Evaluate energy resources: renewable and nonrenewable sources, efficiency, availability and the costs of extraction

Learning goal: Classify energy sources as renewable or nonrenewable and explain what the classification depends on; describe the main conventional and alternative sources with their benefits and environmental costs; calculate energy use and efficiency from real or hypothetical data; explain what determines resource availability; and describe the impacts of mining and resource extraction.

Renewable means replenished, not free. A nonrenewable resource is one that is used far faster than natural processes replace it: coal, oil and natural gas formed from buried organic matter over millions of years, and uranium, which is not replaced at all. A renewable resource is replenished on a human time scale: sunlight, wind, flowing water, tides, geothermal heat and biomass. The classification is about rate, not about being harmless. A forest cut faster than it regrows is being used nonrenewably; a river dam floods a valley; a wind farm needs steel, concrete, rare metals and land. "Renewable" answers one question, whether the source runs out, and none of the others.

The sources. Coal, burned to boil water and spin a turbine, is abundant and Ohio has long mined and burned it; it releases the most carbon dioxide per unit of electricity, plus sulfur dioxide, nitrogen oxides, mercury and ash. Oil is the transport fuel; its costs are spills, refining emissions and carbon dioxide. Natural gas burns cleaner than coal and releases roughly half the carbon dioxide per unit of energy, but unburned methane leaking from wells and pipelines is a powerful greenhouse gas; Ohio's production rose sharply with hydraulic fracturing of the Utica shale. Nuclear fission splits uranium nuclei to release heat; it emits no carbon dioxide during generation and produces a very large amount of energy from a small mass of fuel, but the spent fuel stays dangerously radioactive for thousands of years and has no permanent disposal site in the United States, and accidents, while rare, are severe. Ohio has operating nuclear plants on the Lake Erie shore. Hydroelectric dams are reliable and dispatchable but block fish migration, trap sediment and flood land. Wind turbines emit nothing in operation; they are intermittent, need space and transmission lines, and kill some birds and bats; northwestern Ohio's flat, open farmland hosts the state's largest wind farms. Solar photovoltaic panels convert light directly to electricity, also intermittently; manufacturing them uses energy and some hazardous materials, and large farms take land. Geothermal energy is heat from Earth's interior, used in two very different ways. Geothermal power plants drill into reservoirs of hot water or steam, which are close enough to the surface only in certain places, such as Iceland or parts of the western United States, and use the steam to spin turbines; output is steady rather than intermittent, and emissions are far lower than from fossil-fuel plants, but the reservoir fluids carry hydrogen sulfide, which plants remove with scrubbers, and most plants inject the used water back underground to keep the reservoir from being drawn down. Ground-source heat pumps generate no electricity: they use electricity to move heat between a building and the shallow ground, whose temperature stays nearly steady through the year, so they can heat in winter and cool in summer almost anywhere, including Ohio. Biomass (wood, crop residue, biogas from landfills and manure) is renewable if regrowth keeps pace, but burning it still releases carbon dioxide and particulates, and crops grown for fuel compete with food. Tidal and wave power are predictable and, so far, small.

Efficiency. Efficiency is the fraction of energy input that becomes useful output: efficiency = useful energy out ÷ energy in. No conversion is perfect; the difference leaves as heat. Thermal power plants typically turn only about a third of the fuel's energy into electricity in coal and nuclear plants, and about half or a little more in the best modern combined-cycle gas plants, because the physics of heat engines limits them; the rest warms cooling water or the air. Transmission loses a few percent more. At the end of the wire, an incandescent bulb turns only a few percent of its electricity into light, whereas a light-emitting diode turns a much larger share, so the same light can be had for roughly a fifth of the electricity or less. Multiplying the stages shows why conservation at the point of use is so powerful: saving one unit of electricity in a house supplied by a coal or nuclear plant avoids using about three units of fuel energy at the plant.

A worked model in the prose: an energy audit. Suppose a hypothetical household's daily electricity use, calculated from the wattage and hours of each device (energy in kilowatt-hours = watts × hours ÷ 1,000), is: refrigerator 1.5 kWh; lighting, ten 60 W bulbs for 5 hours, 10 × 60 × 5 ÷ 1,000 = 3.0 kWh; television, 100 W for 4 hours, 0.4 kWh; electric water heater, 4,000 W for 1.5 hours, 6.0 kWh; laptop, 50 W for 6 hours, 0.3 kWh. Total 11.2 kWh per day. At a hypothetical price of $0.14 per kWh, that is $1.57 per day, about $47 for a 30-day month. Replacing the ten bulbs with 9 W LEDs gives 10 × 9 × 5 ÷ 1,000 = 0.45 kWh, a saving of 2.55 kWh per day, which is 2.55 ÷ 11.2 ≈ 23% of the household's use, from one change. The water heater is the largest single user, so a shorter shower or a lower thermostat setting would be the next target. An audit ranks options by data rather than by habit.

Resource availability. A reserve is the quantity of a resource that is known and can be extracted profitably with current technology; a resource is the larger total that exists, including deposits not yet found or not yet economic. Reserves grow when prices rise or technology improves (hydraulic fracturing turned known but unreachable gas into reserves) and shrink as they are used. Dividing reserves by the annual rate of use gives a years-of-supply figure that is only a snapshot: if use grows, supply lasts less time; if new reserves are proven, more. Availability also depends on where the resource is and who controls it, on the infrastructure to move it, and on the energy it takes to get it: a deposit that takes nearly as much energy to extract as it yields is not a useful resource whatever its size.

Mining and resource extraction. Every source begins with extraction. Surface mining strips overburden to reach coal or ore, destroying habitat, generating acid drainage and leaving spoil; mountaintop removal in Appalachia buries valley streams. Underground mining disturbs less surface but risks collapse, subsidence and methane explosions, and Ohio still has land sinking over abandoned mines. Oil and gas drilling brings brine and drilling waste to the surface, and offshore accidents release oil to the sea. Uranium mining leaves radioactive tailings. Renewables are not exempt: turbines, panels and batteries need copper, lithium, cobalt and rare-earth metals whose mining has its own footprint, often in other countries. Feasibility, environmental cost and remediation belong in every comparison: the fair question is not "which source has no cost" but "which mix of sources, used efficiently, has the lowest total cost for the energy actually needed."

What the comparison cannot settle. The science can quantify emissions, land use, water use and risk. It cannot decide how a community weighs a rare severe accident against a continuous small harm, or present costs against future ones. That weighing is a policy choice, informed by the science but not made by it.

Worked chain efficiency: Trace 100 J of chemical energy in coal to light from an incandescent bulb, using hypothetical but typical efficiencies. Plant: 36% efficient, so 36 J of electricity. Transmission: 90% delivered, so 32.4 J reaches the house. Bulb: 5% becomes light, so 1.6 J of light. Overall efficiency 1.6%. With an LED at 30%: 32.4 × 0.30 = 9.7 J of light from the same coal, six times more. Alternatively, for the same 1.6 J of light the LED needs only about 5.3 J at the house (1.6 ÷ 0.30) and about 16.5 J of coal (5.3 ÷ 0.9 ÷ 0.36) instead of 100 J. Condition: the stage efficiencies are typical ranges, not measurements of any particular plant or bulb.

Worked reserve lifetime: A hypothetical region has proven reserves of 1,000 units of a fuel and uses 40 units per year. At constant use: 1,000 ÷ 40 = 25 years. If use grows 3% per year, the cumulative use passes 1,000 units in about 19 years. If exploration proves another 500 units, the constant-use estimate becomes 37.5 years. Conclusion: a years-of-supply figure is a snapshot that depends on assumptions about use and discovery; report the assumptions with the number.

Worked source comparison for a hypothetical town: A town of 5,000 in a windy, flat farming region wants to add local generation. Wind: the resource is present, land is available, but output is intermittent and must be backed by the grid or storage; costs are turbine materials, some bird and bat deaths, and neighbors' objections to noise and views. Solar on rooftops: no new land, output peaks in summer afternoons when demand is high, but is low in winter and at night. Natural gas plant: reliable and dispatchable, but carbon dioxide and methane leakage. A defensible design combines wind and rooftop solar with grid connection, and cuts demand first with an efficiency program, because the cheapest kilowatt-hour is the one not used.

Worked appliance calculation: A space heater rated 1,500 W runs 6 hours a day for a 30-day month. Energy = 1,500 × 6 ÷ 1,000 = 9 kWh per day, 270 kWh per month. At a hypothetical $0.14 per kWh: $37.80. Compare an electric blanket at 100 W for the same hours: 0.6 kWh per day, 18 kWh per month, $2.52. The heater warms the room; the blanket warms the person. Which is appropriate depends on what is actually needed, which is the audit question.

Representation task: Build a comparison table with one row per source (coal, oil, natural gas, nuclear, hydro, wind, solar, geothermal, biomass, tidal) and columns for renewable or nonrenewable, whether output is continuous or intermittent, main environmental costs in extraction, main environmental costs in use, and one place it is used. Fill it from the lesson and from sources you actually consult; leave a cell blank rather than guess, and mark every number as approximate.

Safe investigation, a classroom energy audit: Count the light fixtures in your classroom and read the wattage printed on the bulbs or tubes (with the lights off and cool; do not open fixtures). Record the hours the room is lit on a school day. Calculate daily and weekly kilowatt-hours. Then, with the room empty, count devices left on standby and, if a plug-in power meter is available, measure the standby draw of two of them. Propose two conservation measures with the kilowatt-hours each would save, showing the calculation. If your school will share a monthly electricity bill, compare your lighting total with the building total and state what fraction lighting represents. Do not touch wiring, panels or meters.

Independent performance: Write a letter of at least 250 words to a hypothetical school board recommending one change to the school's energy use or supply. Success criteria: it names the current source of the school's electricity as far as you can determine (or states that you could not); it includes one calculation of energy or cost saved, with the assumptions stated and every price labeled hypothetical unless taken from a bill; it names one environmental cost of the current source and one of the proposed alternative, including extraction; it distinguishes renewable from nonrenewable correctly; and it identifies one question that science alone cannot answer for the board.