Types of Electricity Generating Wind Turbines
Vertical axis wind turbines (VAWTs) have advantages and disadvantages, but overall they have not been commercially successful like their cousins, the horizontal axis wind turbines (HAWTs). This is largely due to the poor performance and reliability of most VAWTs. However, there are practical applications for VAWTs and new research and technology is improving their performance.
Horizontal Axis Wind Turbines (HAWTs), on the other hand, are very advanced, reliable, and economically viable. They come in many sizes and shapes, but they are all descendents of the old windmills used to grind grain or pump water. Today these machines are proven: they are used throughout the world producing clean, affordable, and sustainable electricity. Modern horizontal axis wind turbines produce electricity 70-85% of the time (whenever the wind is over 7-8 mph).
Wind turbines are classified by their size, or "capacity" (how much electricity they can produce). They can be small (< 100 kW), intermediate (100-500 kW), or large (500 kW - 5 MW). Small wind turbines are used for homes, farms, and remote sites where electricity is hard to come by. They can be connected to the electric grid, but often they are just connected to a battery bank instead. Intermediate wind turbines are often used for schools or in hybrid systems with diesel generators used for powering remote towns and villages. Large, or utility scale, wind turbines are used for producing electricity which goes onto the electric grid. We then can use this electricity in our homes, schools, and businesses.
Normally, the rotors of horizontal axis wind turbines are upwind of the tower, so the wind hits the blades before it hits the tower. There are a few models of wind turbines that have rotors downwind of the tower, but the only commercially available downwind turbines are small scale machines.
Yawing - Orienting the Turbine Into the Wind
Horizontal Axis Wind Turbines must be able to rotate to face the wind. This is called "yawing." The blades will not spin if the wind is hitting them from the side, so it is very important that the turbine can yaw. There are two ways a wind turbine can yaw: Active or Passive yawing.
Passive yawing uses the force of the wind to push the turbine into place. For this to work, the turbine must either have a downwind rotor or a vane to orient the rotor perpendicular to the wind. Most small wind turbines use passive yawing systems.
Active yawing is used by all Intermediate and Large turbines produced today, as well as some small turbines made in Europe. A small anemometer and wind vane on top of the wind turbine electronically tell a controller which way to point the rotor into the wind. Then the "yaw drive" mechanism turns gears to point the rotor into the wind.
Modern Small-Scale Wind Turbines
Small scale (also called "residential scale") wind turbines are defined by having a power output of 10 kW or less. They can supply all or part of the power for homes, farms, schools, and businesses. Small wind turbines are high tech, highly reliable, and dependable machines. They require relatively low maintenance because small wind turbines only have 2-3 moving parts. Though the design may seem simple, they are actually very sophisticated. Small wind turbines have been tested thoroughly and have proven their reliability and performance. In the year 2008, over 19,000 small wind turbines were purchased worldwide. More than half of these turbines were sold in the US. There are tens of thousands more already in use, many of which are connected to the electric grid. American companies are currently the technology and market leaders of small wind turbines, while European companies tend to dominate the large wind turbine market.
One major concern for wind turbines is what happens when the wind gets too fast. Violent winds could damage a wind turbine if it did not have a way to protect itself. Small wind turbines built today protect themselves from overspeed conditions through a process called "furling." When a wind turbine furls, it turns the blades out of the direction of the wind to slow or stop the rotation of the rotor. Most small wind turbines furl either by turning the rotor to the side or by tilting the rotor up and back.
Modern Large-Scale Wind Turbines
Large wind turbines can make enough electricity for hundreds, sometimes thousands of homes. An average wind turbine used for a wind farm is 1.5 - 2 MW (Megawatts). As a rule of thumb, 1 MW can power about 300 American homes. The largest wind turbine in the world is the Enercon E-126. This turbine can produce a maximum of 7 MW, enough to power 5,000 homes in the country of Germany where this turbine was built. Here in the US, however, it could only fill the demand of 2,000 homes, because average American homes consume much more energy than homes in other countries.
Industrial scale wind turbines have to protect themselves from overspeed conditions as well. However, unlike small wind turbines, large turbines are not designed to "furl" out of the wind. Instead, most large turbines control their rotation speed by actively "pitching" the blades. The pitch of the blades is the angle at which they are oriented into the wind (see #3 on the diagram below). Just like there is a controller to yaw the turbine into the wind, there is also a device that controlls the pitch of the blades. When the wind is too strong, the the pitch of the blades increases. This creates more drag as the blades spin through the air, which slows them down. When the wind is over a certain cut-off speed, the blades pitch all the way out so that they are parallel to the wind. When the blades are parallel to the wind, the turbine will not rotate at all. You can experiment with the effects of various blade pitches on Kidwind model turbines!
A large wind turbine can be 300-450 feet tall from the base to the tip of the blade. Each blade is usually between 30 - 45 meters long (100-150 feet), while the largest blade in the world is 61.5 feet long (202 feet). The diameter of the blades is often larger than the wingspan of a 747! In total, a large turbine can weigh 160 tons or more!
Wind Turbine Gears
One of the heaviest and most expensive parts of a large wind turbine is the gearbox. Gears give a wind turbine a mechanical advantage in the same way that the way gears on your bicycle do. The gearbox turns the slow rotation of the blades (12-20 RPM) into an extremely fast rotation in the generator (1600 RPM). That means that these turbines need a gear ratio of 100:1 or sometimes more! For comparison, the gear ratio on your bicycle is probably around 3:1. The gearbox on a wind turbine requires a lot of maintenence. Because they are so expensive and require so much maintenence, some wind turbine manufacturers are trying to design wind turbines that do not use gears at all.
Wind Turbine Blades
Most wind turbines made today have three blades. Some have more than three, some have less, but because of the physics of wind energy, turbine designs with three blades have proven to be the most efficient and economical.
Wind turbine blades spin fairly slowly. The average rotation speed for large wind turbine rotors is 12-20 RPM. The slow rotation speed makes the turbines quieter and more aesthetic while having less environmental impact. However, this slow rotation speed makes the heavy-duty, expensive gearbox necessary.
Large wind turbine blades are usually made of a hard, stiff composite material. Fiberglass, balsa wood, and carbon fiber are the most common materials found in wind turbine blades. The weight of the blades is very important, so they are designed using the lightest materials available. A single wind turbine blade can cost upwards of $100,000!