UNDERGROUND COAL MINES IN OHIO AS A POSSIBLE GEOTHERMAL ENERGY RESOURCE

Ground source heat pumps (GSHPs) exploit unsaturated or saturated bedrock/soil layers or surface water bodies as heat exchangers, with an increased efficiency compared to conventional heating and cooling systems. Groundwater and soil/bedrock are typically relatively thermally stable throughout the year and are better conductors of heat compared to air, increasing the efficiency of the heat pump system. While there has been significant research done on GSHPs, there is limited work on studying unconventional reservoirs of heat for GSHPs. This study focused on characterizing the potential for exploitation of flooded mines as heat exchangers within the state of Ohio. Flooded mines offer a large body of water for heat exchange. Additionally, the use of water as the heat source/sink, instead of saturated soil/bedrock, could allow for increased efficiency in GSHP systems.

Mines selected for characterization were chosen based upon proximity to population centers and the availability of water within the mines. Using GIS software and data from the Ohio Geological Survey, mines within roughly 1 mile of cities and towns that are characterized as either below, or partially below drainage were selected for further study. Using data on the physical characteristics of the mines, groundwater recharge, groundwater flow directions, average ambient temperatures, and regional heat flow gradients, a number of parameters were calculated. These include: maximum and minimum residence times, maximum and minimum recharge to the mines, effective mine volumes, linear groundwater velocities, and minimum average mine temperatures. The total maximum theoretical amount of heat extraction possible was calculated based upon the volume of water within the mines, groundwater velocities, and the estimated mine temperatures.

This study identified 147 possible mine sites, spanning 21 counties, for GSHP implementation in Ohio. The mines have an estimated average maximum residence time of 6 years and an estimated average minimum residence time of 3.5 years. It was estimated here that, on average, 10¹⁰ kJ of heat energy could be extracted from the mines per 1⁰C change in mine water temperature. The cities located close to these mines could benefit from this geothermal resource.