AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/6/6-12)
Unit 6 · Topic 6.12
6.12 Wind Energy
Wind turbines turn the kinetic energy of moving air into electricity. Wind is renewable, cheap to run and pollution-free once the turbines are built, but it is intermittent, works best only in windy places, and turbines kill some birds and bats.
Key terms
- wind turbine
- kinetic energy
- wind farm
- intermittent energy source
How a wind turbine works
Moving air has kinetic energy, the energy of motion. Wind pushes on a turbine's long blades, which are shaped like airplane wings so the air flowing past them creates lift and turns the rotor. The rotor spins a shaft, a gearbox speeds up the rotation, and the shaft turns a generator inside the housing at the top of the tower.
Turbines are placed on tall towers because wind is stronger and steadier higher up, away from the friction of the ground. A group of turbines in one area is a wind farm. Onshore wind farms are on land; offshore wind farms are in the ocean, where winds are stronger and steadier but building costs are higher.
Wind speed matters a lot. The power in wind rises with the cube of wind speed, so doubling the wind speed gives about 2³ = 8 times as much power. That is why developers carefully pick the windiest sites.
Where wind comes from, and where to build
Wind is really a form of solar energy. The sun heats Earth's surface unevenly, warm air rises, cooler air moves in to replace it, and that movement is wind (topic 4.5 covers the global pattern). Because wind depends on weather, it varies from hour to hour and season to season.
| Feature | Onshore wind | Offshore wind |
|---|---|---|
| Wind | Weaker and gustier, slowed by land | Stronger and steadier over open water |
| Cost | Cheaper to build and maintain | More expensive foundations, cables and repairs |
| Land use | Land between turbines can still be farmed | Uses no land, but can affect seabirds and marine life during construction |
| Visibility and noise | Close to people, so more complaints | Farther from homes |
Advantages
- Renewable, with no fuel to buy, so operating costs are low.
- No air pollution or CO₂ while generating; most of its lifetime emissions come from making and installing the turbines.
- The land between onshore turbines can still be farmed or grazed, and landowners can earn lease payments.
- Wind is now one of the cheapest new sources of electricity in many places.
Disadvantages
- Intermittent: when the wind stops, so does the power. An intermittent energy source needs backup power, energy storage or a wide grid to fill gaps.
- Bird and bat deaths from collisions with spinning blades; careful siting away from migration routes and pausing turbines in low winds at night reduce this.
- Noise and visual impact, which some nearby residents object to.
- The best sites (plains, ridges, offshore) are often far from cities, so new transmission lines are needed.
- Blades are large and hard to recycle when they wear out.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
How many homes can a turbine supply?
A wind turbine is rated at 2 megawatts (MW), but because the wind varies it produces, on average, 35% of that rated output (its capacity factor). An average home uses 10,500 kWh per year. (a) How much electricity does the turbine produce in a year, in MWh? (b) How many homes can it supply? (1 year = 8,760 hours; 1 MWh = 1,000 kWh)
Show the solutionHide the solution
- Step 1: (a) If the turbine ran at full power all year, it would make 2 MW × 8,760 h = 17,520 MWh.
- Step 2: It averages only 35% of that: 17,520 MWh × 0.35 = 6,132 MWh per year.
- Step 3: (b) Convert to kWh: 6,132 MWh × 1,000 = 6,132,000 kWh.
- Step 4: Divide by use per home: 6,132,000 kWh ÷ 10,500 kWh per home = 584 homes.
Answer: (a) 6,132 MWh per year. (b) About 584 homes.
- Example 2
Trap: forgetting intermittency
A student calculates that 100 wind turbines can supply all the yearly electricity a town uses, and concludes the town can shut down its gas plant. What is wrong with that conclusion?
Show the solutionHide the solution
- Step 1: The calculation matches total yearly energy, but electricity has to be supplied at every moment, not just in total over a year.
- Step 2: On calm days, or during calm hours, the turbines make little or no electricity, yet the town still needs power.
- Step 3: Without energy storage (such as batteries), a connection to a wider grid or a backup source, the town would face blackouts.
Answer: Matching annual totals ignores intermittency. The town still needs storage, grid imports or backup power for calm periods.
Common mistakes
- Multiplying the rated power by hours in a year without the capacity factor. Real turbines produce well below their rated output on average.
- Listing 'pollution' as a disadvantage of running turbines. The real disadvantages are intermittency, wildlife deaths, noise, visual impact and siting.
- Saying wind turbines create energy from nothing. They convert the kinetic energy of air, which ultimately comes from the sun heating Earth unevenly.
On the exam
- Name intermittency, or bird and bat deaths, as the main disadvantage, and pair it with a solution such as battery storage or careful siting.
- Expect energy-output calculations with MW, MWh and kWh. Watch the units and include the capacity factor if one is given.
Connected topics
Videos
Check yourself
4 questions on 6.12 Wind Energy. Pick an answer to see if you got it, and why.
A wind turbine rated at 2 megawatts (MW) runs at full power for 6 hours. How much electricity does it generate? (1 MW = 1,000 kW)
Which is a drawback of relying on wind turbines for a large share of a region's electricity?
| Wind speed (m/s) | Turbine power output (kW) |
|---|---|
| 4 | 60 |
| 6 | 200 |
| 8 | 480 |
| 10 | 940 |
| 12 to 25 | 1,500 (maximum) |
| Above 25 | 0 (shut down for safety) |
Hypothetical data for one large wind turbine
Based on the data, when the wind speed doubles from 4 m/s to 8 m/s, the turbine's power output increases by about how many times?
Based on the data, which site would be the best choice for a new wind farm?
0 of 4 answered