THERMOGEOLOGY ASSESSMENTS OF GEOTHERMAL ENERGY IN THE SHALLOW HETEROGENEOUS SUBSURFACE

The science of thermogeology, the study of the occurrence, movement, and storage of heat in unconsolidated sediments and bedrock, has a long history in mapping sources of low enthalpy energy in Earth's shallow subsurface. Typically, this geothermal resource has been captured using ground source heat pumps and used for space heating and cooling producing 3 to 4 times the amount of energy they consume in the form electricity. Increasingly, heat pumps are being promoted and implemented as source of renewable energy that reduce emissions of greenhouse gases. Although, the high cost of wellbore drilling and the potential chemical and microbiological impacts on the soil and groundwater due to an intensive thermal use of the shallow subsurface has tempered the widespread adoption of the technology.

To assist the geoexchange community in maximizing their efficiency and reduce the overall cost of installation, we have developed a methodology including thermogeology to characterize the complexity and heterogeneity of the shallow subsurface; spanning the critical zone in the upper 100 m. In this depth interval, the mixing of deep and shallow groundwater impacts thermal processes. By integrating geologic mapping, geophysical logging, thermal property analyses, and fiber-optic distributed temperature sensing, we can detect sub-meter scale variations in the physical and thermophysical properties. Our results show that physical properties of sediment and rock, including moisture content, mineralogy and hardness, are strongly correlated with the variability in thermal gradient.

In this talk, we will demonstrate the application of the assessments at various sites in central Illinois where research and applied energy studies are being performed. Besides the influence of near-surface temperature changes and the Earth's geothermal heat flux from below, groundwater flow may be responsible for the conduction of heat, which should be considered in modeling the thermal profile.