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Wind turbines
by Chris Woodford. Last updated: January 4, 2013.
Wind turbines are like airplanes running
on the spot—spinning round but
going nowhere. They're serving a very useful purpose, however.
There's energy locked in wind and these giant propellers can
capture some of it and turn it instantly into electricity.
Have you ever stopped to wonder how wind turbines work? Let's take a
closer look!
Photo: Left: A small wind farm in Colorado, United States. These are relatively small turbines: each one produces about 700kW of energy (enough to supply 225 homes). The turbines are 79m (260ft) high (from the ground to the very top of the rotors) and the rotors themselves are 48.5m (159ft) in diameter. The top part of each turbine (called the nacelle) rotates on the tower beneath so the spinning blades are always facing directly into the wind. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DoE/NREL).
How does a turbine generate electricity?
A turbine is a machine that spins around
in a moving fluid (liquid or
gas) and catches some of the energy passing by. All sorts of machines
use turbines, from jet engines to
hydroelectric power
plants and from diesel railroad locomotives to windmills. Even a
child's toy windmill is a simple form of turbine.
The huge rotor blades (propellers) on the front of a wind turbine
are the
"turbine" part. As wind passes by, the kinetic energy
(energy of movement) it contains makes the blades spin around (usually
quite slowly). The
blades have a special curved shape so they capture as much energy from
the wind as possible.
Although we talk about "wind turbines," the turbine is only one of
the three main parts inside these giant machines. The second part is a gearbox whose gears convert the slow speed of the spinning blades into higher-speed
rotary motion—turning the drive shaft quickly enough to power the
electricity generator.
The generator is the third main part of a
turbine and it's exactly like an enormous, scaled-up version of the dynamo on a bicycle. When you
ride a bicycle, the dynamo touching the back wheel spins around and
generates enough electricity to make a lamp light up. The same thing
happens in a wind turbine, only the "dynamo" generator is driven
by the turbine's rotor blades instead of by a bicycle wheel.
Read more in our main article about generators.
Photo: Head for heights! You can see just how big a wind turbine is compared to this
engineer, who's standing right inside the nacelle (main unit) carrying out maintenance. Notice how
the white blades at the front connect via an axle (gray—under the engineer's feet) to the gearbox and generator
behind (blue). Photo by Lance Cheung courtesy of US Air Force (follow this link to see the original, much bigger version of this picture, but note that
it's about 2MB and may take some time to download).
How does a wind turbine work?
- Wind (moving air that contains kinetic energy) blows toward the turbine's rotor blades.
- The rotors spin around slowly, capturing some of the kinetic energy from the wind, and turning the central drive shaft that supports them.
- The rotor blades can swivel on the hub at the front so they meet the wind at the best angle for harvesting energy.
- Inside the nacelle (the main body of the turbine sitting on top of the tower and behind the blades), the gearbox converts the low-speed rotation of the drive shaft (about 16 revolutions per minute, rpm) into high-speed (1600 rpm) rotation fast enough to drive the generator efficiently.
- The generator, immediately behind the gearbox, takes kinetic energy from the spinning drive shaft and turns it into electrical energy.
- Anemometers (wind-speed monitors) and wind vanes on the back of the nacelle provide measurements
about the wind speed and direction.
- Using these measurements, the entire top part of the turbine (the rotors and nacelle) can be rotated by a yaw motor, mounted between the nacelle and the tower, so it faces directly into the oncoming wind and captures the maximum amount of energy. If the wind speed rises too much, brakes are applied to stop the rotors from turning (for safety reasons).
- The electric current produced by the generator flows through a cable running down through the inside of the turbine tower.
- A substation transforms the voltage of the electricity so it can be transmitted efficiently to nearby communities.
- Homes enjoy clean, green energy.
- Wind carries on blowing past the turbine, but with lower speed and lower energy (for reasons explained below) and more
turbulence (since the turbine has disrupted its flow).
How turbines harvest maximum energy
If you've ever seen a wind turbine, you'll know that they are
absolutely gigantic and mounted on incredibly high towers. The bigger
the rotor blades, the more energy they can capture from the wind. The
giant blades (typically 70 m or 230 feet in diameter, which is about 30
times the wingspan of an eagle) multiply the wind's force like
a wheel and axle, so even
a gentle breeze is enough to make the outer edges of the blades turn
around. Although the blades
rotate quite slowly, the inner axle and turbine rotate with greater
force—enough to turn the generator and make
electricity. (Wind turbines usually have anemometers—automatic speed
measuring devices—built into them and brakes that lock the
blades if the wind speed is too high.)
A typical wind turbines is 85 meters (280 feet) off the
ground—that's like 50 tall adults standing on one another's shoulders! There's
a good reason for this. If you've ever stood on a hill that's the
tallest point for miles around, you'll know that wind travels
much faster when it's clear of the buildings, trees, hills, and other
obstructions at ground level. So if you put a turbine's rotor
blades high in the air, they capture considerably more wind energy
than they would lower down. And capturing energy is what wind turbines
are all about.
Photo: This unusual Darrieus
"egg-beater" wind turbine rotates about a vertical axis, unlike a
normal propeller turbine.
Its main advantage is that it can be mounted nearer to the ground,
without a tower, which makes it cheaper and simpler to construct.
Photo by courtesy of US Department of Energy.
Since the blades of a wind turbine are rotating, they must have
kinetic energy, which they "steal" from the wind. Now
it's a basic law of physics (known as the conservation
of energy) that you can't make energy out of nothing, so the wind
must actually slow down slightly when it passes around a wind
turbine. That's not really a problem, because there's usually
plenty more wind following on behind! It is a problem if you want to
build a wind farm: unless you're in a really windy place, you have
to make sure each turbine is a good distance from the ones around it
so it's not affected by them.
Advantages and disadvantages of wind turbines
One of the drawbacks of wind turbines is that they don't generate
anything like as much energy as a conventional power plant. Each
turbine makes
about 1 megawatt of power, which is enough to power 500 electric
toasters running simultaneously—and enough to supply about 1000 homes.
You'd need about 1000–2000 turbines to make as much power as a
really sizable coal-fired power plant or
a nuclear power station
(either of which can generate enough power to run a million toasters at the same time).
But on the plus side, wind turbines are clean and green: unlike coal
stations, they don't make the carbon dioxide emissions that are
causing global warming or the
sulfur dioxide emissions that cause
acid rain. And they don't have the security and pollution problems
many people associate with nuclear power. Is wind the energy of the
future? It just might be!
But what if the wind doesn't blow?
It's hard to imagine why anyone would object to clean and green wind turbines—especially
when you compare them to dirty coal-fired plants and risky nuclear ones. Some people worry
that because wind is very variable, we might suddenly lose all our electricity and find
ourselves plunged into a "blackout" (a major power outage) if we rely on it too much.
The reality of wind is quite different. Wherever you live, your power comes from a complex grid
(network) of highly interconnected power-generating units (ranging from giant power plants to individual wind turbines).
Utility companies are highly adept at balancing power generated in many different places, in many different ways, to match the load (the total power demand) as it varies from hour to hour and day to day. The power from any one wind turbine will fluctuate as the wind rises and falls, but the total power produced by thousands of turbines, widely dispersed across an entire country, is much more regular and predictable. For a country like the UK, it's pretty much always windy somewhere.
As Graham Sinden of Oxford University's Environmental Change Institute has shown, low wind speeds affect more than half the country
for only 10 percent of the time; for 60 percent of the time, only 20 percent of the UK suffers from low wind speeds;
and only for one hour per year is 90 percent of the UK suffering low speeds (Sinden 2007, figure 7). In other words, having
many wind turbines spread across many different places guarantees a reasonably steady supply of wind energy virtually all year round.
While it's true that you might need 1000 wind turbines to produce as much power as a giant coal or nuclear plant, it's also true that if a single wind turbine fails or stops turning, it causes only 1/1000th (0.1 percent) of the disruption you get when a coal or nuclear plant fails (which happens more often than you might think). It's also worth bearing in mind that wind is extremely predictable several days in advance so it's easy for power planners to take account of its variability as they figure out how to make enough power to meet expected demands.
Opponents of wind power have even suggested that it might be counter-productive, because we'd still need to have backup coal or nuclear plants (or some way of storing wind-generated electricity) for those times when there's not enough wind blowing. That would certainly be true if we made all our energy from one, single mega-sized wind turbine—but we don't! In reality, even countries that have large supplies of wind energy have plenty of other sources of power too (Denmark, for example, makes 20 percent of its electricity—and meets 43 percent of its peak load—with wind); as long as wind power is making less than half of a country's total energy, the variability of the wind is not a problem.
Photo: You can put lots of turbines together
to make a wind farm, but you need to space them out to harvest the
energy effectively. Combining the output from many wind farms
in many different areas produces a smoother and more predictable power supply.
This wind farm is at one of the world's windiest places: Altamont Pass, California, United States.
Photo by courtesy of US Department of Energy.
Acknowledgments
My thanks to Prof. John Twidell of AMSET for suggesting that I add this new material on the variability of wind power (a very broad, very simplified summary of the kind of arguments made in the papers by Michael Milligan et al and John Twidell, which you'll find listed in the references below).
Find out more
On this website
Simple introductions
- How wind turbines work: Ecotricity, the leading British wind energy company has a very informative website, with lots of photos showing how its numerous wind parks were constructed.
- Wind with Miller: A great introduction to wind energy from the Danish Wind Industry Association. This one's for younger readers.
More detail
- Guided tour on wind energy: A deeper introduction to wind energy for older readers, also from the Danish Wind Industry Association. Includes a detailed look at all the parts of a wind turbine and what they do.
- Wind: Physicist David MacKay looks at how much of a contribution wind power can realistically make to the UK's total energy needs in his book Sustainable Energy Without the Hot Air.
- Wind Turbines: Fundamentals, Technologies, Application, Economics by Eric Hau. Springer, 2006. A hugely detailed reference.
- "Wind Power Myths Debunked" by Michael Milligan et al, IEEE Power & Energy Magazine, November/December 2009. A clear, reasonably-easy-to-understand explanation of how wind power can be integrated into a grid network.
- "Assessing Backup Requirements for Wind Power" by John Twidell, Institute of Physics (IOP) Energy Group Newsletter, November 2010.
- Characteristics of the UK wind resource: Long-term patterns and relationship to electricity demand [PDF] by Graham Sinden. Energy Policy, Volume 35, Issue 1, January 2007, Pages 112–127. This detailed paper studies wind power production over a period of 34 years at 66 different UK sites and debunks the myth that wind turbines are unproductive for much of the time.
News articles
Photographs
Need photos for a school project on wind power? Have a look at:
- US Department of Energy/National Renewable Energy Laboratory Photo Library: Enter the search term "wind turbine" and you'll find a couple of thousand photos of turbines. As works of a US Federal Government agency, some of these photos are in the public domain, but others (supplied by turbine manufacturers) are copyright restricted.
- Flickr: Wind turbines: Some Flickr photos are published under Creative Commons licences (allowing limited reuse under certain conditions); others are copyright images.
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