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Environmental Science Unit 5: Energy resources

Societies run on energy. Fossil fuels are cheap but polluting; renewables are cleaner but have their own trade-offs.

Covers: Fossil fuels, nuclear power, renewable energy (solar, wind, hydro, geothermal, biomass), energy efficiency

Study guide

Most of the world's energy still comes from fossil fuels: coal, oil and natural gas, formed from ancient organisms over millions of years. They're nonrenewable. Burning them releases CO₂, the main human-caused greenhouse gas, along with pollutants like sulfur dioxide, nitrogen oxides and particulates. Coal releases the most CO₂ per unit of energy and natural gas the least. Extraction through mining, drilling and fracking damages land and water. In a fossil fuel power plant, burning fuel heats water into steam, which spins a turbine connected to a generator, and only about a third of the energy becomes electricity.

Nuclear power uses fission of uranium-235 to heat water. It produces almost no greenhouse gases or air pollution during operation, but it creates long-lived radioactive waste with no permanent storage solution, carries a risk of accidents (Chernobyl, Fukushima), and requires mining and expensive plants.

Renewable energy sources are replenished naturally. Solar energy uses photovoltaic cells or concentrated solar power. Wind turbines convert moving air into electricity. Both are clean but intermittent, so they need storage or backup. Hydroelectric dams provide steady power but flood habitats and block fish migration. Geothermal energy taps heat from inside Earth and is limited to certain regions. Biomass, including wood and biofuels like ethanol, is renewable but can compete with food crops and still emits CO₂. Hydrogen fuel cells produce only water but need energy to produce the hydrogen.

Energy efficiency and conservation are the cheapest ways to reduce environmental impact: insulation, LED lighting, efficient appliances, fuel-efficient vehicles, combined heat and power, and simply using less.

Power is the rate of energy use: 1 kilowatt (kW) = 1,000 watts. Energy is power multiplied by time, so a 60-watt bulb running for 1,000 hours uses 0.060 kW × 1,000 h = 60 kWh. In multistep problems, write out the units and cancel them (dimensional analysis). Nuclear and fossil fuel plants use large amounts of water for cooling and return it warmer. This thermal pollution lowers dissolved oxygen and can push temperatures beyond what aquatic organisms tolerate, and intake pipes can also kill fish and larvae.

Compared with coal, burning natural gas releases much less sulfur dioxide, particulate matter and mercury, and it produces no ash. That's why switching power plants from coal to gas improves local air quality as well as cutting CO₂.

Key ideas

• Coal, oil and natural gas are nonrenewable and release CO₂ when burned.
• Natural gas emits less CO₂ than coal per unit of energy.
• Nuclear fission makes no CO₂ during operation but produces radioactive waste.
• Solar, wind, hydro and geothermal are renewable; their output can vary by place and time.
• Efficiency and conservation are the cheapest ways to cut energy use.

Formulas and rules

Common mistakes

• Calling nuclear power renewable. Uranium is finite.
• Thinking renewables have no environmental impact (dams flood habitat, for example).

Words to know

Natural gas

The fossil fuel with the lowest CO₂ per unit of energy.

Nuclear waste

Long-lived radioactive waste: nuclear power's main problem.

Intermittency

Wind and solar output varies with weather and time of day.

Geothermal energy

Heat from inside Earth.

Energy conservation

Using less energy; the cheapest way to cut demand.

Fossil fuel power plant

Fuel → heat → steam → turbine → generator.

Nonrenewable resource

Forms far slower than it's used, like fossil fuels.

Coal

The fossil fuel with the most CO₂ per unit of energy.

Fracking

Hydraulic fracturing to release natural gas from shale.

Nuclear fission

Splitting uranium-235 nuclei to release heat.

Photovoltaic cell

Converts sunlight directly into electricity.

Hydroelectric power

Electricity from moving water at dams; floods habitat.

Biomass

Energy from organic matter, like wood and ethanol.

Hydrogen fuel cell

Makes electricity from hydrogen; emits only water.

Cogeneration

Using waste heat from power generation (combined heat and power).

Sample practice questions

15 more questions like these, from level 1 to 5, are in the practice set.