Renewable resources can be replenished on human timescales (solar, wind, biomass, geothermal, tidal). Nonrenewable resources exist in finite quantities and take millions of years to form (fossil fuels, uranium, minerals).
Energy Resources Comparison
Resource
Type
Advantages
Disadvantages
Solar
Renewable
Abundant, no emissions, low maintenance
Intermittent, land use, manufacturing impacts
Wind
Renewable
No emissions, small footprint, cheap
Intermittent, bird/bat mortality, aesthetic
Coal
Nonrenewable
Abundant, reliable, cheap
Most CO₂ per unit energy, mining impacts, air pollution
Natural Gas
Nonrenewable
Cleanest fossil fuel, flexible
Methane leaks, fracking water pollution, still GHG
Nuclear
Nonrenewable
No CO₂ emissions, high energy density
Radioactive waste, meltdown risk, expensive
Key Concept
The distinction isn't always clear: biomass is renewable only if harvested sustainably; soil takes so long to form it's effectively nonrenewable. Nuclear fuel (uranium) is nonrenewable but lasts much longer than fossil fuels.
MCQ · Topic 6.1
Which energy source produces the most CO₂ emissions per unit of energy generated?
(A) Coal
(B) Natural gas
(C) Nuclear
(D) Petroleum
Answer: (A) — Coal produces ~2x the CO₂ per unit energy compared to natural gas and more than petroleum. It is the most carbon-intensive fossil fuel.
Common Mistakes
❌ Thinking all renewable energy is "clean": Hydroelectric dams flood habitats and displace communities; biomass releases CO₂ when burned; solar panel manufacturing uses toxic chemicals; wind turbines kill birds and bats. "Renewable" means replenishable, NOT zero-impact.
❌ Classifying natural gas as non-fossil fuel: Natural gas (methane, CH₄) IS a fossil fuel — it formed from ancient organic matter. It burns cleaner than coal or oil but still emits CO₂ and is a potent GHG when it leaks unburned.
Topic 6.2
Global Energy Consumption
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Global energy consumption is ~580 EJ/year and rising. Fossil fuels provide ~80% of global energy. Developing nations are rapidly increasing consumption as they industrialize, while developed nations consume disproportionately more per capita.
Global Energy Patterns
Per Capita Disparity
USA uses ~300 GJ/person/year; India ~25 GJ/person/year. The top 10% of income earners produce ~50% of global CO₂ emissions.
Energy Mix Trends
Renewables growing fastest (~30% of electricity globally), but fossil fuels still dominate total energy (including transport, heating, industry).
Energy Poverty
~770 million people lack electricity access. Clean cooking fuels unavailable to ~2.6 billion, causing ~4 million deaths/year from indoor air pollution.
MCQ · Topic 6.2
Which statement about global energy consumption is most accurate?
(A) Renewable energy provides most of the world's energy
(B) Fossil fuels provide approximately 80% of global energy
(C) Per capita energy use is roughly equal across all nations
(D) Global energy consumption has been declining since 2000
Answer: (B) — Despite growth in renewables, fossil fuels (coal, oil, natural gas) still supply approximately 80% of the world's total primary energy.
Topic 6.3
Fuel Types and Uses
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Different fuels serve different sectors: petroleum dominates transportation (~92%), coal and natural gas dominate electricity, natural gas heats buildings, and biomass remains the primary cooking fuel in developing nations.
Fuel Characteristics and Uses
Fuel
Primary Use
Energy Density
CO₂ per Unit Energy
Coal
Electricity generation, steel production
24 MJ/kg
Highest
Petroleum/Oil
Transportation, petrochemicals
42 MJ/kg
Medium-high
Natural Gas
Electricity, heating, cooking
55 MJ/kg
Lowest of fossil fuels (~50% of coal)
Uranium
Nuclear electricity
80,000,000 MJ/kg
Zero during operation
MCQ · Topic 6.3
Which fossil fuel emits the least CO₂ per unit of energy produced?
(A) Coal
(B) Petroleum
(C) Natural gas
(D) Wood biomass
Answer: (C) — Natural gas (methane, CH₄) produces ~50% less CO₂ per unit energy than coal because it has a higher hydrogen-to-carbon ratio.
Topic 6.4
Distribution of Natural Energy Resources
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Energy resources are unevenly distributed globally, creating geopolitical dependencies. The Middle East holds ~48% of proven oil reserves; Russia, Iran, and Qatar hold ~50% of natural gas; and the US, Russia, India, China, and Australia dominate coal reserves.
Exam Connection
Uneven distribution drives: international trade and geopolitics, energy security concerns, resource conflicts, and the push for energy independence through renewables (which are more evenly distributed — every country has sun and wind).
MCQ · Topic 6.4
One advantage of renewable energy sources over fossil fuels in terms of global distribution is that
(A) renewable sources have higher energy density
(B) solar and wind resources are available in most regions
(C) renewable sources are cheaper to extract
(D) fossil fuels are found in every country
Answer: (B) — Unlike fossil fuels concentrated in specific regions, solar and wind energy are widely available globally, reducing geopolitical dependencies.
Topic 6.5
Fossil Fuels
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Fossil fuels formed from ancient organic matter buried under heat and pressure for millions of years. Extraction methods include conventional drilling, hydraulic fracturing (fracking), mountaintop removal, and offshore drilling.
Oil spills, habitat disruption, water contamination
Fracking: water use, induced seismicity, methane leaks
Air pollution
SO₂, NOₓ, particulates, mercury
NOₓ, VOCs
Cleanest burning fossil fuel
Fracking Concerns
Hydraulic fracturing injects high-pressure water + sand + chemicals underground to crack shale rock and release trapped gas/oil. Concerns: groundwater contamination, induced earthquakes, methane leaks (potent GHG), massive water use (2-8 million gallons/well).
MCQ · Topic 6.5
Hydraulic fracturing has raised environmental concerns primarily because it can
(A) increase carbon dioxide absorption by oceans
(B) contaminate groundwater and induce seismic activity
(C) deplete the ozone layer
(D) increase ocean acidification
Answer: (B) — Fracking chemicals and methane can migrate into groundwater aquifers, and the injection of wastewater can trigger earthquakes in fault-prone areas.
Common Mistakes
❌ Calling natural gas "clean energy": Natural gas is the CLEANEST fossil fuel (~50% less CO₂ than coal), but it is NOT clean energy. It still emits CO₂ when burned, and methane leaks during extraction/transport are a potent greenhouse gas (~80× stronger than CO₂ over 20 years).
❌ Confusing extraction methods: Fracking = injecting water/chemicals to crack shale for gas/oil. Mountaintop removal = blasting hilltops for coal. Strip mining = removing surface layers for coal/minerals. Each has distinct environmental impacts.
Topic 6.6
Nuclear Energy
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Nuclear power uses nuclear fission (splitting uranium-235 or plutonium-239 atoms) to generate heat → steam → electricity. It provides ~10% of global electricity with zero direct CO₂ emissions during operation.
Advantages
No CO₂ during operation, extremely high energy density, reliable baseload power, small land footprint per kWh.
Disadvantages
Radioactive waste (half-lives: thousands-millions of years), meltdown risk (Chernobyl, Fukushima), high construction cost, uranium mining impacts, weapons proliferation.
Waste Storage
Spent fuel is stored on-site in cooling pools then dry casks. No permanent repository in the US (Yucca Mountain proposed but stalled). Half-life of Pu-239: 24,000 years.
Nuclear Fusion
Fusing hydrogen → helium (like the sun). Potentially unlimited, clean energy. Not yet commercially viable (ITER experimental reactor in France).
MCQ · Topic 6.6
The primary environmental concern with nuclear energy is
(A) high CO₂ emissions during electricity generation
(B) release of sulfur dioxide and particulate matter
(C) long-term storage of radioactive waste
(D) depletion of freshwater resources
Answer: (C) — Radioactive waste remains dangerous for thousands to millions of years and currently has no permanent storage solution in many countries.
Common Mistakes
❌ Confusing fission and fusion: The AP exam tests fission (splitting heavy atoms like U-235 → releases energy, produces radioactive waste). Fusion (combining light atoms like hydrogen) powers the sun but is NOT commercially viable yet. Do not mix them up.
❌ Saying nuclear emits CO₂: Nuclear fission produces ZERO direct CO₂ emissions during electricity generation. Emissions occur only during construction, uranium mining, and fuel processing — the operational phase is carbon-free.
FRQ-Style · Topic 6.6
A country currently generates 70% of its electricity from coal and is considering building nuclear power plants to reduce carbon emissions. (a) Explain how nuclear fission generates electricity. (b) Describe two environmental advantages of nuclear power over coal. (c) Describe two environmental concerns associated with nuclear energy and propose a solution for one of them.
(a) Nuclear fission splits heavy atomic nuclei (uranium-235) when struck by neutrons, releasing enormous heat energy. This heat is used to boil water into steam, which spins turbines connected to generators that produce electricity. The chain reaction is controlled by control rods that absorb excess neutrons.
(b) (1) Zero direct CO₂ emissions — unlike coal, which is the most carbon-intensive fuel (~2× more CO₂ than natural gas per unit energy), nuclear fission produces no greenhouse gases during operation. (2) No air pollutants — coal combustion releases SO₂ (acid rain), NOₓ (smog), particulate matter, and mercury; nuclear power produces none of these, significantly improving air quality and public health.
(c) (1) Radioactive waste — spent fuel remains dangerously radioactive for thousands to millions of years (Pu-239 half-life: 24,000 years), with no permanent storage solution currently operating in most countries. (2) Meltdown risk — reactor failures (Chernobyl 1986, Fukushima 2011) can release radiation over large areas, contaminating land and water for decades. Solution for waste: Develop deep geological repositories (like Finland's Onkalo facility) that store waste in stable rock formations hundreds of meters underground, isolated from groundwater and seismic activity.
Topic 6.7
Energy from Biomass
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Biomass energy comes from organic matter: wood, crop residues, animal waste, biofuels (ethanol, biodiesel). It is considered carbon-neutral IF regrowth absorbs the CO₂ released during combustion — but this depends on sustainable management.
Types of Biomass Energy
Type
Source
Pros
Cons
Ethanol
Corn, sugarcane fermentation
Reduces petroleum dependence, lower CO₂ than gasoline
Land use competition with food, water-intensive, energy input debate
Biodiesel
Vegetable oils, animal fats
Uses waste oils, lower emissions
Land use, palm oil deforestation
Biogas
Anaerobic digestion of waste
Uses waste, reduces methane emissions from landfills
Small scale, odor issues
Wood/Charcoal
Trees, woody plants
Widely available, traditional fuel
Deforestation, indoor air pollution (developing nations)
MCQ · Topic 6.7
A major criticism of using corn-based ethanol as a biofuel is that it
(A) competes with food production for agricultural land
(B) produces more CO₂ per unit energy than coal
(C) requires uranium for processing
(D) cannot be used in existing vehicles
Answer: (A) — Growing corn for ethanol diverts cropland from food production, potentially raising food prices and incentivizing land conversion.
Topic 6.8
Solar Energy
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Solar energy is captured via photovoltaic (PV) cells (convert light directly to electricity) and concentrated solar power (CSP) (mirrors focus sunlight to heat fluid → steam → turbine). Solar is now the cheapest electricity source in many regions.
Passive Solar
Building design that maximizes natural heating/cooling: south-facing windows, thermal mass, overhangs for summer shade. No technology needed.
Active Solar
PV panels, solar water heaters, CSP plants. PV efficiency ~20-25% for commercial panels; improving rapidly.
Limitations
Intermittent (night, clouds), requires energy storage (batteries), land use for large installations, manufacturing uses toxic chemicals (cadmium, silicon).
MCQ · Topic 6.8
The most significant limitation of solar energy as a primary electricity source is that it
(A) produces large amounts of greenhouse gases
(B) is intermittent and requires energy storage
(C) is only available in tropical regions
(D) has a larger carbon footprint than natural gas
Answer: (B) — Solar energy is intermittent — unavailable at night and reduced by clouds — requiring battery storage or backup sources for reliable electricity.
Topic 6.9
Hydroelectric Power
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Hydroelectric power uses the kinetic energy of flowing water to spin turbines. Provides ~16% of global electricity. Largest source of renewable electricity worldwide.
Hydroelectric Pros and Cons
Pros
Cons
No direct CO₂ emissions
Habitat destruction from reservoir flooding
Reliable, controllable output
Blocks fish migration (salmon)
Long lifespan (50-100 years)
Displaces communities (Three Gorges: 1.3M people)
Flood control, recreation, water supply
Sediment trapping downstream, altered river ecology
Can store energy (pumped storage)
Methane from decomposing vegetation in reservoirs (tropical)
MCQ · Topic 6.9
Large hydroelectric dams can negatively affect downstream ecosystems by
(A) increasing water temperature through nuclear processes
(B) trapping sediment and disrupting natural flow patterns
(C) releasing sulfur dioxide into the atmosphere
(D) increasing the salinity of river water
Answer: (B) — Dams trap sediment in reservoirs, depriving downstream areas of nutrient-rich sediment that maintains river deltas, floodplains, and aquatic habitats.
Topic 6.10
Geothermal Energy
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Geothermal energy harnesses heat from Earth's interior. Available 24/7 (not intermittent), but limited to geologically active areas near tectonic plate boundaries or hot spots. Iceland gets ~25% of its electricity from geothermal.
Ground-Source Heat Pumps
Unlike geothermal power plants, ground-source heat pumps can work ANYWHERE. They use the constant underground temperature (~10-15°C) for heating in winter and cooling in summer. Not the same as geothermal electricity generation.
MCQ · Topic 6.10
Geothermal energy is most available in regions with
(A) thick continental crust
(B) active tectonic plate boundaries
(C) large fossil fuel reserves
(D) tropical climates
Answer: (B) — Geothermal energy requires heat close to the surface, which occurs at tectonic plate boundaries, volcanic areas, and hot spots (Iceland, western US, New Zealand).
Common Mistakes
❌ Thinking geothermal works everywhere: Geothermal electricity generation requires volcanic/tectonic activity with heat close to the surface. It is location-dependent — only viable near plate boundaries or hot spots. Do NOT confuse it with ground-source heat pumps, which work anywhere.
❌ Confusing geothermal power plants with ground-source heat pumps: Geothermal plants use underground steam/hot water to generate electricity (limited locations). Ground-source heat pumps use the constant ~10-15°C underground temperature for building heating/cooling (work everywhere). The AP exam tests both.
Topic 6.11
Hydrogen Fuel Cells
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Hydrogen fuel cells combine hydrogen and oxygen to produce electricity, with water as the only byproduct: 2H₂ + O₂ → 2H₂O + energy. The challenge: hydrogen must be produced (it's an energy carrier, not a source), and most current production uses natural gas (gray hydrogen).
Green Hydrogen
Produced by electrolysis of water using renewable electricity. Truly zero-emissions. Currently expensive but costs dropping.
Gray Hydrogen
Produced from natural gas (steam methane reforming). Cheapest but produces CO₂. ~95% of current hydrogen production.
Blue Hydrogen
Gray hydrogen + carbon capture and storage (CCS). Lower emissions than gray, but CCS is not 100% effective.
MCQ · Topic 6.11
The only direct byproduct of a hydrogen fuel cell is
(A) carbon dioxide
(B) water
(C) methane
(D) nitrogen oxides
Answer: (B) — Hydrogen fuel cells combine H₂ and O₂ to produce electricity and water (H₂O) as the only byproduct. However, the environmental impact depends on HOW the hydrogen was produced.
Topic 6.12
Wind Energy
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Wind energy converts kinetic energy of wind into electricity via turbines. Fastest-growing energy source globally. Onshore wind is now one of the cheapest electricity sources ($30-60/MWh). Offshore wind captures stronger, more consistent winds.
Wind Energy Pros and Cons
Pros
Cons
No emissions or water use during operation
Intermittent (no wind = no power)
Small land footprint (farming beneath turbines)
Bird and bat mortality (200,000-500,000 birds/year in US)
Rapidly falling costs
Visual and noise impacts
Domestic energy, rural economic benefits
Requires backup or storage
MCQ · Topic 6.12
An environmental concern unique to wind energy compared to solar energy is
(A) bird and bat mortality from turbine blades
(B) greenhouse gas emissions during operation
(C) large water requirements for cooling
(D) production of radioactive waste
Answer: (A) — Wind turbines can kill birds and bats that collide with spinning blades, a concern not shared by solar panels.
Topic 6.13
Energy Conservation
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Energy conservation (using less energy) and energy efficiency (getting the same output with less energy input) are the cheapest, fastest ways to reduce environmental impact. The cleanest energy is the energy you don't use.
Conservation
Behavioral change: turning off lights, carpooling, lowering thermostat, air-drying clothes. Requires no new technology.
Efficiency
Technological improvement: LED bulbs (90% less energy than incandescent), ENERGY STAR appliances, hybrid vehicles, insulation, smart thermostats.
Cogeneration (CHP)
Combined Heat and Power: uses waste heat from electricity generation for heating/industrial processes. Increases overall efficiency from ~35% to ~80%.
Key Concept
First law of thermodynamics: energy cannot be created or destroyed, only converted. Second law: every conversion loses energy as waste heat. This is why efficiency can never reach 100% and why reducing energy waste is so impactful.
MCQ · Topic 6.13
Replacing incandescent light bulbs with LED bulbs is an example of
(A) energy conservation
(B) energy efficiency
(C) cogeneration
(D) fuel switching
Answer: (B) — Energy efficiency means getting the same service (light) with less energy input. LEDs use ~90% less electricity than incandescent bulbs for the same lumens. Conservation would be turning lights off.
Common Mistakes
❌ Confusing energy efficiency with energy conservation:Efficiency = getting the SAME output with LESS energy input (LED bulbs, hybrid cars, insulation). Conservation = REDUCING energy use through behavior change (turning off lights, carpooling, lowering thermostat). Both reduce consumption but through different mechanisms.
❌ Forgetting cogeneration (CHP): Combined Heat and Power captures waste heat from electricity generation to heat buildings or industrial processes, raising overall efficiency from ~35% to ~80%. Students often overlook this as a key efficiency strategy on the AP exam.
FRQ-Style · Topic 6.13
A school district wants to reduce its energy costs and environmental impact. (a) Distinguish between energy efficiency and energy conservation, providing one specific example of each that the district could implement. (b) Explain the concept of cogeneration and how the district could benefit from it. (c) Using the second law of thermodynamics, explain why energy efficiency can never reach 100%.
(a)Energy efficiency means getting the same service with less energy input. Example: Replacing incandescent lighting with LED bulbs in all classrooms — LEDs produce the same lumens using ~90% less electricity. Energy conservation means reducing energy use through behavioral changes. Example: Implementing a policy to turn off all lights, computers, and HVAC in unoccupied rooms — reducing total energy consumption without new technology.
(b)Cogeneration (Combined Heat and Power) captures waste heat produced during electricity generation and uses it for a useful purpose such as heating buildings. The district could install a CHP system that generates electricity for the school while using the waste heat to warm classrooms and heat water. This raises overall energy efficiency from ~35% (typical power plant) to ~80%, dramatically reducing fuel consumption and costs.
(c) The second law of thermodynamics states that every energy conversion increases entropy — some energy is inevitably lost as waste heat that cannot be recovered. No device can convert 100% of energy input into useful work. For example, even the most efficient LED still converts some electricity into heat rather than light. This fundamental physical law means efficiency improvements can approach but never reach 100%.
Exam Prep
Comprehensive Practice Questions
Mixed MCQ and FRQ in AP APES exam style. Attempt each before revealing the answer.
MCQ · Topics 6.1, 6.5, 6.6
A country currently relies on coal for 60% of its electricity. Officials are debating whether to replace coal plants with nuclear or natural gas. Which statement best compares the environmental trade-offs?
(A) Both nuclear and natural gas produce zero greenhouse gas emissions
(B) Nuclear produces more CO₂ than natural gas but less radioactive waste
(C) Nuclear produces no operational CO₂ but creates radioactive waste; natural gas emits less CO₂ than coal but still contributes to climate change
(D) Natural gas is a renewable resource that produces no greenhouse gases
Answer: (C) — Nuclear fission produces zero CO₂ during operation but generates radioactive waste requiring long-term storage. Natural gas emits ~50% less CO₂ than coal but is still a fossil fuel that contributes to climate change, and methane leaks during extraction amplify its warming impact.
MCQ · Topics 6.8, 6.9, 6.12
A region with abundant sunshine, consistent winds, and a major river is planning its renewable energy portfolio. Which combination of energy sources would provide the most reliable electricity supply throughout the year?
(A) Solar only, with battery storage for nighttime
(B) Wind only, supplemented by diesel generators on calm days
(C) Solar, wind, and hydroelectric together, since their intermittency patterns differ and hydro provides controllable baseload
(D) Hydroelectric only, since it is the most reliable single renewable source
Answer: (C) — Diversifying renewable sources addresses intermittency: solar produces during daytime, wind often peaks at night or in winter, and hydroelectric dams can adjust output on demand (dispatchable). This combination provides more consistent power than any single source alone.
FRQ · Topics 6.1, 6.3, 6.13
Global energy consumption continues to rise, with fossil fuels providing approximately 80% of total energy. (a) Explain the concept of EROI (Energy Return on Investment) and why it matters for comparing energy sources. (b) Describe two specific environmental consequences of the world's heavy reliance on fossil fuels. (c) Propose and explain two strategies (one efficiency-based, one conservation-based) that a nation could implement to reduce its fossil fuel dependence.
(a)EROI is the ratio of energy produced by a source to the energy required to extract and process it. An EROI of 10:1 means you get 10 units of energy for every 1 unit invested. Higher EROI = more energy profit = better energy source. Conventional oil historically had EROI of ~30:1 but tar sands may be as low as 3:1. Solar and wind EROI have improved dramatically to ~10-25:1. If EROI approaches 1:1, the source is not worth pursuing.
(b) (1) Climate change — burning fossil fuels releases CO₂ (coal: ~2 lbs CO₂/kWh; gas: ~1 lb CO₂/kWh), the primary driver of global warming, leading to rising sea levels, extreme weather, ocean acidification, and biodiversity loss. (2) Air pollution — coal combustion releases SO₂ (acid rain), NOₓ (smog and respiratory disease), particulate matter (cardiovascular/lung disease), and mercury (neurotoxin that bioaccumulates in fish), causing millions of premature deaths annually.
(c)Efficiency strategy: Mandate building energy codes requiring modern insulation, LED lighting, ENERGY STAR appliances, and smart thermostats in all new construction — reducing energy demand per building by 30-50% while maintaining the same comfort level. Conservation strategy: Implement congestion pricing in major cities (charging vehicles to enter city centers during peak hours), encouraging carpooling, public transit use, cycling, and remote work — directly reducing fossil fuel consumption in transportation, which accounts for ~28% of energy use.
FRQ · Topics 6.7, 6.10, 6.11
A nation is evaluating three alternative energy options to diversify its energy mix: biomass, geothermal, and hydrogen fuel cells. (a) For each energy source, identify one environmental advantage and one limitation. (b) Explain why the environmental benefit of hydrogen fuel cells depends on HOW the hydrogen is produced. (c) Explain why geothermal energy cannot be adopted everywhere despite being renewable and reliable.
(a)Biomass: Advantage — can be carbon-neutral if regrowth absorbs CO₂ released during combustion; uses organic waste that would otherwise decompose and release methane. Limitation — burning biomass still produces air pollutants (particulates, CO); land used for biofuel crops competes with food production. Geothermal: Advantage — available 24/7 (not intermittent like solar/wind), with minimal CO₂ emissions and tiny land footprint. Limitation — restricted to geologically active regions near tectonic plate boundaries, limiting widespread adoption. Hydrogen: Advantage — fuel cells produce only water as a byproduct, with zero emissions at point of use. Limitation — hydrogen is an energy CARRIER, not a source; producing it requires energy input, and storage/transport of compressed hydrogen is expensive and technically challenging.
(b)Green hydrogen (electrolysis powered by renewable energy) is truly zero-emission. Gray hydrogen (~95% of current production) is made from natural gas via steam methane reforming, which releases CO₂ — so the fuel cell vehicle produces no emissions, but the hydrogen production does. The environmental benefit depends entirely on the production method.
(c) Geothermal electricity generation requires underground reservoirs of steam or hot water close to the surface, which only exist near tectonic plate boundaries, volcanic areas, or hot spots. Most of the planet lacks these geological conditions. Countries like Iceland (on the Mid-Atlantic Ridge) and New Zealand (Pacific Ring of Fire) can use geothermal extensively, but countries in stable continental interiors (like much of Africa or central Asia) cannot. Ground-source heat pumps work everywhere but only provide heating/cooling, not electricity generation.
Exam Prep
High-Frequency Common Mistakes — Full Unit 6
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Efficiency ≠ conservationEfficiency = same output, less input (LED bulbs). Conservation = reducing use (turning lights off). The AP exam frequently asks you to distinguish these — know examples of each.
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Fission ≠ fusionAP APES only tests fission (splitting U-235, produces radioactive waste, powers current nuclear plants). Fusion (combining hydrogen, powers the sun) is NOT commercially viable. Never confuse them on the exam.
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Renewable ≠ zero impactHydroelectric dams flood ecosystems and displace communities. Biomass releases CO₂ and competes with food. Wind kills birds. Solar manufacturing uses toxic chemicals. Every energy source has trade-offs.
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EROI: higher = betterEROI (Energy Return on Investment) is the ratio of energy output to energy input. Higher EROI means more net energy per unit invested. Conventional oil (~30:1) >> tar sands (~3:1). If EROI ≈ 1:1, the source is not viable.
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Natural gas IS a fossil fuelDespite being "cleaner" than coal (~50% less CO₂), natural gas is still a nonrenewable fossil fuel. Methane leaks during extraction make its climate impact worse than CO₂ numbers alone suggest.
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Geothermal is location-dependentGeothermal electricity requires tectonic plate boundaries or hot spots. It does NOT work everywhere. Ground-source heat pumps (different technology) work anywhere but only for heating/cooling, not electricity.
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Hydrogen is a carrier, not a sourceHydrogen must be PRODUCED using energy from another source. It stores and transports energy but does not generate it. The environmental benefit depends entirely on whether the electricity used for production is clean.
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Cogeneration raises efficiency to ~80%Combined Heat and Power (CHP) captures waste heat from electricity generation. Normal power plants are ~35% efficient; cogeneration reaches ~80%. This is a frequently tested concept.
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Hydroelectric dams are NOT impact-freeDams block fish migration, flood upstream habitat, trap sediment (starving downstream ecosystems), displace communities, and can release methane from decomposing vegetation in tropical reservoirs.
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Biomass carbon-neutral claim has conditionsBiomass is only carbon-neutral IF regrowth absorbs the same CO₂ released. Burning forests faster than they regrow is NOT carbon-neutral. Corn ethanol's net carbon benefit is debated due to energy inputs for farming and processing.
Unit 6 Strategy
Unit 6 is 10-15% of the AP exam. Highest-yield topics: energy efficiency vs. conservation distinction, fossil fuel comparison (coal > oil > gas for CO₂), nuclear fission pros/cons, and renewable energy trade-offs. The AP exam loves asking you to compare energy sources — practice creating tables that list advantages, disadvantages, and environmental impacts for each. Always consider the FULL life cycle, not just operational emissions.