Types of Wind Turbines
There are two basic wind turbine designs: those with a vertical axis (sometimes referred to as VAWTs) and those with a horizontal axis (sometimes referred to as HAWTs). There are several manufacturers of vertical axis turbines, but they have not penetrated the "utility scale" (100 kW capacity and larger) market to the same degree as horizontal axis turbines.
Vertical Axis Wind Turbines
VAWTs typically fall into two main design classes: lift-based or drag-based.
- Drag-based, or Savonius, turbines generally have rotors with solid vanes ("scoops") that rotate about a vertical axis. A 3-cup anemometer (which is used to measure wind speed) is an example of a drag-based design.
- Lift-based, or Darrieus, turbines have a tall, vertical airfoil style (some appear to have an "eggbeater" shape). The Windspire is a type of lift-based turbine that is undergoing independent testing at the National Renewable Energy Laboratory's National Wind Technology Center.
<- the Mariah Windspire 1kW wind turbine installation at the National Wind Technology Center (NWTC)
Horizontal Axis Wind Turbines
HAWTs have a propeller-style design, with either two or three blades that rotate around a horizontal axis. Currently, horizontal axis turbines are the most common, particuarly in commercial and utility scale markets (greater than 100 kW capacity). Horizontal axis turbines are either "upwind" (the wind hits the blades before the tower) or "downwind" (the wind hits the tower before the blades).
Wind Turbine Sizes
The "size" of a wind turbine typically refers to its capacity. "Small wind turbines" are generally defined as those with a capacity of 100 kilowatts and less. According to the American Wind Energy Association ( AWEA), residential-scale turbines generally have a rated capacity of 10 kW or less (there are wind turbines with a rated capacity less than 1 kW), and commercial-scale turbines are those with a rated capacity between 21kW and 100kW. There is some grey area to these definitions; there are indeed wind turbines with a rated capacity that falls between 10 kW and 20kW. "Utility-scale" wind turbines typically range in size from greater than 100 kW to as large as several MW.  In 2009, two multimegawatt research turbines were installed at the National Wind Technology Center, a 1.5 MW turbine manufactured by GE and a 2.3 MW turbine manufactured by Siemens.
Components of a Wind Turbine
- Anemometer: Measures the wind speed and transmits wind speed data to the controller.
- Blades: Most turbines have either two or three blades. Wind blowing over the blades causes the blades to "lift" and rotate.
- Brake: A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies.
- Controller:The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds.
- Gear box:Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gear boxes.
- Generator: Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.
- High-speed shaft: Drives the generator.
- Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
- Nacelle: The nacelle sits atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on.
- Pitch: Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.
- Rotor: The blades and the hub together are called the rotor.
- Tower: Towers are made from tubular steel (shown here), concrete, or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity.
- Wind direction: This is an "upwind" turbine, so-called because it operates facing into the wind. Other turbines are designed to run "downwind," facing away from the wind.
- Wind vane: Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.
- Yaw drive: Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don't require a yaw drive, the wind blows the rotor downwind.
- Yaw motor: Powers the yaw drive.
<references>     This article incorporates public domain material from the document: "DOE Wind& Water Power Program: Inside the Wind Turbine"