Geysers, hot springs, and volcanoes in distant places are not the only sources of geothermal energy, or “heat from the earth.” Significant sources of geothermal energy are also found in Ohio, where temperatures increase with depth. For example, a visitor to Ohio Caverns experiences a cool temperature of roughly 55 degrees Fahrenheit (°F) at a depth of less than 100 feet, whereas the temperature that a miner labors in, say 2,000 feet below ground in a northern Ohio salt mine, is nearly 80°F. Temperatures measured in oil and gas wells greater than 8,000 feet deep in eastern Ohio may exceed 160°F. Because heat continually flows from Earth’s hot interior, geothermal energy is renewable, environmentally friendly, and can be used directly for electricity production, space heating, or in shallow geothermal-heat pump systems to control building temperatures.
Geothermal resources are classified based on temperature, where High Temperature is greater than 310°F (150°C), Moderate Temperature ranges from ~200–310°F (90–150°C), and Low Temperature is less than 200°F (90°C; see Duffield and Sass, 2003). In Ohio, shallow ground- and water-source heat pump systems are widely used, and demand continues to grow for this highly efficient heating and cooling technology (see Shallow Sources & Applications of Geothermal Energy). On the other hand, Rankine-cycle turbines use organic working fluids, which operate on low- to moderate-temperature heat sources.
Because binary electrical power production is possible from the upper end of the low-temperature range, electricity coproduction potential exists to recapture heat from deep wells, industrial operations, and conventional electrical power generation (see U.S. DOE, RMOTC). Although recaptured heat from industrial and electricity generation is increasingly being used for low-temperature binary power generation, Ohio also has a long history of oil-and-gas production with many existing and planned deep wells that can have rock temperatures exceeding 160°F. Binary power generation technologies are also being tested that involve energy coproduction with enhanced or secondary oil-and-gas recovery; these technologies also capture and use waters that are usually considered waste products in the energy production cycle. If available in sufficient quantities, relatively warm geothermal water circulated in deep wells or arising from oil-and-gas production and recovery may also be valuable simply for space heat or hot water.
Of course, there are many different geothermal technologies, and the subsurface and surfacegeology and surface-sediment types and thicknesses vary from location to location throughout Ohio, creating many unique geologic and environmental conditions and technical hurdles. Deep wells can involve hot corrosive fluids and oil and gas that must be separated and captured. Any potential geothermal application should be thoroughly investigated with respect to potential costs and geological constraints.