THE UNTAPPED POTENTIAL OF ANTHROPOGENIC GEOTHERMAL RESOURCES

Groundwater aquifers provide a source of low-enthalpy geothermal energy for heating and cooling buildings and other thermal applications that is widespread globally, although minor compared to fossil fuel usage. Impelled by the need to reduce global greenhouse gas emissions, minor gains have been made in harnessing the vast potential of low-enthalpy geothermal resources via direct use and closed-loop ground-source heat pump systems. A particularly promising approach is the use of anthropogenic geothermal resources where the “subsurface urban heat island effect” has raised groundwater temperatures beneath cities. Despite only modest increases in temperature (a few degrees Celsius) documented in the subsurface, research suggests that this thermal resource in the groundwater could meet the entire heating demand of buildings in major cities in North America, Europe, and Asia. At the Bloomington campus of Indiana University, we’ve recently discovered an exceptional anthropogenic heat reservoir having temperatures elevated by more than 10 °C above ambient values in the shallow carbonate bedrock aquifer. These anomalously high temperatures extend to more than 60 m depth in portions of the aquifer. We attribute this remarkable energy resource to long-term heat loss from the university’s district-scale steam delivery system, which heats more than 1 million square meters of building space via a 34 km-long network of buried steam and condensate piping. In this paper, we present results from borehole logging, surface thermal imaging, soil and rock core analyses, and multiphysical modeling to estimate the magnitude and extent of the university’s anthropogenic geothermal resource. In addition, we discuss opportunities for Indiana University to recover this energy for beneficial use through an innovative campus-wide R&D initiative. Finally, we contrast our findings with studies in North America, Europe, and Asia to provide a broad assessment of the untapped potential of anthropogenic geothermal resources. With nearly 700 district-scale energy systems installed across the United States alone, our research suggests that anthropogenic geothermal resources could enable a significant expansion of clean energy technologies throughout the nation.