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). 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. Instead of water, 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. 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 surface geology 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.
Shallow Geothermal Resources
Carl Nielsen, a professor at The Ohio State University (OSU), built the first residential ground-water heat pump in 1948 at his residence in Columbus, Ohio. Nielsen devised a one-ton unit using common plumbing materials and by drilling a well in his back yard to a depth of 80 feet. The constant-temperature well water ran through a simple heat exchanger, regulating his home’s temperature for seven years with no problems. In 1955, Nielsen installed in a home a larger unit that ran for two decades with only a single, minor problem.
Commercial ground-water heat pumps became more widespread in the 1950s and 1960s. One of the largest, early commercial installations was at Battelle Memorial Institute in Columbus. In 1958, Battelle began heating a 317,000-sf building by drilling five 16-inch wells to a depth of 50 feet in a sand-and-gravel aquifer and circulating the constant-temperature water through a series of large heat exchangers. Commercial ground-source heat pump installations increasingly are becoming a viable economic alternative in new or retrofitted commercial and institutional buildings. For instance, OSU installed a geothermal heating-and-cooling system in two new eleven-story dormitories and rehabilitated several older buildings beginning in 2011. The $4M closed-loop system consisted of 450 vertical wells drilled to a depth of 550 feet that circulates fluids to an array of heat exchangers located in the basement of one of the new dormitories. An estimated 34 percent less energy will be needed for heating, cooling, and hot water, saving more than $200,000 annually compared to natural gas.
Residential ground-water heat pumps became more common after the large increase in energy prices in the 1970s. Residential ground-water heat pumps had to overcome several obstacles, including costs of well drilling, lack of experienced installers, corrosion within the geothermal pipes, and other operational or maintenance issues. Technological advances in heat pump efficiency, horizontal and vertical closed-loop systems development (ground-source heat pumps), and better site-specific planning led to more than 450,000 ground-source heat pumps being installed in the United States by the year 2000. Growth in the business has continued, with approximately 115,000 units installed nationwide during 2009. Ohio is a leader in the installation of geothermal heat pumps with approximately 10,000 installed during 2009. Division of Geological Survey efforts include providing additional geotechnical data relevant to heat pumps.
Ultimately, coal-mining regions such as eastern Ohio may become a significant source of geothermal energy, as coal mines are being explored as possible geothermal sources. Not new, the use of abandoned underground mines has been executed abroad and is currently being studied in Pennsylvania and Indiana. These large geothermal systems potentially are some of the most efficient and profitable, but exploitation has been limited in the past due to relatively low conventional-energy costs. The concept also takes advantage of abandoned and flooded mines by capturing mine water, using it in a heat exchange cycle, and then returning it to a mine. Because of Ohio's large number of closed underground mines, the Division of Geological Survey is taking an active interest in investigating their potential for geothermal heating and cooling and for off-peak energy storage.
Deep Geothermal Resources
New technologies are becoming available that produce electricity and space heat from relatively lower-temperature rocks, such as those penetrated by some of Ohio’s deep oil and gas wells. Ohio has produced hydrocarbons from deep boreholes since the 1860s, and approximately 275,000 documented wells have been drilled to depths from a few hundred feet to the current record-setting depth of 13,500 feet in Belmont County. Beginning in the 1940s, the temperature log became one of the first downhole logs available for oil-and-gas exploration. In addition to geothermal energy potential, temperature log data has also become indispensable for understanding heat flow within Earth.
Modern temperature logs can resolve temperature changes as small as 0.05°F and are valuable tools for indicating shale content, fluid and gas flows into a wellbore, and cement tops and for calculating fluid resistivity and other parameters. Currently, the Division of Geological Survey is evaluating its large dataset of bottom-hole temperatures as well as more than 3,200 complete borehole temperature profiles. Selected bottom-hole temperature data will be corrected using best-practice procedures and the results will be used to construct more detailed temperature, gradient, and heat flow maps than are currently available for the state. The division also will make available deep- and shallow-geologic maps and other data relevant for low-temperature geothermal energy production.
In high-temperature geothermal plants, water or steam at very high temperatures (300–700°F [149–371°C]) is used to drive turbines that generate electricity. California has the most (48) installed geothermal power plants in the country, the first installed in 1960. Nevada is second with 20 installed power plants, while seven other states, primarily in the western United States, have at least one geothermal electric-generating project. In 2010, more than 180 geothermal projects in 15 states were in various stages of development. Electricity has been produced primarily from shallow high- and moderate-temperature geothermal sources; but electricity production from the upper end of the low-temperature geothermal range is now possible using binary-cycle power systems.
In the binary power cycle, the working fluid (fluid with a very low boiling point) is selected to optimize the power output from a particular heat source, temperature, and fluid flow. The working fluid is vaporized by the heat flowing through the pre-heater and vaporizer. The vapor expands as it passes through the organic vapor turbine, which is coupled to the electrical generator. The exhaust vapor is subsequently cooled and condensed and is then recycled to the heater(s) and vaporizer.
Electricity production has been possible in conjunction with producing oil and gas wells and from coproduction associated with enhanced- and secondary-oil and gas recovery. New research suggests electricity coproduction is possible withCO2 sequestration or with CO2-induced enhanced recovery projects. These techniques allow for space heating and electrical coproduction using injection fluids, such as brine or CO2, which are usually considered waste products in the energy production cycle. Binary-power technologies will become more important for recapturing energy from waste heat from large commercial electricity production and industrial processes. But this emerging technology may also become important in Ohio and other Appalachian and Illinois Basin states for deep, nonproducing wells; new oil and gas wells drilled in deep rocks; and in association with enhanced- and secondary-oil and gas recovery and CO2 sequestration.
Geothermal Data Resources
In 2010, the Division of Geological Survey joined a coalition to research and integrate geothermal data into a new National Geothermal Data System (NGDS). Sponsored by the Association of American State Geologists, the State Geothermal Data Project collects data from the state geological surveys into the NGDS in an effort to facilitate future development of renewable geothermal energy in the United States. This data will promote additional research by universities, government, industry, and individuals to develop geothermal resources potential nationwide. Geothermal energy represents a substantial but underutilized energy resource. Data and additional information about the NGDS can be found at www.geothermaldata.org.