How a Wind Turbine Works: Page 2 of 5

Beginner

Inside this Article

Wind Turbine
Wind Turbine Up Close
An anemometer for measuring wind speed
An anemometer is a drag device that can’t spin any faster than the wind is moving. The upwind cup hinders it even further. Only through calibration does it register accurate wind speed. Drag devices are inefficient collectors of wind energy.
A three-bladed, permanent-magnet wind turbine
After reading this article, you’d be able to identify this as a three-bladed, upwind, direct-drive, permanent-magnet, side-furling wind turbine if you saw it in action.
A gearbox
In some cases, a gearbox is necessary to optimize blade/rotor rpm with alternator rpm (also note this turbine’s large disk brake).
An old Jacobs field-wound alternator
This old Jacobs field-wound alternator requires brushes to supply energy to the electromagnetic field on the spinning rotor.
Permanent magnets on the outer rotor allow a simple brushless design.
Permanent magnets on the outer rotor allow a simple brushless design. The windings are mounted on the inner immobile stator.
An axial PM alternator
This axial PM alternator has magnets on its rotor that spin past stationary coils face to face.
Brushes transmit electricity from a yawing turbine down a stationary tower
Brushes transmit electricity from a yawing turbine down a stationary tower without twisting wires.
Passive Upwind Yawing
Passive Upwind Yawing
Passive Downwind Yawing
Passive Downwind Yawing
Active Upwind Yawing
Active Upwind Yawing
Centrifugal force on the blades works a linkage to change blade pitch
Centrifugal force on the blades works a linkage to change blade pitch on an old Jacobs generator. This complex but highly effective governor precisely regulates rpm.
A side-furling governor uses the force of the wind to pivot the blades
A side-furling governor uses the force of the wind on an off-center joint to pivot the blades out of the wind. In this case, mechanical furling is used to brake the machine during maintenance.
Dynamic braking works by shorting all three wires together
Dynamic braking uses the alternator’s electromagnetism against itself by shorting all three wires together.
Wind Turbine
An anemometer for measuring wind speed
A three-bladed, permanent-magnet wind turbine
A gearbox
An old Jacobs field-wound alternator
Permanent magnets on the outer rotor allow a simple brushless design.
An axial PM alternator
Brushes transmit electricity from a yawing turbine down a stationary tower
Passive Upwind Yawing
Passive Downwind Yawing
Active Upwind Yawing
Centrifugal force on the blades works a linkage to change blade pitch
A side-furling governor uses the force of the wind to pivot the blades
Dynamic braking works by shorting all three wires together

Many different blade designs and configurations have been tried over the years, and most don’t work terribly well. There are good reasons that support the successful designs. Look around—from sailboat wind generators to utility-scale machines—and you’ll see the results of decades of engineering development. More information on aerodynamics can be found in many places. Hugh Piggott’s book, Windpower Workshop, offers technically accurate, concise explanations.

Limits to Wind Energy

Wind is a cubic resource. The power available in the wind increases as the cube of the wind speed. If we shovel gravel twice as fast, we get twice as much gravel. If the atmospheric heat engine shovels moving air twice as fast, we get eight times as much energy (23, or 2 × 2 × 2).

This helps us understand wind generator design and siting. At the low end of the scale, it’s important to know that anyone making significant performance claims at, say, 4 miles per hour is either clueless about the physics of wind or trying to pull the wool over your eyes. Do the math: Four times four times four is 64; 10 times 10 times 10 is 1,000. There just isn’t very much energy at very low wind speeds. A wind generator doesn’t start doing much until at least 10 mph.

On the other end of the scale, it’s important to remember the forces we are dealing with. Forty times forty times forty is 64,000. That’s 64 times more energy than a 10 mph wind carries. You can see why wind turbines worth buying will protect themselves (govern) at 25 mph to 30 mph, shedding the rare high winds so they can stay alive for the next reasonable wind.

So there’s a practical limit to how high of winds a wind generator is designed to capture. There’s also a physical limit of how much of any wind you can capture. The Betz theorem states that you can only harvest a maximum of 59.3% of the wind before any attempt to take more will decrease what you get.

Think of it this way. A knife blade sticking up will take almost no energy out of the wind (only a little, due to friction). On the other end of the scale, a brick wall will try to take all of the energy out of the wind, blocking its path completely, but it won’t turn any of it into useful motion. In the first case, the wind just passes by the “collector.” In the second case, the wind backs up behind the collector, slows down, goes around, and just doesn’t do any useful work.

Somewhere in between, there’s a sweet spot where we can capture some of the energy in the wind without slowing it down too much. We need to allow enough moving air through the collector to ensure adequate flow, and not back up too much wind against our collector, diminishing the return. Betz says that the maximum is about 60%. In real-world applications, a well-designed, modern utility-scale turbine might hit 50%, while residential turbines fall more in the 20% to 40% range.

Mechanical Transmission

Once we’ve taken our portion of the energy of that moving air and turned it into spinning motion in a shaft, we still need to spin the alternator. That shaft is spinning in a certain speed range, generally peaking at 150 to 900 rpm, depending on the machine’s size and its design. (Low-rpm machines capture the same amount of energy from the wind as faster-turning ones, but the slower pace incurs less wear and tear and makes less noise.)

We can use a generator that works in the same speed range as the spinning shaft coupled to the blades (called the “rotor”). These are “direct-drive” machines, and they are the simplest and most efficient, if the blades and generating device are well-matched.

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