Wind Turbine Technology





There are two different types of wind turbines. These are HAWT (Horizontal Axis Wind Turbines) and VAWT (Vertical Axid Wind Turbines). Within these two groups the most commonly used wind turbines are Savonius and Darrieus VAWTs and the standard HAWT. The image below shows these most common wind turbines and how they move in wind.

Vertical vs Horizontal Wind Turbines

Vertical Axis Wind Turbines are less expensive and easier to maintain as the generator is housed on the ground. They perform better than HAWTs in locations with gusty wind, urban areas, and low wind sites. VAWTs produce minimal noise pollution and wind direction doesn’t have an impact on operation. The primary use of VAWTs is in remote locations where maintenance costs increase significantly, areas with more extreme wind that might damage HAWTs due to their higher height, and places where noise is a major concern (such as residential wind turbines). VAWTs can also be placed much closer together than HAWTs.[5]

Horizontal Axis Wind Turbines are located higher off the ground which results in better efficiency due to the higher wind speeds. Maintenance is more expensive since technicians sometimes have to perform work hundreds of meters off the ground since all the parts are contained up the mast. In spite of this the efficiency advantage provided by the increase in turbine height outweighs this maintenance expense which is why HAWTs continue to be the most common wind turbine used.

HAWT (Horizontal Axis Wind Turbine) Parts

Wind turbine illustration. Image by US DOE.

Wind turbine illustration. Image by US DOE.

Below each of the parts in the image above is explained. The information provided is for horizontal axis wind turbines as they are the most common and only the basics and most common configurations are covered. In reality there is much more to each of the wind turbine components outlined below.

Blades

Medium size wind turbine blades. Image by Alexh.

Medium size wind turbine blades. Image by Alexh. License: GNU FDL

Most HAWTs have two or three blades that connect to a central hub piece. Wind blowing against the blades gives the blades lift and causes them to rotate. Modern wind turbine blades are blade using aluminum and composite materials for their low rotational inertia which means they have less resistance to changes of speed. Blade shape, material, and construction are all analyzed using computer software to obtain the ideal blade that maximizes both efficiency and survivability.[4]

Rotor

When the blades are attached to the hub the result is a wind turbine rotor. The size of the rotor diameter will have a large impact on the amount of wind energy obtained from the overall turbine. It will also dictate the distance that turbines need to be placed from one another. As a rule HAWTs are placed at least 7 times the rotor diameter from each other but it is argued this is too little.[1] It is also used to determine a safe setback distance for other structures where tower collapse, destruction, ice ejection, or other hazards won’t cause collateral damage.

Blade Pitch

Decomissioned wind farm turbines with blades feathered. Image by Christian Razukas.

Decomissioned wind farm turbines with blades feathered showing blade pitch. Image by Christian Razukas. License: CC BY-SA 2.0

Almost all modern HAWTs incorporate blade pitch capabilities that allow for changing the angle of the blades. This allows automated software and operators to adjust the amount of wind energy being caught by the blades.[6] Increasing the blade angle relative to wind direction is known as furling the blades. This allows turbines to operate in higher winds that would damage a static blade system. Decreasing the angle is known as feathering and makes maintenance easier and safer.

This image shows a decommissioned wind farm with all the turbine blades feathered. These Mitsubishi 250 kW wind turbines were later replaced with GE Energy 1.5 MW wind turbines.

Brakes

Dynamic braking resistor for wind turbine. Image by Glogger.

Dynamic braking resistor for wind turbine. Image by Glogger. License: CC BY-SA 3.0

Wind turbines incorporate multiple braking systems with fail safes to prevent run away turbines caused by mechanical and software failures.[3] When operating at high speeds electrical braking is used first to prevent wear the mechanical brakes. This is done by dumping the electricity from the wind turbine generator into a resistor bank which converts the kinetic (motion) energy into heat energy. Another popular brake system used alone or in combination with electrical brakes are blade tip brakes. These are simply adjustable blade tips that can be turned to increase drag. In addition to electrical and air brakes are the mechanical brakes. These are either disc or drum brakes that are usually attached to the high speed shaft. A hydraulic system provides power to the air and mechanical brake system.

The controller is an electronic control device that handles all of the braking systems and blade pitch to provide optimal operation while the turbine is unattended. All of these systems including the controller are setup to be fail safe. This means that hydraulic pressure is required to keep the brake systems from being active and there are mechanical fail safes to prevent run away turbines in the event of software errors.

Low-speed Shaft

The low speed shaft connects the rotor to the gearbox as shown in this video.

Gearbox

A 12 ton wind turbine gearbox and brake assembly. Image by Paul Anderson.

A 12 ton wind turbine gearbox and brake assembly. Image by Paul Anderson. License: CC BY-SA 2.0

Gearing is required because the generators used to produce electricity require significantly more RPM (1,200+) than the rotor spins at (around 30-60).[7] The reason this is required is explained below under the generator section. The low speed shaft connects to the gearbox and the rotation is stepped up to the necessary levels for the generator. A high speed shaft is connected at the high speed side of the gearbox and to the generator. The mechanical brake assembly is usually attached to the gearbox making it a single unit as shown in the image. The disc or drum brake is often attached to the high speed shaft.

Reliability of the gearbox has become a major limiting issue to increasing wind turbine performance. Larger wind turbines and those located in high wind areas can experience significant wear after only 3-5 years of operation with 2 complete gearbox replacements expected over a 20 year period.[2] These replacements are costly and cause extended downtime making them a significant financial burden to wind farm owners.

High-speed shaft

The high speed shaft exits the gearbox and connects to the generator as seen in the video below.

Generator

The most common wind turbine generator is an asynchronous or induction electric motor/generator. If the electric grid is connected to a wind turbine and there is no wind the generator will act as a motor turning the high speed shaft using electricity from the grid. The speed that this shaft turns at is dictated by the grid connection. A gearbox is needed to change the rotor rotation into a speed higher than the grid would turn the shaft. Wind turbines are disconnected and the brakes engaged during low wind to prevent the system from acting like a motor if the shaft speed gets too low. New wind turbine generators sometimes improve efficiency or reduce maintenance by using synchronous generators, gearless systems, and permanent magnets.[8]

Controller

The controller acts as the central monitoring and decision making component of a wind turbine. Both analog and digital signals are received, recorded, and monitored by the controller and used to make decisions about turbine operation. The analog signals include information about the electricity being generated, temperature of various components, speed of the low and high speed shafts, wind speed, and yaw. Digital signals indicate things like the status of the braking system, hydraulic pressure, wind direction, vibration, and other on/off or safety indicators that trip when certain conditions are met.[3] All of this information is processed by the controller to ensure maximum efficiency and safe operation. When wind speed is too low to produce electricity the controller disconnects from the grid and brakes the turbine and when wind speed is too high blade pitch is adjusted, electrical braking is applied, and the turbine is shut down to prevent damage due to overheating or mechanical failure. All of these aspects of turbine operation are handled automatically by the controller. In addition to all of these sensor signals the controller will usually include self monitoring capabilities that check for faulty instrument readings or software errors.

Anemometer & Wind Vane

Combination wind vane and anemometer. Image by US DOE.

Combination wind vane and anemometer. Image by US DOE.

An anemometer mounted to the nacelle (the turbine housing) measures the speed of the wind and transmits this information to the controller. These devices are also used to measure potential for wind turbines by mounting them on large towers made of scaffolding. The anemometers used for wind turbines are much more robust and accurate than your average home weather station because good wind speed readings are critical. A wind vane is also mounted to the nacelle to provide information about wind direction to the yaw drive and controller. This information is required to keep the turbine rotor pointed into the wind as otherwise the efficiency of the turbine is reduced due to less wind flowing through the rotor area.[9]

Yaw Drive & Motor

Schematic of yaw drive. Image by Hanuman Wins.

Schematic of yaw drive. Image by Hanuman Wind. License: GNU FDL

The yaw drive is a simple mechanical system for turning the nacelle. This allows the rotor to face directly into the wind using the wind vane information. If a rotor is not pointed directly into the wind it will operate at a reduced efficiency as less wind is traveling through the rotor area. This is sometimes desirable in high winds where direct wind would overload the system. Usually a small (relative to the wind turbine) electric motor called the yaw motor powers this drive with the assistance of the controller and wind vane. Sometimes hydraulics are used to power the yaw drive instead.

Nacelle

Wind tower nacelles being unloaded. Image by Port of San Diego's Working Waterfront.

Wind tower nacelles being unloaded. Image by Port of San Diego's Working Waterfront. License: CC BY 2.0

The nacelle protects all of the components that are on the tower. This includes the gearbox, low and high speed shafts, generator, controller, and brakes. The wind vane and anemometer are mounted to the top of the nacelle. Design of the nacelle has to adhere to basic aerodynamic principles while large wind turbines also have to add an internal walkway and proper workspace for technicians to perform maintenance. Most commonly a composite material or fiberglass is used to make the nacelle shell as they insulate well against water and can be easily molded to fit any shape. In the past little attention was paid to the shape and design of the nacelle and instead focus was on the mechanical turbine components. Current design maximizes reliability, safety, aerodynamics, and cost efficiency.[10]

Turbine Tower / Mast

The wind turbine tower / mast is used to lift and support the nacelle. Large wind turbine masts are constructed out of tubular steel or steel lattice.[11] Smaller turbines like those found in domestic applications use either combined scaffolding poles to construct a tower or a single pole with guy wires for support.[12] Another less common tower used for small wind turbines is a flip up turbine which is simply a triangular base constructed of metal.[12] Wind speed increases with height so the taller a tower the more energy the turbine will produce but maintenance costs also increase with height.

Sources:

1. New study yields better turbine spacing for large wind farms
2. Wind Turbine Gearbox Reliability
3. Wind Turbine: Components and Operation
4. Wind Turbine Rotor Design
5. Aerodynamics of a Drag-type Vertical Axis Wind Turbine
6. Wind Turbine Yawing and Furling Mechanisms
7. Wind Turbines Today
8. Direct Drive Permanent Magnet Synchronous Generator
9. Soft Yaw Drives for Wind Turbines
10. Wind Turbine Design Cost and Scaling Model
11. Wind and Water Power Program: How Wind Turbines Work
12. Wind Turbine Tower Basics

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