Turning moving air into electricity
Wind is moving air, driven by differences in temperature and pressure across the earth’s surface, which in turn come from uneven heating by the sun. A wind turbine captures part of the kinetic energy in that moving air and converts it into electricity.
The process is conceptually simple:
- Wind flows over the turbine’s blades, which are shaped like aircraft wings. The airflow creates lift that makes the blades rotate around a hub.
- The rotating hub turns a shaft. Most large turbines turn slowly, so a gearbox often increases the speed before it reaches the generator. Some designs use a direct-drive generator and skip the gearbox.
- The generator converts the rotation into electricity.
- Power electronics condition the electricity so it matches the grid, and a transformer raises its voltage for the journey through cables to a substation.
Main parts of a modern turbine
- Blades. Typically three, made from lightweight composite materials. Large modern blades can be longer than a wing of a big passenger aircraft, but exact sizes vary by model.
- Rotor and hub. Hold the blades and allow their angle, or pitch, to be adjusted to control power and protect the turbine in strong wind.
- Nacelle. The housing at the top containing the shaft, gearbox if present, generator and control systems.
- Tower. Raises the rotor to where winds are stronger and steadier. Wind speed generally increases with height above ground.
- Yaw system. Turns the nacelle to face the wind.
- Foundation. Concrete on land, or steel structures fixed to or floating over the seabed at sea.
Turbines have a cut-in wind speed below which they do not generate, a rated speed at which they reach their maximum output, and a cut-out speed above which they shut down or feather the blades for safety.
What affects output
The relationship between wind speed and power is steep. The power available in the wind rises roughly with the cube of its speed, so a modest increase in wind speed can mean a large increase in available power. Doubling the wind speed would, in theory, give eight times the power. Turbines are limited, however, by their rated capacity, which caps output once winds are strong enough.
No turbine can capture all the energy in the wind. A well-known theoretical limit, the Betz limit, puts the maximum at about 59 percent of the wind’s kinetic energy. Real turbines achieve less than this once practical losses are included.
Other influences include:
- Rotor size. The area swept by the blades determines how much wind is intercepted, so longer blades capture more.
- Site and terrain. Hills, trees and buildings create turbulence, while open plains and coasts often provide smoother wind.
- Wake effects. Turbines in a wind farm slow the air behind them, so spacing and layout affect the overall result.
- Air density. Cold air is denser and carries more energy than warm air at the same speed.
- Maintenance and availability. Turbines are occasionally shut down for repairs, icing, or grid constraints.
Because wind varies by day and season, the capacity factor of a wind farm is typically well below 100 percent. Actual numbers differ widely between locations, turbine models and onshore and offshore sites, so check data for the specific region.
Onshore and offshore
Onshore wind is generally cheaper to build and easier to maintain, and it can be sited on farmland and open land. Challenges include local opposition over visual impact and noise, bird and bat effects, and the need for land and access roads.
Offshore wind tends to benefit from stronger and steadier winds and allows very large turbines, with fewer neighbors nearby. It is also more expensive to build and maintain, needs specialized vessels, and involves subsea cables and grid connections. Floating foundations are extending the range of possible sites to deeper waters, although the technology is newer and costs are still being worked out.
Strengths and limits
Wind turbines burn no fuel and produce no emissions while running. Land between turbines can often still be used for farming. Over time, costs of wind generation have fallen in many regions.
The main limit is variability. Wind does not follow demand, and calm periods can last for days. A grid with a lot of wind relies on a mix of tools: other generation, energy storage, long-distance transmission, demand flexibility and good forecasting. The wider effects are discussed under variable generation. Turbine blades are also hard to recycle, and the industry is working on better end-of-life solutions. A life cycle assessment helps compare these impacts with other sources.
New projects typically go through project permitting and environmental review, which look at wildlife, noise, shadow flicker, aviation and community effects.
Safety and careers
Turbines involve heights, rotating machinery and high voltage. Work on them is done by trained, certified personnel using strict procedures, and the public should keep clear of operating sites. A wind technician role is described in general terms elsewhere on this site, and training routes differ between countries.
This article is general information, not engineering advice. Performance and regulation vary by location.