HYDROLOGIC AND THERMAL CONSIDERATIONS ON THE USE OF ABANDONED COAL MINE AS SOURCES/SINKS OF HEAT

The Appalachian region of the United States presents thousands of acres of land that overly abandoned coal mines that produce many environmental problems such as subsidence and acid mine drainage. However, many of them are located close or below urbanized areas and their water could be used as a heat exchanger for geothermal applications such as heat pumps, contributing to ameliorate the energy consumption of fossil fuels in the region. In this work, we discuss the parameters that should be accessed for a scientifically sound use of this resource. Coal mines can be complete or partially saturated with water. The exchange of heat with the stored mine water depends on several parameters such as overall water and heat balances and heat flow circulating within the mine coupled with the water velocity. These variables are a function of the volume of water circulating the mine per unit time, temperature of the mine water, water recharge to the mine, seasonal fluctuations in flow and water levels, chemical composition of the fluid, heat exchanged with the surrounding rocks and overlaying air layer (for the partially saturated mines), and inputs and outputs of water from other sources. Before the use of coal mines as heat exchangers a comprehensive study of the hydrology and thermal properties of the mine should be completed, which includes the monitoring for at least one year of water levels and temperatures at different points within the mine, chemical composition of the circulating fluids, assessment of the meteoric water recharge throughout monitoring of rainfall, temperature, and infiltration, and of the discharge points of the mine to the ground surface. The geology of the area and extension of the coal layer and mined area should also be assessed. With all these information, it is possible to find the spatial distribution of saturated and unsaturated zones, spatial distribution of the thickness of the mine cavities, spatial distribution of hydraulic heads, water temperature, and water quality properties. All these parameters and the accessibility of site to the planned users will determine the best drilling targets for heat pump applications.