Evaluate renewable and nonrenewable energy resources by their geology, efficiency, availability and the cost of mining and extraction, using Ohio's coal, oil and gas

Learning goal: Classify energy resources as renewable or nonrenewable and explain the geology that produces fossil fuels; calculate and compare the efficiency of energy sources, including alternate energy sources; explain resource availability using the difference between a resource and a reserve; describe the main methods of mining and resource extraction and their environmental costs; and apply all of these to Ohio's coal, oil and natural gas and to alternatives available in Ohio.

Renewable and nonrenewable. A resource is renewable if nature replaces it at least as fast as people use it on a human timescale: sunlight, wind, flowing water, heat from the shallow ground, and biomass if it is regrown. A resource is nonrenewable if it forms far more slowly than it is used. Coal, oil and natural gas formed over millions of years, and uranium ore is a fixed amount of an element concentrated by geologic processes, so all are nonrenewable. The condition matters: wood is renewable only when forests are replanted at the rate they are cut, and groundwater used for geothermal systems is renewable only if it is not drawn down faster than it recharges.

The geology of fossil fuels. Coal forms from plant material that accumulated in swamps faster than it decayed, because waterlogged ground is low in oxygen. The plant debris became peat; burial under later sediment raised the temperature and pressure, driving off water and volatile compounds and concentrating carbon, so peat became lignite, then subbituminous and bituminous coal, and under stronger heat anthracite. A thick layer of peat compacts to a much thinner coal seam, often several to more than ten times thinner. Most of Ohio's coal lies in the Pennsylvanian-age rocks of eastern and southeastern Ohio, deposited about 320 to 300 million years ago in coastal swamps on the edge of the Appalachian Basin when Ohio lay near the equator. Ohio coal is bituminous and much of it is high in sulfur, which matters when it is burned. Oil and natural gas form from the remains of microscopic marine organisms buried in fine mud. With burial to a depth where temperatures reach roughly 60 to 120 °C, the organic matter is slowly converted to oil, and at higher temperatures to gas. The rock where this happens is the source rock, usually a dark shale. (Some natural gas also forms at shallow depth from microbes, and coal beds release gas too; the heated-plankton route is the main source of oil.) Oil and gas are less dense than water, so they migrate upward through permeable rock until an impermeable cap rock traps them in a porous reservoir rock such as sandstone or limestone: the conventional three-rock model. In unconventional shale deposits, such as the Utica, the oil and gas never left the source rock. Ohio had an early oil boom in the late 1880s and 1890s from the Ordovician Trenton Limestone around Lima, and a long history of gas production from the Silurian Clinton sandstone. Since about 2011, horizontal drilling combined with hydraulic fracturing has produced gas and oil from the Ordovician Utica and Point Pleasant shale in eastern Ohio, a source rock too tight to yield much by vertical wells.

Efficiency. Efficiency is useful energy out divided by energy in, as a percentage. Every conversion loses some energy, mostly as heat, and the losses multiply along a chain. A fully worked model: a coal power plant converts about 35 percent of the fuel's chemical energy to electricity (typical plants are roughly 33 to 40 percent), and suppose about 5 percent of that electricity is lost in transmission. From 1,000 MJ of coal, 1,000 × 0.35 = 350 MJ of electricity leave the plant and 350 × 0.95 = 332.5 MJ reach homes. The overall efficiency is 0.35 × 0.95 = 33.3 percent. If that electricity runs a light that turns 40 percent of it into light, only about 13 percent of the coal's energy becomes light. The lesson is that where energy is saved at the end of the chain, several times as much fuel is saved at the start.

Alternate energy sources and efficiency. A natural-gas combined-cycle plant, which uses the hot exhaust of a gas turbine to raise steam for a second turbine, reaches about 50 to 60 percent. Wind turbines capture roughly 35 to 45 percent of the kinetic energy of the wind passing through the blades; physics limits any turbine to about 59 percent. Typical solar panels convert about 15 to 22 percent of the sunlight that strikes them. Low efficiency is not the same as waste when the input is free and renewable, so a fair comparison also counts the fuel that does not have to be mined and the emissions not produced. Wind and solar vary with weather and time of day, so they are also compared by capacity factor, the actual output over a year divided by the output if the machine ran at full power all the time. A ground-source heat pump uses the steady temperature of the ground about 10 to 15 m down, roughly 10 to 13 °C (50 to 55 °F) in Ohio, to heat buildings in winter and cool them in summer; it moves about three to four units of heat for each unit of electricity it uses, because it moves heat rather than making it. That ratio is called the coefficient of performance; it is not an efficiency in the useful-out-over-energy-in sense used above, because most of the heat delivered comes from the ground, not from the electricity.

Resource availability. A resource is a concentration of a material in or on Earth in a form from which extraction is, or could one day be, economically feasible, whether it has been discovered yet or not; scattered atoms of an element spread through ordinary rock are not counted. A reserve is the part that is known and can be extracted profitably with present technology and prices. Reserves change even though the rock does not: the Utica shale gas was a resource for decades and became a reserve when horizontal drilling and fracturing made it economic. The reserve-to-production ratio estimates how long a reserve would last at the present rate, but it is a snapshot, not a forecast, because prices, technology and demand change. The same reasoning applies to mineral resources: Ohio produces salt from thick Silurian beds mined beneath Lake Erie, limestone for cement and construction, and sand and gravel from glacial deposits, and each is weighed by use against availability and demand.

Mining and resource extraction. Surface mining removes the overlying rock, the overburden, to reach shallow coal; it recovers most of the seam but disturbs large areas, and United States law since 1977 requires the land to be reclaimed, regraded and replanted. Underground mining follows deeper seams, either leaving pillars of coal to hold the roof or removing whole panels by longwall mining and letting the roof collapse behind; it disturbs less surface land but carries risks of roof falls, methane explosions and subsidence. A lasting problem in southeastern Ohio is acid mine drainage: pyrite (iron sulfide) exposed to air and water in old mines oxidizes to sulfuric acid and dissolved iron, which lowers stream pH and coats stream beds in orange iron hydroxide. Oil and gas extraction requires drilling, and for tight shale, hydraulic fracturing with large volumes of water; the salty wastewater brought back up is often injected into deep disposal wells. That injection has been linked to small earthquakes in eastern Ohio, which shows that extraction costs include effects nobody planned.

The misconception. "Renewable means no environmental cost." Every source has costs: wind and solar farms need land and mined metals, hydroelectric dams flood valleys and trap sediment, and biomass can compete with food crops. The honest comparison includes feasibility, availability and environmental cost across extraction, storage, use and disposal.

Worked chain efficiency: An electric water heater is 95 percent efficient at the house, supplied by a gas combined-cycle plant at 55 percent with 5 percent transmission loss. Overall efficiency = 0.55 × 0.95 × 0.95 = 0.496, about 50 percent. A gas water heater burning the same gas at home at 80 percent is therefore more efficient overall, because it skips the power-plant conversion. The step students skip is multiplying every stage rather than using only the last one.

Worked coal needed for 1 kWh: 1 kWh = 3.6 MJ. With a plant efficiency of 35 percent, the coal must supply 3.6 ÷ 0.35 = 10.3 MJ. Taking a model energy content of 25 MJ per kg for bituminous coal, the mass burned is 10.3 ÷ 25 = 0.41 kg per kWh. A household using a hypothetical 900 kWh per month would need about 900 × 0.41 = 370 kg of coal per month if all its electricity came from such a plant.

Worked reserve-to-production ratio: A hypothetical region has a coal reserve of 1,200 million metric tons and produces 30 million metric tons per year. R/P = 1,200 ÷ 30 = 40 years. If a new technology makes a further 600 million metric tons profitable, R/P becomes 1,800 ÷ 30 = 60 years; if production doubles to 60 million metric tons per year, the original reserve lasts 1,200 ÷ 60 = 20 years. The ratio changes with both technology and demand, which is why it is not a prediction.

Worked capacity factor: A hypothetical 2.0 MW wind turbine in northwestern Ohio produces 5,200 MWh in a year. Maximum possible = 2.0 MW × 8,760 hours = 17,520 MWh. Capacity factor = 5,200 ÷ 17,520 = 0.297, about 30 percent. It does not mean the turbine was broken 70 percent of the time; it ran at less than full power when winds were light and not at all when calm.

Representation task: Draw an energy-flow (Sankey-style) diagram for 1,000 MJ of coal becoming light in a home, with arrow widths proportional to energy: 650 MJ lost at the plant, 17.5 MJ lost in transmission and the light stage losing 60 percent of the rest. Label each value. Then draw a cross-section of a gas trap showing source shale, migration path, reservoir sandstone and cap rock, and label where a vertical well and a horizontal well would go.

Safe investigation, measuring efficiency: With a teacher present, fill an electric kettle with 1.00 kg (1.00 L) of tap water, measure the starting temperature, plug the kettle into a plug-in energy meter and let it heat until it switches itself off at the boil. Unplug it, stir with a thermometer, then record the final temperature and the meter reading in kWh. Useful energy = mass × 4,186 J per kg per °C × temperature rise; energy in = meter reading × 3,600,000 J. For example (model values), 1.00 kg heated from 20 to 90 °C gains 293,000 J; if the meter shows 0.10 kWh (360,000 J), efficiency is 81 percent. A meter that reads to 0.01 kWh makes each result uncertain by about 10 percent, so run three trials and compare the mean with the spread. Keep hands away from steam and let the kettle cool before emptying. What the model cannot show: it measures one conversion only, not the losses at the power plant or in transmission.

Independent performance, Eco-House proposal: Choose an Ohio location and design an off-grid house (sketch or blueprint). Success criteria: it names at least three energy sources suited to the site and classifies each as renewable or nonrenewable; it gives an efficiency or capacity-factor figure with its source for each and shows one calculation; it explains the geology the site offers (ground temperature, wind exposure, sunlight, any local fossil fuel); it addresses storage for times without wind or sun; and it states two environmental costs of the chosen system, including one from mining or extraction of the materials.