTEM DESIGN BASED ON RESULTS FROM THE INDIANA SHALLOW GEOTHERMAL MONITORING NETWORK

Six monitoring sites situated across a diverse range of hydrogeologic settings in Indiana have collected surface and subsurface data since 2011. The purpose of this network of sites is to quantify heat transfer properties of unconsolidated material to optimize horizontal (trench) earth-coupled geothermal system designs. Thermal conductivity and thermal diffusivity are continuously measured at 1.3m depth. Other subsurface instruments placed at multiple depths (up to 2m) are monitoring fluctuations in soil moisture, soil temperature, and matric potential. Surface micrometerological conditions are also recorded to allow calculations of water balance and to estimate potential evapotranspiration. Laboratory experiments and field data indicate that the highest thermal conductivity values are found in quartz-rich soils having high bulk densities (> 1.65g/cm³) and nearly saturated moisture states. Soils generally show an inverse relationship between thermal conductivity and thermal diffusivity related to the specific heat of water. Variability in moisture content can cause thermal conductivity values for a given soil to change by more than 0.5 W/m²K based on field measurements, and by as much as 1.8 W/m²K based on laboratory experiments. Bulk density and thermal conductivity are positively correlated at most sites, and both variables are inversely correlated with porosity. Clay content has a weak positive correlation with bulk density and dry thermal conductivity, but is inversely correlated with saturated thermal conductivity. Results suggest that methods for improving the heat transfer efficiency of a soil include adding dense, conductive material and artificially increasing bulk density while decreasing porosity and thermal contact resistance by compacting the soil. The ability of systems in unsaturated soils to absorb heat (summer cooling applications) may also be improved by irrigating the soil. However, this practice could yield reduced efficiency for winter heating applications in some climates unless the irrigated trench is lined with an impermeable layer and the residence time of water in the soil is controlled.