Notes
Energy basics
- Energy (kWh) $=$ power (kW) $\times$ time (h). Efficiency $\displaystyle = \frac{\text{useful output}}{\text{input}} \times 100\%$; much energy becomes waste heat.
- Fossil fuels (about 80% of world energy) are nonrenewable. Renewables are replenished naturally.
Nonrenewable sources
- Coal emits the most $CO_2$ per unit energy plus $SO_2$ and mercury. Oil brings spill and air risks. Natural gas is cleanest of the three but is mostly methane, a greenhouse gas.
- Nuclear fission emits almost no $CO_2$ in operation but leaves long-lived radioactive waste and has accident risks. Half-life is the time for half of a radioactive sample to decay. Views differ on nuclear power.
Renewable sources
- Solar (PV and thermal), wind, hydro, geothermal, biomass, and hydrogen each have trade-offs: intermittency, land use, habitat impacts, location limits, or food competition.
Efficiency and conservation
- Conservation is using less; efficiency is getting the same service with less energy. Examples: LEDs, insulation, hybrid cars, cogeneration. The rebound effect can offset savings.
Topics in this unit
Each topic has its own short notes and practice.
Practice
$2 \times 6 = 12$ kWh, and $12 \times $0.10 = $1.20$.
Earth's internal heat is continually available on a human timescale.
Three half-lives: $160 \to 80 \to 40 \to 20$.
Coal's carbon content and impurities increase emissions.
$300 \times 0.33 = 99$. The rest is waste heat.
Dams change flow, trap sediment, and flood land.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A town of 20,000 people wants to replace its coal-fired power plant. Options are (1) a natural gas plant, (2) a solar and wind farm with battery storage, or (3) a nuclear plant.
(a) Describe one environmental advantage and one disadvantage of the natural gas option.
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Advantage: it releases less $CO_2$ and fewer air pollutants than coal. Disadvantage: it still emits $CO_2$, and leaks of methane add to climate change; fracking can affect water.
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(b) Explain one challenge of the solar and wind option and a way to address it.
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Solar and wind are intermittent because they depend on weather and time of day. Batteries, other storage, connecting to a larger grid, or combining with other sources can address this.
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(c) The nuclear plant would have 35% efficiency and use 600 MW of thermal energy. Calculate the electrical power produced and explain what happens to the remaining energy.
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Electrical power $= 600 \times 0.35 = 210$ MW. The other 390 MW is released as waste heat, which is removed by cooling water and can cause thermal pollution.
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