Offshore wind farms use large wind turbines placed in coastal waters to generate electricity from strong sea winds. Winds over the ocean are often faster and steadier than winds over land because there are fewer obstacles to slow the moving air. This makes offshore wind a powerful renewable energy source for cities and industries near coastlines.
Understanding these machines connects physics, engineering, weather, and electric power systems.
Key Facts
- Wind power available to a turbine is Pwind = 1/2 ρ A v^3, where ρ is air density, A is swept area, and v is wind speed.
- The swept area of a turbine rotor is A = πr^2, so longer blades capture energy from a much larger area.
- Turbine electrical output is approximately Pelectric = Cp η 1/2 ρ A v^3, where Cp is the power coefficient and η is drivetrain and generator efficiency.
- No wind turbine can capture all wind energy because the Betz limit gives Cp max = 0.593.
- Offshore wind farms use subsea power cables to carry electricity from turbines to an offshore substation and then to shore.
- Fixed foundations are common in shallow water, while floating platforms are used in deeper water and are held in place by mooring lines.
Vocabulary
- Offshore wind farm
- A group of wind turbines installed in the ocean or large lakes to generate electricity from moving air.
- Rotor swept area
- The circular area covered by the spinning turbine blades, which determines how much wind energy the turbine can intercept.
- Nacelle
- The housing at the top of a wind turbine tower that contains the gearbox, generator, brakes, and control systems.
- Subsea cable
- An insulated power cable laid on or under the seabed to transmit electricity from offshore turbines toward land.
- Floating foundation
- A buoyant support structure that keeps a wind turbine upright in deep water while mooring lines anchor it to the seabed.
Common Mistakes to Avoid
- Treating wind speed as a small detail is wrong because wind power depends on v^3, so doubling wind speed can increase available power by a factor of eight.
- Forgetting to square the blade radius when finding swept area is wrong because A = πr^2, not πr, and blade length has a large effect on energy capture.
- Assuming turbines produce rated power at all times is wrong because output changes with wind speed and turbines shut down in very low or dangerously high winds.
- Thinking all offshore turbines are attached the same way is wrong because shallow sites often use fixed foundations, while deep sites usually need floating platforms and moorings.
Practice Questions
- 1 A turbine has blade radius 80 m. Calculate its swept area using A = πr^2. Use π = 3.14.
- 2 Wind speed at a site increases from 8 m/s to 10 m/s. By what factor does the available wind power increase, assuming air density and rotor area stay the same?
- 3 Explain why an offshore wind farm might send power through an offshore substation before the electricity travels to shore by subsea cable.