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Unit 6
10–15% of examAlmost everything you do runs on energy, and most of the world's energy still comes from burning fossil fuels. This unit compares the main energy sources (coal, oil, natural gas, nuclear, biomass, solar, water, geothermal, hydrogen and wind) by how they work and what they cost the environment. It ends with what is often the cheapest option: using less.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Nonrenewable resources like coal, oil, natural gas and uranium form far more slowly than you use them, so they eventually run out. Renewable resources like sunlight, wind and moving water are replaced naturally, though some, like wood, only stay renewable if you don't use them faster than they regrow.
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Fossil fuels supply most of the energy the world uses. Wealthy industrialized countries use far more energy per person than developing countries, and demand is growing fastest as developing countries industrialize; price, availability and government policy all shape which fuels a country uses.
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Wood is still a main fuel in many developing countries. Coal forms in stages from peat to lignite, bituminous and finally anthracite, which holds the most energy; natural gas is mostly methane and burns cleaner than coal or oil; crude oil is refined into gasoline, diesel and other products. Cogeneration uses one fuel to make both electricity and useful heat.
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Fossil fuels, uranium, and good places for solar, wind, geothermal and hydro power aren't spread evenly around the world. That uneven spread decides which energy sources a region can use and how much it has to rely on imports.
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Burning coal, oil or natural gas releases stored chemical energy that heats water into steam; the steam spins a turbine connected to a generator to make electricity, and the burning releases CO₂ and other air pollutants. Getting the fuel out has costs too: hydraulic fracturing (fracking) can contaminate groundwater and release volatile organic compounds, and oil from tar sands takes a lot of water and energy to extract.
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Nuclear plants split uranium-235 atoms (nuclear fission) to release heat, which boils water to spin a turbine. They release no CO₂ while running, but they leave radioactive waste that stays dangerous for thousands of years, and accidents like Three Mile Island, Chernobyl and Fukushima show the risks.
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Biomass is fuel from living or recently living things, like wood, charcoal, crop waste and dried manure. Burning it is cheap but releases CO₂, carbon monoxide, nitrogen oxides and particulates, and cutting too many trees for fuel causes deforestation. Biofuels like ethanol (often from corn or sugarcane) and biodiesel can replace some gasoline and diesel, but corn ethanol gives back only a little more energy than it takes to grow and make.
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Photovoltaic (PV) solar cells turn sunlight directly into electricity, while active and passive solar designs capture the sun's heat for buildings and water. Solar makes no air pollution while running, but panels need mined metals, and large solar farms take up land and can disturb desert habitats.
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Hydroelectric power uses flowing or falling water, usually held behind a dam, to spin turbines; tidal power uses the rise and fall of tides. It's renewable and makes no air pollution once built, but dams flood land, block fish migration, trap sediment and change the river downstream.
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Geothermal power taps heat from inside the earth, which comes partly from radioactive decay deep underground, to make steam that spins turbines or to heat buildings directly. It only works well where hot rock is close to the surface, and it can release hydrogen sulfide gas.
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A hydrogen fuel cell combines hydrogen with oxygen to make electricity, and its only exhaust is water. The catch is that pure hydrogen has to be produced first, often from natural gas or by using electricity to split water, and it's hard to store and transport.
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Wind turbines turn the energy of moving air into electricity by spinning a generator. Wind is renewable, cheap to run and pollution-free while operating, but turbines can kill birds and bats, and the wind doesn't always blow.
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Using less energy can be as simple as changing the thermostat or as big as how a building is designed. Efficient appliances and LED bulbs, better insulation, passive solar design, fuel economy (CAFE) standards, hybrid and electric cars, and public transportation all cut energy use and pollution.
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